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A cephalopod is any member of the molluscan
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...
Cephalopoda (
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
plural , ; "head-feet") such as a
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting ...
,
octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefis ...
,
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control ...
, or
nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of
arms Arms or ARMS may refer to: *Arm or arms, the upper limbs of the body Arm, Arms, or ARMS may also refer to: People * Ida A. T. Arms (1856–1931), American missionary-educator, temperance leader Coat of arms or weapons *Armaments or weapons **Fi ...
or
tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s (
muscular hydrostat A muscular hydrostat is a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in ...
s) modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt
ink Ink is a gel, sol, or solution that contains at least one colorant, such as a dye or pigment, and is used to color a surface to produce an image, text, or design. Ink is used for drawing or writing with a pen, brush, reed pen, or quill. ...
. The study of cephalopods is a branch of
malacology Malacology is the branch of invertebrate zoology that deals with the study of the Mollusca (mollusks or molluscs), the second-largest phylum of animals in terms of described species after the arthropods. Mollusks include snails and slugs, clams ...
known as
teuthology Teuthology (from Greek , "cuttlefish, squid", and , ''-logia'') is the study of cephalopods such as octopus, squid, and cuttlefish.
. Cephalopods became dominant during the
Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...
period, represented by primitive
nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...
s. The class now contains two, only distantly related,
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
subclasses:
Coleoidea Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whos ...
, which includes
octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefis ...
es,
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting ...
, and
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control ...
; and
Nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...
ea, represented by ''
Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
'' and ''
Allonautilus The genus ''Allonautilus'' contains two species of nautiluses, which have a significantly different morphology from those placed in the sister taxon ''Nautilus''. ''Allonautilus'' is now thought to be a descendant of ''Nautilus'', rendering the ...
''. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
of cephalopods have been identified. Two important extinct
taxa In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
are the
Ammonoidea Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttl ...
(ammonites) and
Belemnoidea Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of r ...
(belemnites). Extant cephalopods range in size from the 10 mm (0.3 in) '' Idiosepius thailandicus'' to the 14 m (45.1 ft)
colossal squid The colossal squid (''Mesonychoteuthis hamiltoni'') is part of the family Cranchiidae. It is sometimes called the Antarctic squid or giant cranch squid and is believed to be the largest squid species in terms of mass. It is the only recognize ...
, the largest extant
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
.


Distribution

There are over 800
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
species of cephalopod, although new species continue to be described. An estimated 11,000 extinct
taxa In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
have been described, although the soft-bodied nature of cephalopods means they are not easily fossilised. Cephalopods are found in all the oceans of Earth. None of them can tolerate
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
, but the brief squid, '' Lolliguncula brevis'', found in
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the ...
, is a notable partial exception in that it tolerates
brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
. Cephalopods are thought to be unable to live in fresh water due to multiple biochemical constraints, and in their >400 million year existence have never ventured into fully freshwater habitats. Cephalopods occupy most of the depth of the ocean, from the
abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. ...
to the sea surface. Their diversity is greatest near the equator (~40 species retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N).


Biology


Nervous system and behavior

Cephalopods are widely regarded as the most intelligent of the
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s, and have well developed senses and large
brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...
s (larger than those of gastropods). The
nervous system In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...
of cephalopods is the most complex of the invertebrates and their brain-to-body-mass ratio falls between that of
endotherm An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
ic and
ectotherm An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life a ...
ic vertebrates. Captive cephalopods have also been known to climb out of their aquaria, maneuver a distance of the lab floor, enter another aquarium to feed on the crabs, and return to their own aquarium. The brain is protected in a
cartilaginous Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck a ...
cranium. The giant
nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...
fibers of the cephalopod mantle have been widely used for many years as experimental material in
neurophysiology Neurophysiology is a branch of physiology and neuroscience that studies nervous system function rather than nervous system architecture. This area aids in the diagnosis and monitoring of neurological diseases. Historically, it has been dominated b ...
; their large diameter (due to lack of
myelination Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...
) makes them relatively easy to study compared with other animals. Many cephalopods are social creatures; when isolated from their own kind, some species have been observed shoaling with fish. Some cephalopods are able to fly through the air for distances of up to 50 m. While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion; water continues to be expelled from the funnel while the organism is in the air. The animals spread their fins and tentacles to form wings and actively control lift force with body posture. One species, ''
Todarodes pacificus The Japanese flying squid, Japanese common squid or Pacific flying squid, scientific name ''Todarodes pacificus'', is a squid of the family Ommastrephidae. This animal lives in the northern Pacific Ocean, in the area surrounding Japan, along th ...
'', has been observed spreading tentacles in a flat fan shape with a mucus film between the individual tentacles while another, '' Sepioteuthis sepioidea'', has been observed putting the tentacles in a circular arrangement.


Senses

Cephalopods have advanced vision, can detect gravity with
statocyst The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. A similar structure is also found in ''Xenoturbella''. The statocyst cons ...
s, and have a variety of chemical sense organs. Octopuses use their arms to explore their environment and can use them for depth perception.


Vision

Most cephalopods rely on vision to detect predators and prey, and to communicate with one another. Consequently, cephalopod vision is acute: training experiments have shown that the
common octopus The common octopus (''Octopus vulgaris'') is a mollusc belonging to the class Cephalopoda. ''Octopus vulgaris'' is one of the most studied of all octopus species, and also one of the most intelligent. It ranges from the eastern Atlantic, exten ...
can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects. The morphological construction gives cephalopod eyes the same performance as shark eyes, however their construction differs, as cephalopods lack a cornea and have an everted retina. Cephalopods' eyes are also sensitive to the plane of
polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
of light. Unlike many other cephalopods,
nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
es do not have good vision; their eye structure is highly developed, but lacks a solid
lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'' ...
. They have a simple "
pinhole A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
" eye through which water can pass. Instead of vision, the animal is thought to use olfaction as the primary sense for
foraging Foraging is searching for wild food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce. Foraging theory is a branch of behavioral ecology that studies the foraging behavi ...
, as well as locating or identifying potential mates. Surprisingly, given their ability to change color, all octopuses and most cephalopods are considered to be
color blind Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
. Coleoid cephalopods (octopus, squid, cuttlefish) have a single photoreceptor type and lack the ability to determine color by comparing detected photon intensity across multiple spectral channels. When
camouflaging Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...
themselves, they use their chromatophores to change brightness and pattern according to the background they see, but their ability to match the specific color of a background may come from cells such as
iridophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contra ...
s and
leucophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contra ...
s that reflect light from the environment. They also produce visual pigments throughout their body, and may sense light levels directly from their body. Evidence of
color vision Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
has been found in the
sparkling enope squid The firefly squid (''Watasenia scintillans''), also commonly known as the sparkling enope squid or hotaru-ika in Japan, is a species of squid in the family Enoploteuthidae. It is the sole species in the monotypic genus ''Watasenia''. These tiny ...
(''Watasenia scintillans''). It achieves color vision with three photoreceptors, which are based on the same
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
, but use distinct
retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use re ...
molecules as chromophores: A1 (retinal), A3 (3-dehydroretinal), and A4 (4-hydroxyretinal). The A1-photoreceptor is most sensitive to green-blue (484 nm), the A2-photoreceptor to blue-green (500 nm), and the A4-photoreceptor to blue (470 nm) light. In 2015, a novel mechanism for spectral discrimination in cephalopods was described. This relies on the exploitation of
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the ...
(wavelength-dependence of focal length). Numerical modeling shows that chromatic aberration can yield useful chromatic information through the dependence of image acuity on accommodation. The unusual off-axis slit and annular pupil shapes in cephalopods enhance this ability by acting as prisms which are scattering white light in all directions.


Photoreception

In 2015, molecular evidence was published indicating that cephalopod chromatophores are photosensitive; reverse transcription polymerase chain reactions (RT-PCR) revealed transcripts encoding
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduct ...
and retinochrome within the retinas and skin of the
longfin inshore squid The longfin inshore squid (''Doryteuthis pealeii'') is a species of squid of the family Loliginidae. Description This species of squid is often seen with a reddish hue, but like many types of squid can manipulate its color, varying from a deep ...
(''Doryteuthis pealeii''), and the
common cuttlefish #REDIRECT Common cuttlefish #REDIRECT Common cuttlefish #REDIRECT Common cuttlefish {{redirect category shell, {{R from other capitalisation{{R from move ... {{redirect category shell, {{R from other capitalisation{{R from move ...
{{redire ...
(''Sepia officinalis'') and broadclub cuttlefish (''
Sepia latimanus ''Sepia latimanus'', also known as the broadclub cuttlefish, is widely distributed from the Andaman Sea, east to Fiji, and south to northern Australia. It is the most common cuttlefish species on coral reefs, living at a depth of up to 30 m ...
''). The authors claim this is the first evidence that cephalopod dermal tissues may possess the required combination of molecules to respond to light.


