Starfish or sea stars are
star-shaped echinoderm
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s belonging to the
class
Class, Classes, 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 d ...
Asteroidea (). Common usage frequently finds these names being also applied to
ophiuroids, which are correctly referred to as
brittle star
Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...
s or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the
seabed
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
in all the world's
ocean
The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...
s, from warm,
tropical zones to frigid,
polar regions
The polar regions, also called the frigid geographical zone, zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North Pole, North and South Poles), lying within the pol ...
. They are found from the
intertidal zone
The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various ...
down to
abyssal depths, at below the surface.
Starfish are
marine invertebrates
Marine invertebrates are invertebrate animals that live in marine habitats, and make up most of the macroscopic life in the oceans. It is a polyphyletic blanket term that contains all marine animals except the marine vertebrates, including the ...
. They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have
tube feet
Tube or tubes may refer to:
* Tube (2003 film), ''Tube'' (2003 film), a 2003 Korean film
* "Tubes" (Peter Dale), performer on the Soccer AM#Tubes, Soccer AM television show
* Tube (band), a Japanese rock band
* Tube & Berger, the alias of dance/e ...
operated by a
hydraulic system and a mouth at the centre of the oral or lower surface. They are
opportunistic
300px, ''Opportunity Seized, Opportunity Missed'', engraving by Theodoor Galle, 1605
Opportunism is the practice of taking advantage of circumstances — with little regard for principles or with what the consequences are for others. Opport ...
feeders and are mostly
predators on
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and
suspension feeding. They have complex
life cycles and can reproduce both sexually and
asexually. Most can
regenerate damaged parts or lost arms and they can shed arms as a means of defense.
The Asteroidea occupy several significant
ecological roles. Starfish, such as the
ochre sea star
''Pisaster ochraceus'', generally known as the purple sea star, ochre sea star, or ochre starfish, is a common seastar found among the waters of the Pacific Ocean. Identified as a keystone species, ''P. ochraceus'' is considered an important indi ...
(''Pisaster ochraceus'') and the
reef sea star (''Stichaster australis''), have become widely known as examples of the
keystone species
A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in main ...
concept in ecology. The tropical
crown-of-thorns starfish (''Acanthaster planci'') is a voracious predator of
coral
Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
throughout the Indo-Pacific region, and the
Northern Pacific seastar is on the list of the
World's 100 Worst Invasive Alien Species.
The
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 preserve ...
record for starfish is ancient, dating back to the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
around 450 million years ago, but it is rather sparse, as starfish tend to disintegrate after death. Only the
ossicles
The ossicles (also called auditory ossicles) are three irregular bones in the middle ear of humans and other mammals, and are among the smallest bones in the human body. Although the term "ossicle" literally means "tiny bone" (from Latin ''ossi ...
and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as
curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.
Anatomy
Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms.
In Antarctic ''
Labidiaster annulatus'' the number of arms can reach over fifty. Evidence from
gene expression
Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...
finds that the starfish body corresponds to a head externally (with lips attacted to the tube feet) and a
torso
The torso or trunk is an anatomical terminology, anatomical term for the central part, or the core (anatomy), core, of the body (biology), body of many animals (including human beings), from which the head, neck, limb (anatomy), limbs, tail an ...
internally.
Body wall
The body wall layers include a thin cuticle, an
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...
consisting of a single layer of cells, a thick
dermis
The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
formed of
connective tissue
Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...
and thin
coelom
The coelom (or celom) is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, i ...
ic
myoepithelial layer for the muscles, and a
peritoneum
The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesotheli ...
. The dermis contains an
endoskeleton
An endoskeleton (From Ancient Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is a structural frame (skeleton) — usually composed of mineralized tissue — on the inside of an animal, overlaid by soft ...
of
calcium carbonate
Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...
components known as ossicles. These are honeycomb-like structures composed of
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
microcrystals arranged in a lattice.
[Ruppert et al., 2004. pp. 876–880] They vary in form, from flat plates to granules to spines, and cover the aboral surface.
Some are specialised structures such as the
madreporite (the entrance to the water vascular system),
pedicellariae and
paxillae. Paxillae are umbrella-like structures found on starfish that live buried in substrate. The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. The ossicles are located under the epidermal layer, even those emerging externally.
[
Several groups of starfish, including ]Valvatida
The Valvatida are an order of starfish in the class Asteroidea, which contains 695 species in 172 genera in 17 families.
Description
The order encompasses both tiny species, which are only a few millimetres in diameter, like those in the gen ...
and Forcipulatida
The Forcipulatida are an order (biology), order of sea stars, containing three families and 49 genera.
Description
Forcipulatids share with the Brisingida, brisingid sea stars distinctive pedicellariae, consisting of a short stalk with three ske ...
, possess pedicellariae. These are scissor-like ossicles at the tip of the spine which displace organisms from resting on the starfish's surface. Some species like ''Labidiaster annulatus'' and '' Novodinia antillensis'' use their pedicellariae to catch prey. There may also be papulae, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory function. The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices. This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress.