Hearing

Some squids have been shown to detect sound using their
statocyst The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. A similar structure is also found in ''Xenoturbella''. The statocyst cons ...
s, but, in general, cephalopods are deaf.


Use of light

Most cephalopods possess an assemblage of skin components that interact with light. These may include iridophores, leucophores,
chromatophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, ...
s and (in some species)
photophore A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors, h ...
s. Chromatophores are colored pigment cells that expand and contract in accordance to produce color and pattern which they can use in a startling array of fashions. As well as providing camouflage with their background, some cephalopods bioluminesce, shining light downwards to disguise their shadows from any predators that may lurk below. The
bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some ...
is produced by bacterial symbionts; the host cephalopod is able to detect the light produced by these organisms.
Bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some ...
may also be used to entice prey, and some species use colorful displays to impress mates, startle predators, or even communicate with one another.


Coloration

Cephalopods can change their colors and patterns in milliseconds, whether for
signalling In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
(both within the species and for
warning Warning may refer to: Signal * Precautionary statement * Warning sign * Warning system * Warning (traffic stop), issued by a police officer in lieu of a citation following a traffic stop Books * A Warning (book), ''A Warning'' (book), a 2019 ...
) or
active camouflage Active camouflage or adaptive camouflage is camouflage that adapts, often rapidly, to the surroundings of an object such as an animal or military vehicle. In theory, active camouflage could provide perfect concealment from visual detection. Activ ...
, as their chromatophores are expanded or contracted."Integument (mollusks)". Encyclopædia Britannica. 2009.
Encyclopædia Britannica 2006 Ultimate Reference Suite DVD An encyclopedia (American English) or encyclopædia (British English) is a reference work or compendium providing summaries of knowledge either general or special to a particular field or discipline. Encyclopedias are divided into articles ...
. Although color changes appear to rely primarily on vision input, there is evidence that skin cells, specifically
chromatophores Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, ...
, can detect light and adjust to light conditions independently of the eyes. The octopus changes skin color and texture during quiet and active sleep cycles. Cephalopods can use chromatophores like a muscle, which is why they can change their skin hue as rapidly as they do. Coloration is typically stronger in near-shore species than those living in the open ocean, whose functions tend to be restricted to disruptive camouflage. These chromatophores are found throughout the body of the octopus, however, they are controlled by the same part of the brain that controls elongation during jet propulsion to reduce drag. As such, jetting octopuses can turn pale because the brain is unable to achieve both controlling elongation and controlling the chromatophores. Most octopuses mimic select structures in their field of view rather than becoming a composite color of their full background. Evidence of original coloration has been detected in cephalopod fossils dating as far back as the
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago (Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozoi ...
; these orthoconic individuals bore concentric stripes, which are thought to have served as camouflage. Devonian cephalopods bear more complex color patterns, of unknown function.


Ink

With the exception of the
Nautilidae The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species i ...
and the species of
octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefis ...
belonging to the
suborder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...
Cirrina Cirrina or Cirrata is a suborder and one of the two main divisions of octopuses. Cirrate octopuses have a small, internal shell and two fins on their head, while their sister suborder Incirrina has neither. The fins of cirrate octopods are ass ...
, all known cephalopods have an ink sac, which can be used to expel a cloud of dark ink to confuse
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...
s. This sac is a muscular bag which originated as an extension of the hindgut. It lies beneath the gut and opens into the anus, into which its contents – almost pure
melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amin ...
– can be squirted; its proximity to the base of the funnel means the ink can be distributed by ejected water as the cephalopod uses its jet propulsion. The ejected cloud of melanin is usually mixed, upon expulsion, with
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
, produced elsewhere in the mantle, and therefore forms a thick cloud, resulting in visual (and possibly chemosensory) impairment of the predator, like a smokescreen. However, a more sophisticated behavior has been observed, in which the cephalopod releases a cloud, with a greater mucus content, that approximately resembles the cephalopod that released it (this decoy is referred to as a
pseudomorph In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form ( crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced b ...
). This strategy often results in the predator attacking the pseudomorph, rather than its rapidly departing prey. For more information, see Inking behaviors. The ink sac of cephalopods has led to a common name of "inkfish", formerly the pen-and-ink fish.


Circulatory system

Cephalopods are the only molluscs with a closed circulatory system.
Coleoid Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whose ...
s have two gill
heart The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon diox ...
s (also known as
branchial heart Branchial hearts are accessory pumps that supplement the action of the systemic heart in a cephalopod's body. They are myogenic in nature. Branchial hearts are always in pairs located at the base of the gills. Each branchial heart consists of a ...
s) that move blood through the capillaries of the
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s. A single systemic heart then pumps the oxygenated blood through the rest of the body. Like most molluscs, cephalopods use
hemocyanin Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2 ...
, a copper-containing protein, rather than
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
, to transport oxygen. As a result, their blood is colorless when deoxygenated and turns blue when bonded to oxygen. In oxygen-rich environments and in acidic water, hemoglobin is more efficient, but in environments with little oxygen and in low temperatures, hemocyanin has the upper hand. The hemocyanin molecule is much larger than the hemoglobin molecule, allowing it to bond with 96 or molecules, instead of the hemoglobin's just four. But unlike hemoglobin, which are attached in millions on the surface of a single red blood cell, hemocyanin molecules float freely in the bloodstream.


Respiration

Cephalopods exchange gases with the seawater by forcing water through their gills, which are attached to the roof of the organism. Water enters the mantle cavity on the outside of the gills, and the entrance of the mantle cavity closes. When the mantle contracts, water is forced through the gills, which lie between the mantle cavity and the funnel. The water's expulsion through the funnel can be used to power jet propulsion. If respiration is used concurrently with jet propulsion, large losses in speed or oxygen generation can be expected. The gills, which are much more efficient than those of other mollusks, are attached to the ventral surface of the mantle cavity. There is a trade-off with gill size regarding lifestyle. To achieve fast speeds, gills need to be small – water will be passed through them quickly when energy is needed, compensating for their small size. However, organisms which spend most of their time moving slowly along the bottom do not naturally pass much water through their cavity for locomotion; thus they have larger gills, along with complex systems to ensure that water is constantly washing through their gills, even when the organism is stationary. The water flow is controlled by contractions of the radial and circular mantle cavity muscles. The gills of cephalopods are supported by a skeleton of robust fibrous proteins; the lack of mucopolysaccharides distinguishes this matrix from cartilage. The gills are also thought to be involved in excretion, with NH4+ being swapped with K+ from the seawater.