File:Luidia maculata, Ras Sedr, Egypt.jpg, '' Luidia maculata'', a seven armed starfish
File:Astropecten aranciacus Naxos08 1775 dett.jpg, '' Astropecten aranciacus'' ossicles
The ossicles (also called auditory ossicles) are three irregular bones in the middle ear of humans and other mammals, and are among the smallest bones in the human body. Although the term "ossicle" literally means "tiny bone" (from Latin ''ossi ...
File:Pédicellaires d' Acanthaster Planci.JPG, Pedicellariae and retracted papulae among the spines of '' Acanthaster planci''
File:Asterias forbesi pedicellaria and papulae.jpg, Pedicellaria and papulae of '' Asterias forbesi''
Water vascular system
The water vascular Vascular can refer to:
* blood vessels, the vascular system in animals
* vascular tissue
Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue ...
system of the starfish is a hydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a b ...
. Water enters the system through the madreporite, a porous, often conspicuous, sieve-like ossicle on the aboral surface. It is linked through a stone canal, often lined with calcareous material, to a ring canal around the mouth opening. A set of radial canals leads off this; one radial canal runs along the ambulacral groove in each arm. There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows. The interior of the whole canal system is lined with cilia
The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
.[Ruppert et al., 2004. pp. 879–883, 889]
Water is pushed into the tube face when longitudinal muscles in the ampullae contract, and shut the valves in the lateral canals. This causes the tube feet to stretch and touch the substrate. Although the tube feet resemble suction cups in appearance, the gripping action is a function of adhesive chemicals rather than suction. Other chemicals and relaxation of the ampullae allow for release from the substrate. The tube feet latch on to surfaces and move in a wave, with one arm section attaching to the surface as another releases.[ To expose the sensory tube feet and the eyespot to external stimuli, some starfish turn up the tips of their arms while moving.
Having descended from bilateral organisms, starfish may move in a bilateral fashion, particularly when hunting or threated. When crawling, certain arms act as the leading arms, while others trail behind. When a starfish finds itself upside down, its raises its arms and then two adjacent arms and an opposite arm along press against the ground to lift up the two remaining arms; the opposite arm leaves the ground as the starfish turns itself over and recovers its normal stance.]
Apart from their function in locomotion, the tube feet act as accessory gills. The water vascular system serves to transport oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion. Fluid movement is bidirectional and initiated by cilia
The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
.
File:Seastar.webm, Video showing the tube feet movement of a starfish
File:Détail bras d'étoile de mer.jpg, alt=Arm tip with tube feet, Arm tip of '' Leptasterias polaris'' showing tube feet and eyespot
Digestive system and excretion
The gut of a starfish fills most of the disc and extends into the arms. The mouth occupies the centre of the oral surface, where it is surrounded by a tough peristomial membrane and closed with a sphincter
A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the human bo ...
. A short oesophagus
The esophagus (American English), oesophagus (British English), or œsophagus ( archaic spelling) ( see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), colloquially known also as the food pipe, food tube, or gullet, ...
connects the mouth to a stomach
The stomach is a muscular, hollow organ in the upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...
which consists of an eversible cardiac portion and a smaller pyloric portion. The cardiac stomach is glandular and pouched, and is supported by ligament
A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...
s attached to ossicles in the arms so it can be pulled back into position after it has been everted. The pyloric stomach has two extensions into each arm: the pyloric caeca. These long, hollow tubes that are lined by a series of glands, which secrete digestive enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s and absorb nutrients from the food. A short intestine
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...
and rectum
The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...
run from the pyloric stomach to the anus
In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...
at the apex of the aboral surface of the disc.[Ruppert et al., 2004. p. 885]
Primitive starfish, such as ''Astropecten'' and ''Luidia'', shallow their prey
Predation is a biological interaction in which 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 ki ...
whole, and start to digest it in their cardiac stomachs, spitting out hard material like shells. The semi-digested fluid flows into the caeca for more digestion as well as absorption. In more advanced species of starfish, the cardiac stomach can be everted from the organism's body to engulf and digest food, which is passed to the pyloric stomach. The retraction and contraction of the cardiac stomach is activated by a neuropeptide
Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like the ...
known as NGFFYamide.
The main nitrogenous waste product is ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
, which is removed diffusion through the tube feet, papulae and other thin-walled areas. Other waste material include urates. Their body fluid contains phagocytic
Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...
cells called coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water.[Ruppert et al., 2004. pp. 886–887]
Starfish keep their body fluids at the same salt concentration as the surrounding water, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments.[
]
Sensory and nervous systems
Although starfish do not have many well-defined sense organs, they are sensitive to touch, light, temperature, orientation and the status of the water around them. The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at the tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food.[ There are eyespots at the ends of the arms, each one made of 80–200 simple ]ocelli
A simple eye or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates. These eyes are called "simple" to distinguish the ...
composed of pigment
A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
ed epithelial cells. Individual photoreceptor cells are present in other parts of their bodies and respond to light. Whether they advance or retreat depends on the species.[Ruppert et al., 2004. pp. 883–884]
While a starfish lacks a centralized brain, it has a complex nervous system
In 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 body. Th ...
with a nerve ring around the mouth and a radial nerve running along the ambulacral region of each arm parallel to the radial canal. The peripheral nerve system consists of two nerve nets: one in the epidermis and the other in the lining of the coelomic cavity, which are the sensory and motor systems respectively. Neurons passing through the dermis join the two. Both the ring and radial nerves function in movement and sensory. The sensory component is supplied with information from the sensory organs while the motor nerves control the tube feet and musculature. If one arm detects something, it becomes dominant and temporarily over-rides the other arms to initiate movement towards it.
Circulatory and gas exchange system
The body cavity contains the circulatory or haemal system. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring) and the third near the aboral surface (the genital ring). The heart beats about six times a minute and is at the apex of a vertical channel (the axial vessel) that connects the three rings. Blood does not contain a pigment but is probably used to transport nutrients around the body. Gas exchange aminly takes place through gills known as papulae, which are thin-walled bulges along the aboral surface of the arms. Oxygen is transferred from these to the coelomic fluid, which moves gas around the body.
Secondary metabolites
Starfish produce a large number of secondary metabolite
Secondary metabolites, also called ''specialised metabolites'', ''secondary products'', or ''natural products'', are organic compounds produced by any lifeform, e.g. bacteria, archaea, fungi, animals, or plants, which are not directly involved ...
s in the form of lipid
Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
s, including steroid
A steroid is an organic compound with four fused compound, fused rings (designated A, B, C, and D) arranged in a specific molecular configuration.
Steroids have two principal biological functions: as important components of cell membranes t ...
al derivatives of cholesterol
Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils.
Cholesterol is biosynthesis, biosynthesized by all anima ...
, and fatty acid
In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...
s of sphingosine. The steroids are mostly saponin
Saponins (Latin ''sapon'', 'soap' + ''-in'', 'one of') are bitter-tasting, usually toxic plant-derived secondary metabolites. They are organic chemicals that become foamy when agitated in water and have high molecular weight. They are present ...
s, known as asterosaponins, and their sulphated derivatives. They vary between species and are typically formed from up to six sugar molecules (usually glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
and galactose
Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', ), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epime ...
) connected by up to three glycosidic
A glycosidic bond or glycosidic linkage is a type of ether bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal group of a ...
chains. Long-chain fatty acid amides of sphingosine occur frequently with some having known biological activity
In pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or ...
. Starfish also contain various ceramide
Ceramides are a family of waxy lipid molecules. A ceramide is composed of sphingosine and a fatty acid joined by an amide bond. Ceramides are found in high concentrations within the cell membrane of Eukaryote, eukaryotic cells, since they are co ...
s and a small number of alkaloid
Alkaloids are a broad class of natural product, naturally occurring organic compounds that contain at least one nitrogen atom. Some synthetic compounds of similar structure may also be termed alkaloids.
Alkaloids are produced by a large varie ...
s. These these chemicals in the starfish may function in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are antifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish's aboral surface. Some are alarm pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s and escape-eliciting chemicals, the release of which trigger responses in conspecific
Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species.
Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organism ...
starfish but often stimulate flight in potential prey. Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.
Life cycle
Sexual reproduction
Most species of starfish are gonochorous, there being separate male and female individuals. Some species are simultaneous hermaphrodites, producing eggs and sperm at the same time, and in a few of these the same gonad, called an ovotestis
An ovotestis is a gonad with both Testicle, testicular and Ovary, ovarian aspects. In humans, ovotestes are an infrequent anatomical variation associated with gonadal dysgenesis. The only mammals where ovotestes are not characteristic of an infrequ ...
, produces both eggs and sperm. Other starfish are sequential hermaphrodites. Protandrous individuals of species like '' Asterina gibbosa'' start life as males before changing sex into females as they grow older. In some species such as '' Nepanthia belcheri'', a large female can split in half and the resulting offspring are males. When these grow large enough they change back into females.
Each starfish arm contains two gonads that release gametes
A gamete ( ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. The name gamete was introduced by the Ge ...
through openings called gonoducts, located on the central disc between the arms. Fertilization
Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...
is generally external but in a few species, internal fertilization takes place. In most species, the buoyant eggs and sperm are simply released into the water (free spawning) and the resulting embryo
An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
s and larva
A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
e live as part of the plankton
Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
. In others, the eggs may be stuck to the undersides of rocks. In certain species of starfish, the females brood their eggs – either by simply enveloping them or by holding them in specialised structures in different parts of the body, externally or internally.[ Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate.] '' Pteraster militaris'' broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch.[ In these brooding species, the eggs are relatively large, and supplied with ]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 ...