Locomotion and buoyancy

While most cephalopods can move by
jet Jet, Jets, or The Jet(s) may refer to: Aerospace * Jet aircraft, an aircraft propelled by jet engines ** Jet airliner ** Jet engine ** Jet fuel * Jet Airways, an Indian airline * Wind Jet (ICAO: JET), an Italian airline * Journey to Enceladus a ...
propulsion, this is a very energy-consuming way to travel compared to the tail propulsion used by fish. The efficiency of a
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
-driven waterjet (i.e. Froude efficiency) is greater than a
rocket A rocket (from it, rocchetto, , bobbin/spool) is a vehicle that uses jet propulsion to accelerate without using the surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entire ...
. The relative efficiency of
jet propulsion Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the moving body is propelled in the opposite direction to the jet. Reaction engines operating on ...
decreases further as animal size increases;
paralarva Paralarvae (singular: ''paralarva'') are young cephalopods in the planktonic stages between hatchling and subadult. This stage differs from the larval stage of animals that undergo true metamorphosis. Paralarvae have been observed only in mem ...
e are far more efficient than juvenile and adult individuals. Since the
Paleozoic era The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ''z ...
, as competition with
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...
produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with
fins A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. F ...
and
tentacles In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mai ...
used to maintain a steady velocity. Whilst jet propulsion is never the sole mode of locomotion, the stop-start motion provided by the jets continues to be useful for providing bursts of high speed – not least when capturing
prey Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
or avoiding
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...
s. Indeed, it makes cephalopods the fastest marine invertebrates, and they can out-accelerate most fish. The jet is supplemented with fin motion; in the squid, the fins flap each time that a jet is released, amplifying the thrust; they are then extended between jets (presumably to avoid sinking). Oxygenated water is taken into the
mantle cavity The mantle (also known by the Latin word pallium meaning mantle, robe or cloak, adjective pallial) is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form o ...
to the
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s and through muscular contraction of this cavity, the spent water is expelled through the
hyponome A siphon is an anatomical structure which is part of the body of aquatic molluscs in three classes: Gastropoda, Bivalvia and Cephalopoda (members of these classes include saltwater and freshwater snails, clams, octopus, squid and relatives). ...
, created by a fold in the mantle. The size difference between the posterior and anterior ends of this organ control the speed of the jet the organism can produce. The velocity of the organism can be accurately predicted for a given mass and morphology of animal. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions. Some cephalopods accompany this expulsion of water with a gunshot-like popping noise, thought to function to frighten away potential predators. Cephalopods employ a similar method of propulsion despite their increasing size (as they grow) changing the dynamics of the water in which they find themselves. Thus their paralarvae do not extensively use their fins (which are less efficient at low Reynolds numbers) and primarily use their jets to propel themselves upwards, whereas large adult cephalopods tend to swim less efficiently and with more reliance on their fins. Early cephalopods are thought to have produced jets by drawing their body into their shells, as ''Nautilus'' does today. ''Nautilus'' is also capable of creating a jet by undulations of its funnel; this slower flow of water is more suited to the extraction of oxygen from the water. When motionless, ''Nautilus'' can only extract 20% of oxygen from the water. The jet velocity in ''Nautilus'' is much slower than in
coleoid Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whose ...
s, but less musculature and energy is involved in its production. Jet thrust in cephalopods is controlled primarily by the maximum diameter of the funnel orifice (or, perhaps, the average diameter of the funnel) and the diameter of the mantle cavity. Changes in the size of the orifice are used most at intermediate velocities. The absolute velocity achieved is limited by the cephalopod's requirement to inhale water for expulsion; this intake limits the maximum velocity to eight body-lengths per second, a speed which most cephalopods can attain after two funnel-blows. Water refills the cavity by entering not only through the orifices, but also through the funnel. Squid can expel up to 94% of the fluid within their cavity in a single jet thrust. To accommodate the rapid changes in water intake and expulsion, the orifices are highly flexible and can change their size by a factor of twenty; the funnel radius, conversely, changes only by a factor of around 1.5. Some octopus species are also able to walk along the seabed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of
muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...
around the mantle. While most cephalopods float (i.e. are neutrally buoyant or nearly so; in fact most cephalopods are about 2–3% denser than seawater), they achieve this in different ways. Some, such as ''
Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
'', allow gas to diffuse into the gap between the mantle and the shell; others allow purer water to ooze from their kidneys, forcing out denser salt water from the body cavity; others, like some fish, accumulate oils in the liver; and some octopuses have a gelatinous body with lighter
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s replacing
sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
in the body chemistry. Squids are the primary sufferers of negative buoyancy in cephalopods. The negative buoyancy means that some squids, especially those whose habitat depths are rather shallow, have to actively regulate their vertical positions. This means that they must expend energy, often through jetting or undulations, in order to maintain the same depth. As such, the cost of transport of many squids are quite high. That being said, squid and other cephalopod that dwell in deep waters tend to be more neutrally buoyant which removes the need to regulate depth and increases their locomotory efficiency. The ''Macrotritopus defilippi'', or the sand-dwelling octopus, was seen mimicking both the coloration and the swimming movements of the sand-dwelling flounder ''Bothus lunatus'' to avoid predators. The octopuses were able to flatten their bodies and put their arms back to appear the same as the flounders as well as move with the same speed and movements. Females of two species, Ocythoe tuberculata and Haliphron atlanticus, have evolved a true
swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth w ...
.


Octopus vs. Squid Locomotion

Two of the categories of cephalopods, octopus and squid, are vastly different in their movements despite being of the same class. Octopuses are generally not seen as active swimmers; they are often found scavenging the sea floor instead of swimming long distances through the water. Squids, on the other hand, can be found to travel vast distances, with some moving as much as 2000 km in 2.5 months at an average pace of 0.9 body lengths per second. There is a major reason for the difference in movement type and efficiency: anatomy. Both octopuses and squids have mantles (referenced above) which function towards respiration and locomotion in the form of jetting. The composition of these mantles differs between the two families, however. In octopuses, the mantle is made up of three muscle types: Longitudinal, radial, and circular. The longitudinal muscles run parallel to the length of the octopus and they are used in order to keep the mantle the same length throughout the jetting process. Given that they are muscles, it can be noted that this means the octopus must actively flex the longitudinal muscles during jetting in order to keep the mantle at a constant length. The radial muscles run perpendicular to the longitudinal muscles and are used to thicken and thin the wall of the mantle. Finally, the circular muscles are used as the main activators in jetting. They are muscle bands that surround the mantle and expand/contract the cavity. All three muscle types work in unison to produce a jet as a propulsion mechanism. Squids do not have the longitudinal muscles that octopus do. Instead, they have a tunic. This tunic is made of layers of collagen and it surrounds the top and the bottom of the mantle. Because they are made of collagen and not muscle, the tunics are rigid bodies that are much stronger than the muscle counterparts. This provides the squids some advantages for jet propulsion swimming. The stiffness means that there is no necessary muscle flexing to keep the mantle the same size. In addition, tunics take up only 1% of the squid mantle's wall thickness, whereas the longitudinal muscle fibers take up to 20% of the mantle wall thickness in octopuses. Also because of the rigidity of the tunic, the radial muscles in squid can contract more forcefully. The mantle is not the only place where squids have collagen. Collagen fibers are located throughout the other muscle fibers in the mantle. These collagen fibers act as elastics and are sometimes named "collagen springs". As the name implies, these fibers act as springs. When the radial and circular muscles in the mantle contract, they reach a point where the contraction is no longer efficient to the forward motion of the creature. In such cases, the excess contraction is stored in the collagen which then efficiently begins or aids in the expansion of the mantle at the end of the jet. In some tests, the collagen has been shown to be able to begin raising mantle pressure up to 50ms before muscle activity is initiated. These anatomical differences between squid and octopuses can help explain why squid can be found swimming comparably to fish while octopuses usually rely on other forms of locomotion on the sea floor such as bipedal walking, crawling, and non-jetting swimming.


Shell

Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
es are the only extant cephalopods with a true external shell. However, all molluscan shells are formed from the
ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
(outer layer of the embryo); in
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control ...
(''Sepia'' spp.), for example, an invagination of the ectoderm forms during the embryonic period, resulting in a shell (
cuttlebone Cuttlebone, also known as cuttlefish bone, is a hard, brittle internal structure (an internal shell) found in all members of the family Sepiidae, commonly known as cuttlefish, within the cephalopods. In other cephalopod families it is calle ...
) that is internal in the adult. The same is true of the chitinous
gladius ''Gladius'' () is a Latin word meaning "sword" (of any type), but in its narrow sense it refers to the sword of ancient Roman foot soldiers. Early ancient Roman swords were similar to those of the Greeks, called '' xiphe'' (plural; singular ''xi ...
of squid and octopuses.
Cirrate Cirrina or Cirrata is a suborder and one of the two main divisions of octopuses. Cirrate octopuses have a small, internal shell and two fins on their head, while their sister suborder Incirrina has neither. The fins of cirrate octopods are asso ...
octopods have arch-shaped cartilaginous fin supports, which are sometimes referred to as a "shell vestige" or "gladius". The
Incirrina Incirrata (or Incirrina) is a suborder of the order Octopoda. The suborder contains the classic " benthic octopuses," as well as many pelagic octopus families, including the paper nautiluses. The incirrate octopuses are distinguished from th ...
have either a pair of rod-shaped stylets or no vestige of an internal shell, and some squid also lack a gladius. The shelled coleoids do not form a clade or even a paraphyletic group. The ''Spirula'' shell begins as an organic structure, and is then very rapidly mineralized. Shells that are "lost" may be lost by resorption of the calcium carbonate component. Females of the octopus genus '' Argonauta'' secrete a specialized paper-thin egg case in which they reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal and has a separate evolutionary origin. The largest group of shelled cephalopods, the
ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...
s, are extinct, but their shells are very common as
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s. The deposition of carbonate, leading to a mineralized shell, appears to be related to the acidity of the organic shell matrix (see
Mollusc shell The mollusc (or molluskOften spelled mollusk shell in the USA; the spelling "mollusc" are preferred by ) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which ...
); shell-forming cephalopods have an acidic matrix, whereas the gladius of squid has a basic matrix. The basic arrangement of the cephalopod outer wall is: an outer (spherulitic) prismatic layer, a laminar (nacreous) layer and an inner prismatic layer. The thickness of every layer depends on the taxa. In modern cephalopods, the Ca carbonate is aragonite. As for other mollusc shells or coral skeletons, the smallest visible units are irregular rounded granules.