, and they generally develop directly into miniature starfish without an intervening larval stage,[ called "lecithotrophic" . In '' Parvulastra parvivipara'', an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch.] Brooding occurs in species that live in colder waters.[Ruppert et al., 2004. pp. 887–888] and in smaller species that produce just a few eggs.[ in Lawrence (2013)]
Contributations to the timing of spawning may include lighting conditions, water temperature and food availability. Individuals may gather together to release their gametes as once, using pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s to attract each other.[ In some species, a male and female may come together and form a pair.] They engage in pseudocopulation
Pseudocopulation is a behavior similar to Copulation (zoology), copulation that serves a reproductive function for one or both participants but does not involve actual sexual union between the individuals. It is most generally applied to a pollin ...
which involves the male crawling on the female and fertilising her gametes as she releases them.[
]
Larval development
Starfish embryos typically hatch as blastulas. Invagination
Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination of Epithelium, epithelial sheets occurs in many contexts during Animal embryonic development, embryonic developme ...
s take place, the first creating the anus is created from blastopore, while a second, taking place in the ectodermic layer, creates the mouth. The archenteron
The archenteron, also called the gastrocoel, is the internal cavity formed in the gastrulation stage in early embryonic development that becomes the cavity of the primitive gut.
Formation in sea urchins
As primary mesenchyme cells detach fro ...
stretches towards the mouth and connects with it, forming the gut. A band of cilia
The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a bipinnaria. The cilia are used for locomotion and feeding, their rhythmic beat wafting phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
towards the mouth.[
The next stage in development is a brachiolaria larva and involves the growth of three short ventral-anterior arms with adhesive tips and surrounding a sucker. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk made from the ventral arms and sucker. Metamorphosis now takes place with a radical rearrangement of tissues. The larvae develops an oral surface on the left and a aboral surface on the right. While the gut remains, the mouth and anus move to new positions. Some of the body cavities disappear but others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical. It casts off its stalk and becomes a free-living juvenile starfish up to in diameter.]
Asexual reproduction
Some species of starfish can reproduce asexually as adults either by fission of their central discs or by autotomy of one or more of their arms. Single arms that regenerate a whole individual are called comet forms. The larvae of several species of starfish can reproduce asexually before they reach maturity. They do this by autotomising some parts of their bodies or by budding
Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...
. Larva increase asexual reproduction when they sense that food is plentiful. Though this costs it time and energy and delays maturity, it allows a single larva to give rise to multiple adults when the conditions are appropriate.[
]
Regeneration
Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.[ A few can regrow a complete new disc from a single arm, while others need at least part of the central disc to be attached to the detached part.][ Regrowth can take several months, and starfish are vulnerable to infections during the early stages after the loss of an arm.] Other than fragmentation carried out for the purpose of reproduction, the division of the body may happen as a defense mechanism
In psychoanalytic theory, defence mechanisms are unconscious psychological processes that protect the self from anxiety-producing thoughts and feelings related to internal conflicts and external stressors.
According to this theory, healthy ...
.[ The loss of parts of the body is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. This type of tissue is called catch connective tissue and is found in most ]echinoderm
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s. An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.
Lifespan
The lifespan of a starfish varies considerably between species. For example, '' Leptasterias hexactis'' at reaches sexual maturity at and in two years and lives for about ten years. '' Pisaster ochraceus'' matures at , and in five years and has a maximum recorded lifespan of 34 years.
Ecology
Distribution and habitat
Starfish live in marine waters around the world and include both tropical and polar waters.[ They are mainly ]benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
animals and live in sandy, muddy and rocky substrates and range from shallow, intertidal waters[ to the deep-sea floor down to at least . Starfish are most common along the coast.][
]
Diet
Most species are generalist predators, eating microalgae
Microalgae or microphytes are microscopic scale, microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine life, marine systems, living in both the water column and sediment. They are unicellul ...
, sponge
Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...
s, bivalve
Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
s, snail
A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...
s and other small animals.[ The crown-of-thorns starfish consumes ]coral
Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
polyps,[ while other species are ]detritivore
Detritivores (also known as detrivores, detritophages, detritus feeders or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...
s, feeding on decomposing organic material and faecal matter.[ in Lawrence (2013)][ in Lawrence (2012)] A few are suspension feeders, gathering in phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
; '' Henricia'' and ''Echinaster
''Echinaster'' is a well-studied and common genus of starfish containing ~30 species and is the second-largest genus found within the family Echinasteridae. The genera ''Henricia'' and ''Echinaster'' encompass 90% of all the species found within ...
'' often feed near with sponges, using from the water current they produce. Various species have been shown to be able to absorb organic nutrients from the surrounding water, and this may form a significant portion of their diet.
The processes of feeding and capture may be aided by special parts; '' Pisaster brevispinus'', the short-spined pisaster from the West Coast of America, can use a set of specialized tube feet to dig itself deep into the soft substrate to extract prey (usually clam
Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...
s). Grasping the shellfish, the starfish slowly pries open the prey's shell, wearing out its adductor muscle, and then inserts its everted stomach into the crack to digest the soft tissues. The gap between the valves need only be a fraction of a millimetre wide for the stomach to gain entry.