Head appendages

Cephalopods, as the name implies, have muscular appendages extending from their heads and surrounding their mouths. These are used in feeding, mobility, and even reproduction. In
coleoids Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whose ...
they number eight or ten. Decapods such as cuttlefish and squid have five pairs. The longer two, termed ''
tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s'', are actively involved in capturing prey; they can lengthen rapidly (in as little as 15 milliseconds). In
giant squid The giant squid (''Architeuthis dux'') is a species of deep-ocean dwelling squid in the family (biology), family Architeuthidae. It can grow to a tremendous size, offering an example of deep-sea gigantism, abyssal gigantism: recent estimates ...
they may reach a length of 8 metres. They may terminate in a broadened, sucker-coated club. The shorter four pairs are termed ''
arms Arms or ARMS may refer to: *Arm or arms, the upper limbs of the body Arm, Arms, or ARMS may also refer to: People * Ida A. T. Arms (1856–1931), American missionary-educator, temperance leader Coat of arms or weapons *Armaments or weapons **Fi ...
'', and are involved in holding and manipulating the captured organism. They too have suckers, on the side closest to the mouth; these help to hold onto the prey. Octopods only have four pairs of sucker-coated arms, as the name suggests, though developmental abnormalities can modify the number of arms expressed. The tentacle consists of a thick central nerve cord (which must be thick to allow each sucker to be controlled independently) surrounded by circular and radial muscles. Because the volume of the tentacle remains constant, contracting the circular muscles decreases the radius and permits the rapid increase in length. Typically a 70% lengthening is achieved by decreasing the width by 23%. The shorter arms lack this capability. The size of the tentacle is related to the size of the buccal cavity; larger, stronger tentacles can hold prey as small bites are taken from it; with more numerous, smaller tentacles, prey is swallowed whole, so the mouth cavity must be larger. Externally shelled nautilids (''
Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
'' and ''
Allonautilus The genus ''Allonautilus'' contains two species of nautiluses, which have a significantly different morphology from those placed in the sister taxon ''Nautilus''. ''Allonautilus'' is now thought to be a descendant of ''Nautilus'', rendering the ...
'') have on the order of 90 finger-like appendages, termed ''tentacles'', which lack suckers but are sticky instead, and are partly retractable.


Feeding

All living cephalopods have a two-part
beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for fo ...
; most have a
radula The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...
, although it is reduced in most octopus and absent altogether in ''Spirula''. They feed by capturing prey with their tentacles, drawing it into their mouth and taking bites from it. They have a mixture of toxic digestive juices, some of which are manufactured by symbiotic algae, which they eject from their salivary glands onto their captured prey held in their mouths. These juices separate the flesh of their prey from the bone or shell. The salivary gland has a small tooth at its end which can be poked into an organism to digest it from within. The digestive gland itself is rather short. It has four elements, with food passing through the crop, stomach and caecum before entering the intestine. Most digestion, as well as the absorption of nutrients, occurs in the digestive gland, sometimes called the liver. Nutrients and waste materials are exchanged between the gut and the digestive gland through a pair of connections linking the gland to the junction of the stomach and caecum. Cells in the digestive gland directly release pigmented excretory chemicals into the lumen of the gut, which are then bound with mucus passed through the anus as long dark strings, ejected with the aid of exhaled water from the funnel. Cephalopods tend to concentrate ingested heavy metals in their body tissue. However, octopus arms use a family of cephalopod-specific chemotactile receptors (CRs) to be their "taste by touch" system.


Radula

The cephalopod radula consists of multiple symmetrical rows of up to nine teeth – thirteen in fossil classes. The organ is reduced or even vestigial in certain octopus species and is absent in ''
Spirula ''Spirula spirula'' is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus ''Spirula'', the family Spirulidae, and the order Spirulida. Because of the shape of its internal shell, it is commonly kno ...
''. The teeth may be homodont (i.e. similar in form across a row), heterodont (otherwise), or ctenodont (comb-like). Their height, width and number of cusps is variable between species. The pattern of teeth repeats, but each row may not be identical to the last; in the octopus, for instance, the sequence repeats every five rows. Cephalopod radulae are known from fossil deposits dating back to the Ordovician. They are usually preserved within the cephalopod's body chamber, commonly in conjunction with the mandibles; but this need not always be the case; many radulae are preserved in a range of settings in the Mason Creek. Radulae are usually difficult to detect, even when they are preserved in fossils, as the rock must weather and crack in exactly the right fashion to expose them; for instance, radulae have only been found in nine of the 43 ammonite genera, and they are rarer still in non-ammonoid forms: only three pre-Mesozoic species possess one.


Excretory system

Most cephalopods possess a single pair of large
nephridia The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...
. Filtered
nitrogenous waste Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, w ...
is produced in the pericardial cavity of the
branchial heart Branchial hearts are accessory pumps that supplement the action of the systemic heart in a cephalopod's body. They are myogenic in nature. Branchial hearts are always in pairs located at the base of the gills. Each branchial heart consists of a ...
s, each of which is connected to a nephridium by a narrow canal. The canal delivers the excreta to a bladder-like renal sac, and also resorbs excess water from the filtrate. Several outgrowths of the lateral
vena cava In anatomy, the venae cavae (; singular: vena cava ; ) are two large veins (great vessels) that return deoxygenated blood from the body into the heart. In humans they are the superior vena cava and the inferior vena cava, and both empty into the ...
project into the renal sac, continuously inflating and deflating as the branchial hearts beat. This action helps to pump the secreted waste into the sacs, to be released into the mantle cavity through a pore. ''Nautilus'', unusually, possesses four nephridia, none of which are connected to the pericardial cavities. The incorporation of
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...
is important for shell formation in terrestrial molluscs and other non-molluscan lineages. Because
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...
(i.e. flesh) is a major constituent of the cephalopod diet, large amounts of
ammonium ion The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary amm ...
s are produced as waste. The main organs involved with the release of this excess ammonium are the gills. The rate of release is lowest in the shelled cephalopods ''
Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
'' and '' Sepia'' as a result of their using
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...
to fill their shells with gas to increase buoyancy. Other cephalopods use ammonium in a similar way, storing the
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s (as ammonium chloride) to reduce their overall density and increase buoyancy.


Reproduction and life cycle

Cephalopods are a diverse group of species, but share common life history traits, for example, they have a rapid growth rate and short life spans. Stearns (1992) suggested that in order to produce the largest possible number of viable offspring, spawning events depend on the ecological environmental factors of the organism. The majority of cephalopods do not provide parental care to their offspring, except, for example, octopus, which helps this organism increase the survival rate of their offspring. Marine species' life cycles are affected by various environmental conditions. The development of a cephalopod embryo can be greatly affected by temperature, oxygen saturation, pollution, light intensity, and salinity. These factors are important to the rate of embryonic development and the success of hatching of the embryos. Food availability also plays an important role in the reproductive cycle of cephalopods. A limitation of food influences the timing of spawning along with their function and growth. Spawning time and spawning vary among marine species; it's correlated with temperature, though cephalopods in shallow water spawn in cold months so that the offspring would hatch at warmer temperatures. Breeding can last from several days to a month.


Sexual maturity

Cephalopods that are sexually mature and of adult size begin spawning and reproducing. After the transfer of genetic material to the following generation, the adult cephalopods then die. Sexual maturation in male and female cephalopods can be observed internally by the enlargement of gonads and accessory glands. Mating would be a poor indicator of sexual maturation in females; they can receive sperm when not fully reproductively mature and store them until they are ready to fertilize the eggs. Males are more aggressive in their pre-mating competition when in the presence of immature females than when competing for a sexually mature female. Most cephalopod males develop a hectocotylus, an arm tip which is capable of transferring their spermatozoa into the female mantle cavity. Though not all species use a hectocotylus; for example, the adult nautilus releases a spadix. An indication of sexual maturity of females is the development of brachial photophores to attract mates.