Ecological impact
Starfish are keystone species
A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in main ...
in their respective marine communities
A community is a Level of analysis, social unit (a group of people) with a shared socially-significant characteristic, such as place (geography), place, set of Norm (social), norms, culture, religion, values, Convention (norm), customs, or Ide ...
. Their relatively large sizes, diverse diets and ability to adapt to different environments makes them ecologically important. The term "keystone species" was in fact first used by Robert Paine in 1966 to describe a starfish, ''Pisaster ochraceus''. When studying the low intertidal coasts of Washington state
Washington, officially the State of Washington, is a state in the Pacific Northwest region of the United States. It is often referred to as Washington State to distinguish it from the national capital, both named after George Washington ...
, Paine found that predation by ''P. ochraceus'' was a major factor in the diversity of species. Experimental removals of this top predator from a stretch of shoreline resulted in lower species diversity and the eventual domination of '' Mytilus'' mussels, which were able to outcompete other organisms for space and resources. Similar results were found in a 1971 study of '' Stichaster australis'' on the intertidal coast of the South Island
The South Island ( , 'the waters of Pounamu, Greenstone') is the largest of the three major islands of New Zealand by surface area, the others being the smaller but more populous North Island and Stewart Island. It is bordered to the north by ...
of New Zealand
New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
. ''S. australis'' was found to have removed most of a batch of transplanted mussels within two or three months of their placement, while in an area from which ''S. australis'' had been removed, the mussels increased in number dramatically, overwhelming the area and threatening biodiversity
Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...
.
The feeding activity of the omnivorous
An omnivore () is an animal that regularly consumes significant quantities of both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize ...
starfish '' Oreaster reticulatus'' on sandy and seagrass bottoms in the Virgin Islands
The Virgin Islands () are an archipelago between the Atlantic Ocean, North Atlantic Ocean and northeastern Caribbean Sea, geographically forming part of the Leeward Islands of the Lesser Antilles in the Caribbean, Caribbean islands or West Indie ...
effects the composition of communities of microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles. Organisms that dislike this disturbance are replaced by others better able to rapidly recolonise "clean" sediment. In addition, foraging by these starfish creates diverse patches of organic matter, which may attract larger organisms such as fish, crabs and sea urchins that feed on the sediment.
Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and French Polynesia
French Polynesia ( ; ; ) is an overseas collectivity of France and its sole #Governance, overseas country. It comprises 121 geographically dispersed islands and atolls stretching over more than in the Pacific Ocean, South Pacific Ocean. The t ...
. A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping over 40% to under 5% in four years. This in turn had a cascade effect
A cascade effect is an inevitable and sometimes unforeseen chain of events due to an act affecting a system. If there is a possibility that the cascade effect will have a negative impact on the system, it is possible to analyze the effects with a ...
on both sessile bottom-dwelling animals and reef fish.[ '' Asterias amurensis'' is a rare example of an invasive echinoderm . Its larvae likely arrived in ]Tasmania
Tasmania (; palawa kani: ''Lutruwita'') is an island States and territories of Australia, state of Australia. It is located to the south of the Mainland Australia, Australian mainland, and is separated from it by the Bass Strait. The sta ...
from central Japan via water discharged from ships in the 1980s. The species has since grown in numbers to the point where they threaten important bivalve
Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
s fisheries in Australia. As such, they are considered pests,[ in Lawrence (2013)] and are on the Invasive Species Specialist Group's list of the world's 100 worst invasive species. Some species that prey on bivalve molluscs can transmit paralytic shellfish poisoning.
Threats
Starfish may be preyed on by conspecifics, sea anemones, other starfish species, tritons, crabs, fish, gull
Gulls, or colloquially seagulls, are seabirds of the subfamily Larinae. They are most closely related to terns and skimmers, distantly related to auks, and even more distantly related to waders. Until the 21st century, most gulls were placed ...
s and sea otter
The sea otter (''Enhydra lutris'') is a marine mammal native to the coasts of the northern and eastern Pacific Ocean, North Pacific Ocean. Adult sea otters typically weigh between , making them the heaviest members of ...
s.[ in Lawrence (2013)][ in Lawrence (2013)][ in Lawrence (2013)] Their first lines of defence are the saponin
Saponins (Latin ''sapon'', 'soap' + ''-in'', 'one of') are bitter-tasting, usually toxic plant-derived secondary metabolites. They are organic chemicals that become foamy when agitated in water and have high molecular weight. They are present ...
s present in their body walls, which have unpleasant flavours. Some starfish such as '' Astropecten polyacanthus'' also include powerful toxins such as tetrodotoxin
Tetrodotoxin (TTX) is a potent neurotoxin. Its name derives from Tetraodontiformes, an Order (biology), order that includes Tetraodontidae, pufferfish, porcupinefish, ocean sunfish, and triggerfish; several of these species carry the toxin. Alt ...