Fertilization

Cephalopods are not
broadcast spawner Spawn is the eggs and sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of releasing the eggs and sperm, and the act of both sexes is called spawning. Most aquatic animals, except for aquat ...
s. During the process of fertilization, the females use sperm provided by the male via
external fertilization External fertilization is a mode of reproduction in which a male organism's sperm fertilizes a female organism's egg outside of the female's body. It is contrasted with internal fertilization, in which sperm are introduced via insemination and the ...
.
Internal fertilization Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For intern ...
is seen only in octopuses. The initiation of copulation begins when the male catches a female and wraps his arm around her, either in a "male to female neck" position or mouth to mouth position, depending on the species. The males then initiate the process of fertilization by contracting their mantle several times to release the spermatozoa. Cephalopods often mate several times, which influences males to mate longer with females that have previously, nearly tripling the number of contractions of the mantle. To ensure the fertilization of the eggs, female cephalopods release a sperm-attracting peptide through the gelatinous layers of the egg to direct the spermatozoa. Female cephalopods lay eggs in clutches; each egg is composed of a protective coat to ensure the safety of the developing embryo when released into the water column. Reproductive strategies differ between cephalopod species. In giant Pacific octopus, large eggs are laid in a den; it will often take several days to lay all of them. Once the eggs are released and attached to a sheltered substrate, the females then die, making them
semelparous Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characteri ...
. In some species of cephalopods, egg clutches are anchored to substrates by a mucilaginous adhesive substance. These eggs are swelled with perivitelline fluid (PVF), a hypertonic fluid that prevents premature hatching. Fertilized egg clusters are neutrally buoyant depending on the depth that they were laid, but can also be found in substrates such as sand, a matrix of corals, or seaweed. Because these species do not provide parental care for their offspring, egg capsules can be injected with ink by the female in order to camouflage the embryos from predators.


Male–male competition

Most cephalopods engage in aggressive sex: a protein in the male capsule sheath stimulates this behavior. They also engage in male–male aggression, where larger males tend to win the interactions. When a female is near, the males charge one another continuously and flail their arms. If neither male backs away, the arms extend to the back, exposing the mouth, followed by the biting of arm tips. During mate competition males also participate in a technique called flushing. This technique is used by the second male attempting to mate with a female. Flushing removes spermatophores in the buccal cavity that was placed there by the first mate by forcing water into the cavity. Another behavior that males engage in is sneaker mating or mimicry – smaller males adjust their behavior to that of a female in order to reduce aggression. By using this technique, they are able to fertilize the eggs while the larger male is distracted by a different male. During this process, the sneaker males quickly insert drop-like sperm into the seminal receptacle.


Mate choice

Mate choice Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
is seen in cuttlefish species, where females prefer some males over others, though characteristics of the preferred males are unknown. A hypothesis states that females reject males by olfactory cues rather than visual cues. Several cephalopod species are polyandrous- accepting and storing multiple male spermatophores, which has been identified by DNA fingerprinting. Females are no longer receptive to mating attempts when holding their eggs in their arms. Females can store sperm in two places (1) the buccal cavity where recently mated males place their spermatophores, and (2) the internal sperm-storage receptacles where sperm packages from previous males are stored. Spermatophore storage results in sperm competition; which states that the female controls which mate fertilizes the eggs. In order to reduce this sort of competition, males develop agonistic behaviors like mate guarding and flushing. The ''Hapalochlaena lunulata'', or the blue-ringed octopus, readily mates with both males and females.


Sexual dimorphism

In a variety of marine organisms, it is seen that females are larger in size compared to the males in some closely related species. In some lineages, such as the
blanket octopus ''Tremoctopus'' is a genus of pelagic cephalopods, containing four species that occupy surface to mid-waters in subtropical and tropical oceans. They are commonly known as blanket octopuses, in reference to the long, transparent webs that connect ...
, males become structurally smaller and smaller resembling a term, "dwarfism" dwarf males usually occurs at low densities. The blanket octopus male is an example of sexual-evolutionary dwarfism; females grow 10,000 to 40,000 times larger than the males and the sex ratio between males and females can be distinguished right after hatching of the eggs.


Embryology

Cephalopod eggs span a large range of sizes, from 1 to 30 mm in diameter. The fertilised
ovum The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete i ...
initially divides to produce a disc of germinal cells at one pole, with the
yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
remaining at the opposite pole. The germinal disc grows to envelop and eventually absorb the yolk, forming the embryo. The tentacles and arms first appear at the hind part of the body, where the foot would be in other molluscs, and only later migrate towards the head. The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the opposite surface. The early embryological stages are reminiscent of ancestral gastropods and extant
Monoplacophora Monoplacophora , meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments . Extant representatives were not recognized as such until 1952; previously they were known only from ...
. The shells develop from the ectoderm as an organic framework which is subsequently mineralized. In ''Sepia'', which has an internal shell, the ectoderm forms an invagination whose pore is sealed off before this organic framework is deposited.


Development

The length of time before hatching is highly variable; smaller eggs in warmer waters are the fastest to hatch, and newborns can emerge after as little as a few days. Larger eggs in colder waters can develop for over a year before hatching. The process from spawning to hatching follows a similar trajectory in all species, the main variable being the amount of yolk available to the young and when it is absorbed by the embryo. Unlike most other molluscs, cephalopods do not have a morphologically distinct
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. Th ...
l stage. Instead, the juveniles are known as
paralarvae Paralarvae (singular: ''paralarva'') are young cephalopods in the planktonic stages between hatchling and subadult. This stage differs from the larval stage of animals that undergo true metamorphosis Metamorphosis is a biological proces ...
. They quickly learn how to hunt, using encounters with prey to refine their strategies. Growth in juveniles is usually
allometric Allometry is the study of the relationship of body size to shape, anatomy, physiology and finally behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in ''On Growth and Form'' and by Julian Huxley in 1932. Overview Allome ...
, whilst adult growth is
isometric The term ''isometric'' comes from the Greek for "having equal measurement". isometric may mean: * Cubic crystal system, also called isometric crystal system * Isometre, a rhythmic technique in music. * "Isometric (Intro)", a song by Madeon from ...
.


Evolution

The traditional view of cephalopod evolution holds that they evolved in the Late Cambrian from a
monoplacophoran Monoplacophora , meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments . Extant representatives were not recognized as such until 1952; previously they were known only from t ...
-like ancestor with a curved, tapering shell, which was closely related to the gastropods (snails). The similarity of the early shelled cephalopod '' Plectronoceras'' to some gastropods was used in support of this view. The development of a
siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and ...
would have allowed the shells of these early forms to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separated the true cephalopods from putative ancestors such as '' Knightoconus'', which lacked a siphuncle. Neutral or positive buoyancy (i.e. the ability to float) would have come later, followed by swimming in the
Plectronocerida Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic Pres Inc. Oc ...
and eventually jet propulsion in more derived cephalopods. Possible early Cambrian remains have been found in the Avalon Peninsula, matching genetic data for a pre-Cambrian origin. However, some morphological evidence is difficult to reconcile with this view, and the redescription of ''
Nectocaris pteryx ''Nectocaris'' is a genus of squid-like animal of controversial affinities known from the Cambrian period. The initial fossils were described from the Burgess Shale of Canada. Other similar remains possibly referrable to the genus are known from ...
'', which did not have a shell and appeared to possess jet propulsion in the manner of "derived" cephalopods, complicated the question of the order in which cephalopod features developed – provided ''Nectocaris'' is a cephalopod at all. Early cephalopods were likely predators near the top of the food chain. After the late Cambrian extinction led to the disappearance of many
Anomalocaridid Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the last two origina ...
s, predatory niches became available for other animals. During the Ordovician period the primitive cephalopods underwent pulses of diversification to become diverse and dominant in the
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...
and
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
seas. In the Early Palaeozoic, their range was far more restricted than today; they were mainly constrained to sublittoral regions of shallow shelves of the low latitudes, and usually occurred in association with
thrombolites Thrombolites (from Ancient Greek θρόμβος ''thrómbos'' meaning "clot" and λῐ́θος ''líthos'' meaning "stone") are clotted accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary g ...
. A more pelagic habit was gradually adopted as the Ordovician progressed. Deep-water cephalopods, whilst rare, have been found in the Lower Ordovician – but only in high-latitude waters. The mid-Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m. The direction of shell coiling would prove to be crucial to the future success of the lineages; endogastric coiling would only permit large size to be attained with a straight shell, whereas exogastric coiling – initially rather rare – permitted the spirals familiar from the fossil record to develop, with their corresponding large size and diversity. (Endogastric means the shell is curved so as the ventral or lower side is longitudinally concave (belly in); exogastric means the shell is curved so as the ventral side is longitudinally convex (belly out) allowing the funnel to be pointed backward beneath the shell.) The ancestors of coleoids (including most modern cephalopods) and the ancestors of the modern nautilus, had diverged by the Floian Age of the Early Ordovician Period, over 470 million years ago. The
Bactritida The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in Pragian stage before 409 million ye ...
, a Silurian–Triassic group of orthocones, are widely held to be paraphyletic without the coleoids and ammonoids, that is, the latter groups arose from within the Bactritida. An increase in the diversity of the coleoids and ammonoids is observed around the start of the Devonian period and corresponds with a profound increase in fish diversity. This could represent the origin of the two derived groups. Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern
nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...
species. Competitive pressure from fish is thought to have forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonize shallow waters. However, some of the straight-shelled
nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...
s evolved into belemnites, out of which some evolved into
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting ...
and
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control ...
. The loss of the shell may also have resulted from evolutionary pressure to increase maneuverability, resulting in a more fish-like habit. There has been debate on the embryological origin of cephalopod appendages. Until the mid-twentieth century, the "Arms as Head" hypothesis was widely recognized. In this theory, the arms and tentacles of cephalopods look similar to the head appendages of gastropods, suggesting that they might be
homologous structures In biology, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats and birds, the arms of prima ...
. Cephalopod appendages surround the mouth, so logically they could be derived from embryonic head tissues. However, the "Arms as Foot" hypothesis, proposed by
Adolf Naef Adolf Naef (1 May 1883 – 11 May 1949) was a Swiss zoologist and palaeontologist who worked on cephalopods and systematics. Although he struggled with academic politics throughout his career and difficult conditions during World War I and II ...
in 1928, has increasingly been favoured; for example,
fate mapping Fate mapping is a method used in developmental biology to study the embryonic origin of various adult tissues and structures. The "fate" of each cell or group of cells is mapped onto the embryo, showing which parts of the embryo will develop into ...
of limb buds in the
chambered nautilus The chambered nautilus (''Nautilus pompilius''), also called the pearly nautilus, is the best-known species of nautilus. The shell, when cut away, reveals a lining of lustrous nacre and displays a nearly perfect equiangular spiral, although it ...
indicates that limb buds originate from "foot" embryonic tissues.