, and the slime star can ooze out large quantities of repellent mucus The crown-of-thorns starfish possesses sharp spines, toxins and bright warning colours.
Several species sometimes suffer from a wasting
In medicine, wasting, also known as wasting syndrome, refers to the process by which a debilitating disease causes muscle and fat tissue to "waste" away. Wasting is sometimes referred to as "acute malnutrition" because it is believed that epis ...
condition caused by bacteria in the genus '' Vibrio'';[ however, a more widespread wasting disease, causing mass mortalities among starfish, appears sporadically. A paper published in November 2014 revealed the most likely cause of this disease to be a densovirus the authors named sea star-associated densovirus (SSaDV).
The results of a 2025 study of starfish off the coast of central British Columbia suggest that those living in the fjords can better survive outbreaks of the disease due to the lower temperatures and higher salinity of their environment. The protozoan '' Orchitophrya stellarum'' is known to infect and damage the gonads of starfish.][ Starfish are vulnerable to high temperatures. Experiments have shown that the feeding and growth rates of ''P. ochraceus'' reduce greatly when their body temperatures rise above and that they die when their temperature rises to .] This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide. It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like the stomach.
Starfish and other echinoderms can be vulnerable to marine pollution
Marine pollution occurs when substances used or spread by humans, such as industrial waste, industrial, agricultural pollution, agricultural, and municipal solid waste, residential waste; particle (ecology), particles; noise; excess carbon dioxi ...
. The common starfish is considered to be a bioindicator
A bioindicator is any species (an indicator species) or group of species whose function, population, or status can reveal the qualitative status of the environment. The most common indicator species are animals. For example, copepods and other sma ...
for marine ecosystems. A 2009 study found that ''P. ochraceus'' is unlikely to be affected by ocean acidification
Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ...
as severely as other marine animals with calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines.
In zoology
''Calcare ...
skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when the pH is lowered. Researchers found that when ''P. ochraceus'' were exposed to and 770 ppm carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
(beyond rises expected in the next century), they were relatively unaffected. Their survival is likely due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue.
Evolution
Fossil record
The earliest fossil echinoderms date to the Cambrian
The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
, with the first asterozoans (group that includes starfish and brittle stars) being the Somasteroidea, which exhibit traits of both groups. Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. Despite this, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in Lagerstätte
A Fossil-Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that preserves an exceptionally high amount of palaeontological information. ''Konzentrat-Lagerstätten'' preserv ...
n – so-called "starfish beds".
By the late Paleozoic
The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
, the crinoids and blastoid
Blastoids (class Blastoidea) are an extinct type of stemmed echinoderm, often referred to as sea buds. They first appear, along with many other echinoderm classes, in the Ordovician period, and reached their greatest diversity in the Mississi ...
s were the predominant echinoderms, fragments of which are almost the only fossil found in some limestones. In the two major extinction event
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occ ...
s that occurred during the late Devonian
The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
and late Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
, the blastoids were wiped out and only a few species of crinoids survived.[ Many starfish species also became extinct in these events, but afterwards the surviving few species quickly diversified rapidly within sixty million years during from between the beginning and middle of the ]Middle Jurassic
The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...
. A 2012 study found that 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 within ...
in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development of ''Cryptasterina hystera
''Cryptasterina hystera'' is a species of starfish. It is found in a limited region of the coast of Australia and is very similar in appearance to '' Cryptasterina pentagona''. The two appear to have diverged from a common ancestral line a few th ...
'' and ''Cryptasterina pentagona
''Cryptasterina pentagona'' is a species of starfish in the family Asterinidae. It is found in shallow waters in north eastern Australia. Its life cycle includes the release of large-yolked eggs and the development of planktonic larvae which is ...
'' has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.
Diversity
The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830. It is derived from the Greek
Greek may refer to:
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 of all kno ...
''aster'', ἀστήρ (a star) and the Greek ''eidos'', εἶδος (form, likeness, appearance). Starfish are included in the subphylum Asterozoa along with brittle and basket stars (order Ophiuroidea
Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...
), the characteristics of which include a star-shaped body as adults, with multiple arms surrounding central disc. The arms of asteroids are skeletally connected to the disc by ossicles in the body wall while ophiuroids have clearly demarcated arms.
The starfish are a large and diverse class with over 1,900 living species. There are seven extant
Extant or Least-concern species, least concern 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
* Exta ...
orders, Brisingida
The Brisingids are deep-sea-dwelling starfish in the order Brisingida.
Description
These starfish have between 6 and 16 long, attenuated arms which they use for suspension feeding. Other characteristics include a single series of marginals, a ...
, Forcipulatida
The Forcipulatida are an order (biology), order of sea stars, containing three families and 49 genera.
Description
Forcipulatids share with the Brisingida, brisingid sea stars distinctive pedicellariae, consisting of a short stalk with three ske ...
, Notomyotida, Paxillosida
The Paxillosida are a large order of sea stars.
Characteristics
Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development.Matsubara, M., Komatsu, M., Araki, T ...