Genetics

The sequencing of a full Cephalopod genome has remained challenging to researchers due to the length and repetition of their DNA. The characteristics of Cephalopod genomes were initially hypothesized to be the result of entire
genome duplications Polyploidy is a condition in which the cells of an organism have more than one pair of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...
. Following the full sequencing of a California two-spot octopus, the genome showed similar patterns to other marine invertebrates with significant additions to the genome assumed to be unique to Cephalopods. No evidence of full genome duplication was found. Within the California two-spot octopus genome there are substantial replications of two gene families. Significantly, the expanded gene families were only previously known to exhibit replicative behaviour within vertebrates. The first gene family was identified as the
Protocadherin Protocadherins (Pcdhs) are the largest mammalian subgroup of the cadherin superfamily of homophilic cell-adhesion proteins. They were discovered by Shintaro Suzuki's group, when they used PCR to find new members of the cadherin family. The PCR fr ...
s which are attributed to neuron development. Protocadherins function as cell adhesion molecules, essential for synaptic specificity. The mechanism for Protocadherin gene family replication in vertebrates is attributed to complex splicing, or cutting and pasting, from a locus. Following the sequencing of the California two-spot octopus, researchers found that the Prorocadherin gene family in Cephalopods has expanded in the genome due to tandem gene duplication. The different replication mechanisms for Protocadherin genes indicate an independent evolution of Protocadherin gene expansion in vertebrates and invertebrates. Analysis of individual Cephalopod Protocadherin genes indicate independent evolution between species of Cephalopod. A species of shore squid ''
Doryteuthis pealeii The longfin inshore squid (''Doryteuthis pealeii'') is a species of squid of the family Loliginidae. Description This species of squid is often seen with a reddish hue, but like many types of squid can manipulate its color, varying from a deep ...
'' with expanded Protocadherin gene families differ significantly from those of the California two-spot octopus suggesting gene expansion did not occur before
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...
within Cephalopods. Despite different mechanisms for gene expansion, the two-spot octopus Protocadherin genes were more similar to vertebrates than squid, suggesting a
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
mechanism. The second gene family known as are small proteins that function as zinc transcription factors. are understood to moderate DNA, RNA and protein functions within the cell. The sequenced California two spot octopus genome also showed a significant presence of
transposable element A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Trans ...
s as well as transposon expression. Although the role of transposable elements in marine vertebrates is still relatively unknown, significant expression of transposons in nervous system tissues have been observed. In a study conducted on vertebrates, the expression of transposons during development in the fruitfly ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or " pomace fly". Starting with ...
'' activated genomic diversity between neurons. This diversity has been linked to increased memory and learning in mammals. The connection between transposons and increased neuron capability may provide insight into the observed intelligence, memory and function of Cephalopods. Using long-read sequencing, researchers have decoded the cephalopod genomes and discovered they have been churned and scrambled. The genes were compared to those of thousands of other species and while blocks of three or more genes co-occurred between squid and octopus, the blocks of genes were not found together in any other animals'. Many of the groupings were in the nervous tissue, suggesting the course they adapted their intelligence.


Phylogeny

The approximate consensus of extant cephalopod phylogeny, after Strugnell ''et al''. 2007, is shown in the
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
. Mineralized taxa are in bold. The attachment of the clade including ''Sepia'' and ''Spirula'' is unclear; either of the points marked with an asterisk may represent the root of this clade. The internal phylogeny of the cephalopods is difficult to constrain; many molecular techniques have been adopted, but the results produced are conflicting. ''Nautilus'' tends to be considered an outgroup, with '' Vampyroteuthis'' forming an outgroup to other squid; however in one analysis the nautiloids, octopus and teuthids plot as a
polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...
. Some molecular phylogenies do not recover the mineralized coleoids (''Spirula'', ''Sepia'', and ''Metasepia'') as a clade; however, others do recover this more parsimonious-seeming clade, with ''Spirula'' as a sister group to ''Sepia'' and ''Metasepia'' in a clade that had probably diverged before the end of the Triassic. Molecular estimates for clade divergence vary. One 'statistically robust' estimate has ''Nautilus'' diverging from ''Octopus'' at .


Taxonomy

The classification presented here, for recent cephalopods, follows largely fro
Current Classification of Recent Cephalopoda
(May 2001), for fossil cephalopods takes from Arkell et al. 1957, Teichert and Moore 1964, Teichert 1988, and others. The three subclasses are traditional, corresponding to the three orders of cephalopods recognized by Bather. Class Cephalopoda († indicates extinct groups) * Subclass
Nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...
ea: Fundamental ectocochliate cephalopods that provided the source for the Ammonoidea and Coleoidea. ** Order †
Plectronocerida Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic Pres Inc. Oc ...
: the ancestral cephalopods from the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...
Period ** Order †
Ellesmerocerida The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician. Morphology The Ellesmerocerida are characterized by shells that ...
() ** Order †
Endocerida Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Siluri ...
() ** Order †
Actinocerida The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposit ...
() ** Order †
Discosorida Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and c ...
() ** Order †
Pseudorthocerida Pseudorthocerida is an order of generally straight longiconic orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous pa ...
() ** Order †
Tarphycerida The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician (middle and upper Canad) to the Middle Devonian. Some, such as ''Aphetoceras'' and ''Estonioceras'', are loosely coiled and gyroconic; ...
() ** Order †
Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible member is known from the Early Permian), in which the connecting rings are ...
() ** Order
Nautilida The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, ''Nautilus'' and '' Allonau ...
(extant; 410.5 Ma to present) ** Order †
Orthocerida Orthocerida is an order of extinct Orthoceratoid cephalopods also known as the Michelinocerida that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until t ...
() ** Order †
Ascocerida The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflated ...
() ** Order †
Bactritida The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in Pragian stage before 409 million ye ...
() * Subclass †
Ammonoidea Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttl ...
: Ammonites () ** Order †
Goniatitida Goniatids, informally goniatites, are ammonoid cephalopods that form the order Goniatitida, derived from the more primitive Agoniatitida during the Middle Devonian some 390 million years ago (around Eifelian stage). Goniatites (goniatitids) sur ...
() ** Order †
Ceratitida Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from t ...
() ** Order †
Ammonitida Ammonitida is an order of ammonoid cephalopods that lived from the Jurassic through Paleocene time periods, commonly with intricate ammonitic sutures. Ammonitida is divided into four suborders, the Phylloceratina, Lytoceratina, Ancylocerati ...
() * Subclass
Coleoidea Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whos ...
(410.0 Ma-Rec) ** Cohort †
Belemnoidea Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of r ...
: Belemnites and kin *** Genus † ''
Jeletzkya ''Jeletzkya douglassae'' is a fossil coleoid from the early Pennsylvanian Mazon Creek lagerstätten and represents the earliest known crown-group squid. Non-mineralized anatomy is preserved and comprises ten hooked tentacles and a radula ...
'' *** Order †
Aulacocerida Aulacocerida is an order of primitive coleoid cephalopods, possibly derived from michelinoceraitids (Orthocerida) early in the Devonian, which in turn gave rise to the Belemnites. Aulococerids are characterized by a straight, slim, conical she ...
() *** Order †
Phragmoteuthida Phragmoteuthida is an order of extinct coleoid cephalopods characterized by a fan-like teuthoid pro-ostracum attached to a belemnoid-like phragmocone. Diagnosis Jeletzky characterized phragmoteuthids as having a large tripartite, fanlike pro ...
() *** Order †
Hematitida Hematitida is a group of coleoid cephalopods known from the early Carboniferous Period. They are the oldest definite coleoids, although there are controversial claims for even older coleoids from the Devonian. Fossil hematitidans have so far bee ...
() *** Order †
Belemnitida Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most to ...
() *** Genus † '' Belemnoteuthis'' () ** Cohort
Neocoleoidea Neocoleoidea is a large group of marine cephalopods. This cohort contains two extant groups: Decapodiformes (squid, cuttlefish, and relatives) and Octopodiformes (octopuses and the vampire squid). Species within this group exist in all major hab ...
*** Superorder
Decapodiformes Decapodiformes is a superorder of Cephalopoda comprising all cephalopod species with ten limbs, specifically eight short arms and two long tentacles. It is hypothesized that the ancestral coleoid had five identical pairs of limbs, and that one ...
(also known as Decabrachia or Decembranchiata) **** Order
Spirulida Spirulida is an order of cephalopods comprising one extant species ('' Spirula spirula'') and several extinct taxa. Fossil record * Oldest representative: Carboniferous, though contested: see '' Shimanskya'' * Oldest uncontested representati ...
: Ram's horn squid **** Order
Sepiida Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
: cuttlefish **** Order
Sepiolida Bobtail squid (order Sepiolida) are a group of cephalopods closely related to cuttlefish. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. They have eight suckered arms and two tentacles and are generally quit ...
: pygmy, bobtail and bottletail squid **** Order Teuthida: squid *** Superorder
Octopodiformes Octopodiformes is a superorder of the subclass Coleoidea, comprising the octopuses and the vampire squid. All living members of Octopodiformes have eight arms, either lacking the two tentacles of squid (as is the case in octopuses) or modifying t ...
(also known as Vampyropoda) **** Family † Trachyteuthididae **** Order
Vampyromorphida Vampyromorphida is an order of cephalopods comprising one known extant species ('' Vampyroteuthis infernalis'') and many extinct taxa. Physically, they somewhat resemble octopuses (their closest relatives), but the eight arms are united by a web ...
: Vampire squid **** Order Octopoda: octopus *** Superorder †
Palaeoteuthomorpha Palaeoteuthomorpha is a superorder of primitive coleoid cephalopods, containing the Boletzkyida, characterized by an orthoceroid-like phragmocone early in ontogeny Ontogeny (also ontogenesis) is the origination and development of an orga ...
**** Order †
Boletzkyida Boletzkyida is a primitive order of teuthid coleoid cephalopod: the boletzkyids are thought to be the earliest forms of coleoid cephalopods, and appear to form a link between nautiloid orthocerids and more advanced coleoids. Boletzkyida was na ...
Other classifications differ, primarily in how the various decapod orders are related, and whether they should be orders or families.


Suprafamilial classification of the Treatise

This is the older classification that combines those found in parts K and L of the ''Treatise on Invertebrate Paleontology'', which forms the basis for and is retained in large part by classifications that have come later. Nautiloids in general (Teichert and Moore, 1964) sequence as given. : Subclass †
Endoceratoidea Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian ...
. Not used by Flower, e.g. Flower and Kummel 1950, interjocerids included in the Endocerida. :: Order †
Endocerida Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Siluri ...
:: Order †
Intejocerida Intejocerida is the name given to a group of generally straight shelled nautiloid cephalopods originally found in Lower and Middle Ordovician sediments in the Angara River basin in Russia; defined in the Treatise as an order, and combined there w ...
: Subclass † Actinoceratoidea Not used by Flower, ibid :: Order †
Actinocerida The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposit ...
: Subclass
Nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...
ea Nautiloidea in the restricted sense. :: Order †
Ellesmerocerida The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician. Morphology The Ellesmerocerida are characterized by shells that ...
Plectronocerida subsequently split off as separate order. :: Order †
Orthocerida Orthocerida is an order of extinct Orthoceratoid cephalopods also known as the Michelinocerida that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until t ...
Includes orthocerids and pseudorthocerids :: Order †
Ascocerida The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflated ...
:: Order †
Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible member is known from the Early Permian), in which the connecting rings are ...
:: Order †
Discosorida Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and c ...
:: Order †
Tarphycerida The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician (middle and upper Canad) to the Middle Devonian. Some, such as ''Aphetoceras'' and ''Estonioceras'', are loosely coiled and gyroconic; ...
:: Order † Barrandeocerida A polyphyletic group now included in the Tarphycerida :: Order
Nautilida The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, ''Nautilus'' and '' Allonau ...
: Subclass †
Bactritoidea The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in Pragian stage before 409 million y ...
:: Order †
Bactritida The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in Pragian stage before 409 million ye ...
Paleozoic Ammonoidea (Miller, Furnish and Schindewolf, 1957) :: Suborder † Anarcestina :: Suborder †
Clymeniina ''Clymeniina'' is an extinct suborder of ammonites that existed during the Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago ( ...
:: Suborder †
Goniatitina Goniatitina is one of two suborders included in the order Goniatitida; extinct Paleozoic ammonoid cephalopods only distantly related to the Nautiloidea. Taxonomy The Goniatitina contains 17 defined superfamilies, listed below. * Adrianitoidea ...
:: Suborder † Prolecanitina Mesozoic Ammonoidea (Arkel et al., 1957) :: Suborder † Ceratitina :: Suborder †
Phylloceratina The Phyllocertina comprise a suborder of ammonoid cephalopods, belonging to the Ammonitida, whose range extends from the Lower Triassic to the Upper Cretaceous. Shells of the Phylloceratina are generally smooth with small to large umbilici and c ...
:: Suborder †
Lytoceratina Lytoceratina is a suborder of Jurassic and Cretaceous ammonites that produced loosely coiled, evolute and gyroconic shells in which the sutural element are said to have complex moss-like endings. Morphologic characteristics Shells are generally ...
:: Suborder †
Ammonitina Ammonitina comprises a diverse suborder of ammonite cephalopods that lived during the Jurassic and Cretaceous periods of the Mesozoic Era. They are excellent index fossils, and it is often possible to link the rock layer in which they are found t ...
Subsequent revisions include the establishment of three Upper Cambrian orders, the Plectronocerida, Protactinocerida, and Yanhecerida; separation of the pseudorthocerids as the Pseudorthocerida, and elevating orthoceratid as the Subclass Orthoceratoidea.