, Spinulosida, Valvatida
The Valvatida are an order of starfish in the class Asteroidea, which contains 695 species in 172 genera in 17 families.
Description
The order encompasses both tiny species, which are only a few millimetres in diameter, like those in the gen ...
and Velatida
The Velatida are an order of sea stars containing about 200 species in five families. These sea stars normally have thick bodies with large discs.
Description and characteristics
This order contains mostly deep or cold seas sea stars, often w ...
. Living asteroids, the Neoasteroidea, are distinct from their forerunners in the Paleozoic. Their classification has changed little but there is debate in regards to Paxillosida
The Paxillosida are a large order of sea stars.
Characteristics
Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development.Matsubara, M., Komatsu, M., Araki, T ...
, and the deep-water sea daisies, though clearly Asteroidea and currently included in Velatida
The Velatida are an order of sea stars containing about 200 species in five families. These sea stars normally have thick bodies with large discs.
Description and characteristics
This order contains mostly deep or cold seas sea stars, often w ...
, do not fit easily in any accepted lineage. Phylogenetic data suggests that they may be a sister group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and ...
, the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group.[
]
Living groups
; Brisingida
The Brisingids are deep-sea-dwelling starfish in the order Brisingida.
Description
These starfish have between 6 and 16 long, attenuated arms which they use for suspension feeding. Other characteristics include a single series of marginals, a ...
(2 families, 17 genera, 111 species)
:Species in this order have a small, rigid disc and 6–20 long, thin arms, which they use for suspension feeding. They have one series of marginal plates, disc plates merged in a ring, fewer numbers of aboral plates, crossed pedicellariae, and several series of long spines on the arms. They live almost exclusively in deep-sea habitats, although a few live in shallow waters in the Antarctic. In some species, the tube feet have rounded tips and lack suckers.
; Forcipulatida
The Forcipulatida are an order (biology), order of sea stars, containing three families and 49 genera.
Description
Forcipulatids share with the Brisingida, brisingid sea stars distinctive pedicellariae, consisting of a short stalk with three ske ...
(6 families, 63 genera, 269 species)
:Species in this order have distinctive pedicellariae, consisting of a short stalk with forceps like tips. and tube feet with flat-tipped suckers usually arranged in four rows. The order includes well-known species from temperate and cold-water regions, ranging from intertidal and abyssal zones.
; Notomyotida (1 family, 8 genera, 75 species)
:These starfish are deep-sea dwelling and have particularly flexible arms with distinctive lines of musculature along the sides of the dorsal region. In some species, the tube feet lack suckers.[
]
; Paxillosida
The Paxillosida are a large order of sea stars.
Characteristics
Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development.Matsubara, M., Komatsu, M., Araki, T ...
(7 families, 48 genera, 372 species)
:This is a primitive order and members do not extrude their stomach when feeding and both anus and tube feet suckers are absent.[ Papulae are present on their aboral surface and they possess marginal plates and paxillae. They mostly inhabit soft substrates.][Ruppert et al, 2004. pp. 887–889] There is no brachiolaria stage in their larval development. The comb starfish ('' Astropecten polyacanthus'') is a member of this order.
; Spinulosida (1 family, 8 genera, 121 species)
:Most species in this order lack pedicellariae and all have a delicate skeletal arrangement with small or no marginal plates on the disc and arms. They have numerous groups of short spines on the aboral surface.[
; ]Valvatida
The Valvatida are an order of starfish in the class Asteroidea, which contains 695 species in 172 genera in 17 families.
Description
The order encompasses both tiny species, which are only a few millimetres in diameter, like those in the gen ...
(16 families, 172 genera, 695 species)
:Most species in this order have five arms and two rows of tube feet with suckers. There are conspicuous marginal plates on the arms and disc. Some species have paxillae and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates.
; Velatida
The Velatida are an order of sea stars containing about 200 species in five families. These sea stars normally have thick bodies with large discs.
Description and characteristics
This order contains mostly deep or cold seas sea stars, often w ...
(4 families, 16 genera, 138 species)
:This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. The skeleton is underdeveloped.
Extinct groups
Extinct groups within the Asteroidea include:
* † Calliasterellidae, with the type genus '' Calliasterella'' from the Devonian and Carboniferous
* † Palastericus, a Devonian genus
* † Trichasteropsida, with the Triassic
The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
genus '' Trichasteropsis'' (at least 2 species)[
]
Phylogeny
External
Starfish are deuterostome
Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...
animals, like the chordate
A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree
A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...
. The times at which the clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s diverged are shown under the labels in millions of years ago (mya).
Internal
The phylogeny of the Asteroidea has been difficult to resolve, with visible (morphological) features proving inadequate, and the question of whether traditional taxa
In biology, a taxon (back-formation from ''taxonomy''; : 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 name and ...
are clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s in doubt.[ The phylogeny proposed by Gale in 1987 is:][
The phylogeny proposed by Blake in 1987 is:][
Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.][ In 2011, on further molecular evidence, Janies and colleagues noted that the phylogeny of the echinoderms "has proven difficult", and that "the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods". They presented a phylogenetic tree for the living Asteroidea only; using the traditional names of starfish orders where possible, and indicating "part of" otherwise, the phylogeny is shown below. The Solasteridae are split from the Velatida, and the old Spinulosida is broken up.]
Human relations
In research
Starfish have been used in reproductive and developmental studies. Female starfish produce large numbers of oocyte
An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ger ...
s that are easily isolated; these can be stored in a pre-meiosis phase and stimulated to complete division by the use of 1-methyladenine. Starfish oocytes are well suited for this research as they are easy to handle and maintain in sea water at room temperature, are transparent and develop quickly. '' Asterina pectinifera'', used as a model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...
for this purpose, is resilient and easy to breed and maintain in the laboratory.
Another area of research is the ability of starfish to regenerate lost body parts. The stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell ...
s of adult humans are incapable of much differentiation and understanding the regrowth, repair and cloning processes in starfish may have implications for human medicine.
Starfish also have an unusual ability to displace foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.
In legend and culture
An aboriginal Australian
Aboriginal Australians are the various indigenous peoples of the Australian mainland and many of its islands, excluding the ethnically distinct people of the Torres Strait Islands.
Humans first migrated to Australia 50,000 to 65,000 year ...
fable retold by the Welsh school headmaster William Jenkyn Thomas
William Jenkyn Thomas (5 July 1870 – 14 March 1959) was a Welsh headmaster and author best known for his ''The Welsh Fairy Book''. He was an undergraduate student at the University of Cambridge
The University of Cambridge is a Publi ...
(1870–1959) tells how some animals needed a canoe to cross the ocean. Whale had one but refused to lend it, so Starfish kept him busy, telling him stories and grooming him to remove parasites, while the others stole the canoe. When Whale realized the trick he beat Starfish ragged, which is how Starfish still is today.
In 1900, the scholar Edward Tregear documented ''The Creation Song'', which he describes as "an ancient prayer for the dedication of a high chief" of Hawaii
Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
. Among the "uncreated gods" described early in the song are the male Kumulipo ("Creation") and the female Poele, both born in the night, a coral insect, the earthworm, and the starfish.
Georg Eberhard Rumpf's 1705 ''The Ambonese Curiosity Cabinet'' describes the tropical varieties of ''Stella Marina'' or ''Bintang Laut'', "Sea Star", in Latin and Malay respectively, known in the waters around Ambon. He writes that the ''Histoire des Antilles'' reports that when the sea stars "see thunder storms approaching, heygrab hold of many small stones with their little legs, looking to ... hold themselves down as if with anchors".
As food
Starfish are sometimes eaten in China, Japan and in Micronesia. Georg Eberhard Rumpf found few starfish being used for food in the Indonesia
Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...
n archipelago, other than as bait in fish traps, but on the island of "Huamobel" the people cut them up, squeeze out the "black blood" and cook them with sour tamarind
Tamarind (''Tamarindus indica'') is a Legume, leguminous tree bearing edible fruit that is indigenous to tropical Africa and naturalized in Asia. The genus ''Tamarindus'' is monotypic taxon, monotypic, meaning that it contains only this spe ...
leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in coconut milk
Coconut milk is a plant milk extracted from the grated pulp of mature coconuts. The opacity and rich taste of the milky-white liquid are due to its high oil content, most of which is saturated fat. Coconut milk is a traditional food ingred ...
.[
]
As collectables
Starfish are in some cases taken from their habitat and sold to tourists as souvenir
A souvenir ( French for 'a remembrance or memory'), memento, keepsake, or token of remembrance is an object a person acquires for the memories the owner associates with it. A souvenir can be any object that can be collected or purchased and trans ...
s, ornaments
An ornament is something used for decoration.
Ornament may also refer to:
Decoration
*Ornament (art), any purely decorative element in architecture and the decorative arts
*Ornamental turning
*Biological ornament, a characteristic of animals tha ...
, curios or for display in aquariums. In particular, ''Oreaster reticulatus'', with its easily accessed habitat and bright coloration, is widely collected in the Caribbean. In the early to mid 20th century, this species was numerous along the West Indian coasts, but collection and trade have severely diminished its numbers. In the State of Florida, ''O. reticulatus'' is listed as endangered
An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching, inv ...
and its collection is illegal. Nevertheless, it is still sold both in and outside its range.[ A similar phenomenon exists in the Indo-Pacific for species such as '']Protoreaster nodosus
''Protoreaster nodosus'', commonly known as the horned sea star or chocolate chip sea star, is a species of sea star found in the warm, shallow waters of the Indo-Pacific region. They are sometimes seen in the marine aquarium trade or dried and ...
''.
References
Bibliography
*
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External links
* A blog about sea stars by a passionate and professional specialist.
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Articles containing video clips
Late Ordovician first appearances
Extant Ordovician first appearances
Taxa named by Henri Marie Ducrotay de Blainville