Shevyrev classification

Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms, although this classification has been criticized as arbitrary, lacking evidence, and based on misinterpretations of other papers. Class Cephalopoda * Subclass † Ellesmeroceratoidea **Order †
Plectronocerida Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic Pres Inc. Oc ...
() **Order † Protactinocerida **Order † Yanhecerida **Order †
Ellesmerocerida The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician. Morphology The Ellesmerocerida are characterized by shells that ...
() * Subclass †
Endoceratoidea Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian ...
() **Order †
Endocerida Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Siluri ...
() **Order †
Intejocerida Intejocerida is the name given to a group of generally straight shelled nautiloid cephalopods originally found in Lower and Middle Ordovician sediments in the Angara River basin in Russia; defined in the Treatise as an order, and combined there w ...
() * Subclass † Actinoceratoidea ** Order †
Actinocerida The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposit ...
() * Subclass
Nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...
ea (490.0 Ma- Rec) ** Order †
Basslerocerida Basslerocerida is an order (biology), order of Nautiloidea, nautiloid cephalopods from the Ordovician comprising exogastric longiconic cyrtocones,R. H. Flower and B. Kummel. 1950. A Classification of the Nautiloidea. Journal of Paleontology 24(5) ...
() ** Order †
Tarphycerida The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician (middle and upper Canad) to the Middle Devonian. Some, such as ''Aphetoceras'' and ''Estonioceras'', are loosely coiled and gyroconic; ...
() ** Order †
Lituitida Lituitida is an order of orthoceratoid cephalopods. They correspond to the family Lituitidae of the Treatise (Furnish & Glenister, 1964), reranked as an order and combined with other orthoceratoids. They are considered to be more closely related ...
() ** Order †
Discosorida Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and c ...
() ** Order †
Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible member is known from the Early Permian), in which the connecting rings are ...
() ** Order
Nautilida The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, ''Nautilus'' and '' Allonau ...
(410.5 Ma-Rec) * Subclass †
Orthoceratoidea Orthoceratoidea is a major subclass of nautiloid cephalopods. Members of this subclass usually have orthoconic (straight) to slightly cyrtoconic (curved) shells, and central to subcentral siphuncles which may bear internal deposits. Orthocerato ...
() ** Order †
Orthocerida Orthocerida is an order of extinct Orthoceratoid cephalopods also known as the Michelinocerida that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until t ...
() ** Order †
Ascocerida The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflated ...
() ** Order † Dissidocerida () ** Order † Bajkalocerida * Subclass †
Bactritoidea The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in Pragian stage before 409 million y ...
() * Subclass †
Ammonoidea Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttl ...
() * Subclass
Coleoidea Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whos ...
(410.0 Ma-rec)


Cladistic classification

Another recent system divides all cephalopods into two clades. One includes nautilus and most fossil nautiloids. The other clade (
Neocephalopoda Neocephalopods are a group of cephalopod mollusks that include the coleoids and all extinct species that are more closely related to extant coleoids than to the nautilus. In cladistic terms, it is the total group of Coleoidea. In contrast, the ...
or Angusteradulata) is closer to modern coleoids, and includes belemnoids, ammonoids, and many
orthocerid Orthocerida is an order of extinct Orthoceratoid cephalopods also known as the Michelinocerida that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until t ...
families. There are also
stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
cephalopods of the traditional
Ellesmerocerida The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician. Morphology The Ellesmerocerida are characterized by shells that ...
that belong to neither clade. The coleoids, despite some doubts, appear from molecular data to be monophyletic.


In culture

Ancient seafaring people were aware of cephalopods, as evidenced by artworks such as a stone carving found in the archaeological recovery from Bronze Age
Minoan Crete The Minoan civilization was a Bronze Age Aegean civilization on the island of Crete and other Aegean Islands, whose earliest beginnings were from 3500BC, with the complex urban civilization beginning around 2000BC, and then declining from 1450B ...
at
Knossos Knossos (also Cnossos, both pronounced ; grc, Κνωσός, Knōsós, ; Linear B: ''Ko-no-so'') is the largest Bronze Age archaeological site on Crete and has been called Europe's oldest city. Settled as early as the Neolithic period, the na ...
(1900 – 1100 BC) has a depiction of a fisherman carrying an octopus. The terrifyingly powerful
Gorgon A Gorgon ( /ˈɡɔːrɡən/; plural: Gorgons, Ancient Greek: Γοργών/Γοργώ ''Gorgṓn/Gorgṓ'') is a creature in Greek mythology. Gorgons occur in the earliest examples of Greek literature. While descriptions of Gorgons vary, the te ...
of
Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities of ...
may have been inspired by the octopus or squid, the octopus's body representing the severed head of
Medusa In Greek mythology, Medusa (; Ancient Greek: Μέδουσα "guardian, protectress"), also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those ...
, the beak as the protruding tongue and fangs, and its tentacles as the snakes. The
Kraken The kraken () is a legendary sea monster of enormous size said to appear off the coasts of Norway. Kraken, the subject of sailors' superstitions and mythos, was first described in the modern age at the turn of the 18th century, in a travel ...
are legendary sea monsters of giant proportions said to dwell off the coasts of Norway and Greenland, usually portrayed in art as giant cephalopods attacking ships.
Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, t ...
included it in the first edition of his 1735 ''
Systema Naturae ' (originally in Latin written ' with the ligature æ) is one of the major works of the Swedish botanist, zoologist and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy. Although the system, now known as binomial nom ...
''. A Hawaiian
creation myth A creation myth (or cosmogonic myth) is a symbolic narrative of how the world began and how people first came to inhabit it., "Creation myths are symbolic stories describing how the universe and its inhabitants came to be. Creation myths develop ...
says that the present cosmos is the last of a series which arose in stages from the ruins of the previous universe. In this account, the octopus is the lone survivor of the previous, alien universe. The Akkorokamui is a gigantic tentacled
monster A monster is a type of fictional creature found in horror, fantasy, science fiction, folklore, mythology and religion. Monsters are very often depicted as dangerous and aggressive with a strange, grotesque appearance that causes terror and fe ...
from
Ainu Ainu or Aynu may refer to: *Ainu people, an East Asian ethnic group of Japan and the Russian Far East *Ainu languages, a family of languages **Ainu language of Hokkaido **Kuril Ainu language, extinct language of the Kuril Islands **Sakhalin Ainu la ...
folklore. A battle with an octopus plays a significant role in
Victor Hugo Victor-Marie Hugo (; 26 February 1802 – 22 May 1885) was a French Romantic writer and politician. During a literary career that spanned more than sixty years, he wrote in a variety of genres and forms. He is considered to be one of the great ...
's book ''Travailleurs de la mer'' ('' Toilers of the Sea''), relating to his time in exile on Guernsey. Ian Fleming's 1966 short story collection ''
Octopussy and The Living Daylights ''Octopussy and The Living Daylights'' (sometimes published as ''Octopussy'') is the 14th and final James Bond book written by Ian Fleming in the Bond series. The book is a collection of short stories published posthumously in the United Ki ...
'', and the 1983 ''James Bond'' film were partly inspired by Hugo's book. Japanese erotic art, ''
shunga is a type of Japanese language, Japanese erotic art typically executed as a kind of ukiyo-e, often in Woodcut, woodblock print format. While rare, there are also extant erotic painted handscrolls which predate ukiyo-e. Translated literally, t ...
'', includes
ukiyo-e Ukiyo-e is a genre of Japanese art which flourished from the 17th through 19th centuries. Its artists produced woodblock prints and paintings of such subjects as female beauties; kabuki actors and sumo wrestlers; scenes from history and folk ...
woodblock prints such as Katsushika Hokusai's 1814 print ''Tako to ama'' (The Dream of the Fisherman's Wife), in which an ama diver is sexually intertwined with a large and a small octopus. The print is a forerunner of tentacle erotica. The biologist P. Z. Myers noted in his science blog, Pharyngula (blog), Pharyngula, that octopuses appear in "extraordinary" graphic illustrations involving women, tentacles, and bare breasts. Its many arms that emanate from a common center means that the octopus is sometimes used to symbolize a powerful and manipulative organization.


See also

*Cephalopod size *Cephalopod eye *Cephalopod intelligence *Pain in cephalopods *
Kraken The kraken () is a legendary sea monster of enormous size said to appear off the coasts of Norway. Kraken, the subject of sailors' superstitions and mythos, was first described in the modern age at the turn of the 18th century, in a travel ...
*List of nautiloids *List of ammonites


References


Further reading

* A comprehensive overview of Paleozoic cephalopods. * * Felley, J., Vecchione, M., Roper, C. F. E., Sweeney, M. & Christensen, T., 2001–2003: ''Current Classification of Recent Cephalopoda''
National Museum of Natural History: Department of Systematic Biology: Invertebrate Zoology: Cephalopods
* * N. Joan Abbott, Roddy Williamson, Linda Maddock. ''Cephalopod Neurobiology''. Oxford University Press, 1995. * Marion Nixon & John Zachary Young, John Z. Young. ''The brains and lives of Cephalopods''. Oxford University Press, 2003. * Hanlon, Roger T. & John B. Messenger.
Cephalopod Behaviour
'. Cambridge University Press, 1996. * Martin Stevens & Sami Merilaita. ''Animal camouflage: mechanisms and function''. Cambridge University Press, 2011. * * Classification key to modern cephalopods: ftp://ftp.fao.org/docrep/fao/009/a0150e/a0150e03.pdf


External links


TONMO.COM – The Octopus News Magazine Online – cephalopod articles and discussionScientific American: Can a Squid Fly Out of the Water?Roger Hanlon's Seminar: "Rapid Adaptive Camouflage and Signaling in Cephalopods"Deep Sea Dwelling Bristle Worms
{{Authority control Cephalopods, Marine molluscs Extant Cambrian first appearances Taxa named by Georges Cuvier