Chondrichthyes (; ) is a
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 ...
of
jawed fish
Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant taxon, extant vertebrates, including all living bony fishes (bot ...
that contains the cartilaginous fish or chondrichthyans, which all have
skeletons primarily composed of
cartilage. They can be contrasted with the
Osteichthyes
Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...
or ''bony fish'', which have skeletons primarily composed of
bone tissue. Chondrichthyes are
aquatic vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s with
paired fins, paired
nares,
placoid scales,
conus arteriosus in the
heart
The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...
, and a lack of
opercula and
swim bladders. Within the infraphylum
Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
The class is divided into two subclasses:
Elasmobranchii (
shark
Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s,
rays,
skates and
sawfish) and
Holocephali
Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...
(
chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Extant chondrichthyans range in size from the
finless sleeper ray to the over
whale shark
The whale shark (''Rhincodon typus'') is a slow-moving, filter feeder, filter-feeding carpet shark and the largest known Extant taxon, extant fish species. The largest confirmed individual had a length of . The whale shark holds many records for ...
.
Anatomy
Skeleton
The skeleton is cartilaginous. The
notochord
The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
is gradually replaced by a vertebral column during development, except in
Holocephali
Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...
, where the notochord stays intact. In some deepwater sharks, the column is reduced.
As they do not have
bone marrow,
red blood cells are produced in the
spleen and the epigonal organ (special tissue around the
gonad
A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...
s, which is also thought to play a role in the immune system). They are also produced in the
Leydig's organ, which is only found in certain cartilaginous fishes. The subclass
Holocephali
Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...
, which is a very specialized group, lacks both the Leydig's and epigonal organs.
Appendages
Apart from
electric rays, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called
placoid scales (or ''dermal denticles''), making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other.
Originally, the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In
rays, the pectoral fins are connected to the head and are very flexible.
One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion.
Body covering
Chondrichthyans have tooth-like scales called
dermal denticles or placoid scales. Denticles usually provide protection, and in most cases, streamlining. Mucous glands exist in some species, as well.
It is assumed that their oral teeth evolved from dermal denticles that migrated into the mouth, but it could be the other way around, as the
teleost
Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...
bony fish ''
Denticeps clupeoides'' has most of its head covered by dermal teeth (as does, probably, ''
Atherion elymus'', another bony fish). This is most likely a secondary evolved characteristic, which means there is not necessarily a connection between the teeth and the original dermal scales.
The old
placoderms did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown whether the dermal or oral teeth evolved first. It has even been suggested that the original bony plates of ''all'' vertebrates are now gone and that the present scales are just modified teeth, even if both the teeth and body armor had a common origin a long time ago. However, there is currently no evidence of this.
Respiratory system
All chondrichthyans breathe through five to seven pairs of
gill
A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, 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 r ...
s, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can actively pump water in through their
spiracles and out through their gills. However, this is only a general rule and many species differ.
A spiracle is a small hole found behind each eye. These can be tiny and circular, such as found on the nurse shark (''Ginglymostoma cirratum''), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them.
Nervous system

In chondrichthyans, the nervous system is composed of a small brain, 8–10 pairs of cranial nerves, and a spinal cord with spinal nerves. They have several sensory organs which provide information to be processed.
Ampullae of Lorenzini are a network of small jelly filled pores called
electroreceptors which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The
lateral line system has modified epithelial cells located externally which sense motion, vibration, and pressure in the water around them. Most species have large well-developed eyes. Also, they have very powerful nostrils and
olfactory organs. Their inner ears consist of 3 large
semicircular canals which aid in balance and orientation. Their sound detecting apparatus has limited range and is typically more powerful at lower frequencies. Some species have
electric organs which can be used for defense and predation. They have relatively simple brains with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development.
Immune system
Like all other jawed vertebrates, members of Chondrichthyes have an
adaptive immune system
The adaptive immune system (AIS), also known as the acquired immune system, or specific immune system is a subsystem of the immune system that is composed of specialized cells, organs, and processes that eliminate pathogens specifically. The ac ...
.
Reproduction
Fertilization is internal. Development is usually live birth (
ovoviviparous species) but can be through eggs (
oviparous
Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...
). Some rare species are
viviparous. There is no parental care after birth; however, some chondrichthyans do guard their eggs.
Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks/rays when fished.
Capture-induced parturition is often mistaken for natural birth by recreational fishers and is rarely considered in commercial fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date).
Classification
The class Chondrichthyes has two subclasses: the subclass
Elasmobranchii (
shark
Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s,
rays, skates, and sawfish) and the subclass
Holocephali
Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...
(
chimaeras). To see the
full list of the species, click
here.
Evolution
Cartilaginous fish are considered to have evolved from
acanthodians. The discovery of ''
Entelognathus'' and several examinations of acanthodian characteristics indicate that bony fish evolved directly from placoderm like ancestors, while acanthodians represent a paraphyletic assemblage leading to Chondrichthyes. Some characteristics previously thought to be exclusive to acanthodians are also present in basal cartilaginous fish. In particular, new phylogenetic studies find cartilaginous fish to be well nested among acanthodians, with ''
Doliodus'' and ''
Tamiobatis'' being the closest relatives to Chondrichthyes. Recent studies vindicate this, as ''
Doliodus'' had a mosaic of chondrichthyan and acanthodian traits. Dating back to the Middle and Late
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 ...
Period, many isolated scales, made of
dentine and bone, have a structure and growth form that is chondrichthyan-like. They may be the remains of
stem-chondrichthyans, but their classification remains uncertain.
The earliest unequivocal fossils of acanthodian-grade cartilaginous fishes are ''
Qianodus'' and ''
Fanjingshania'' from the early Silurian (
Aeronian) of
Guizhou
)
, image_skyline =
, image_caption =
, image_map = Guizhou in China (+all claims hatched).svg
, mapsize = 275px
, map_alt = Map showing the location of Guizhou Province
, map_caption = Map s ...
, China around 439 million years ago, which are also the oldest unambiguous remains of any jawed vertebrates.
''Shenacanthus vermiformis'', which lived 436 million years ago, had thoracic armour plates resembling those of placoderms.
By the start of the Early Devonian, 419 million years ago,
jawed fish
Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant taxon, extant vertebrates, including all living bony fishes (bot ...
es had divided into three distinct groups: the now extinct
placoderm
Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...
s (a paraphyletic assemblage of ancient armoured fishes), the
bony fish
Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...
es, and the clade that includes
spiny sharks and early
cartilaginous fish
Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...
. The modern bony fishes, class
Osteichthyes
Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...
, appeared in the late
Silurian
The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...
or early Devonian, about 416 million years ago. The first abundant genus of shark, ''
Cladoselache
''Cladoselache'' ("branch shark") is an Extinction, extinct genus of shark-like chondrichthyan (cartilaginous fish) from the Late Devonian (Famennian) of North America. It was similar in body shape to modern Lamnidae, lamnid sharks (such as Isuru ...
'', appeared in the oceans during the Devonian Period. The first cartilaginous fishes evolved from ''
Doliodus''-like
spiny shark ancestors.
Taxonomy
Subphylum
Vertebrata
└─Infraphylum Gnathostomata
├─
Placodermi — ''extinct'' (armored gnathostomes)
└
Eugnathostomata (true jawed vertebrates)
├─
Acanthodii (stem cartilaginous fish)
└─Chondrichthyes (true cartilaginous fish)
├─
Holocephali
Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...
(chimaeras + several extinct clades)
└
Elasmobranchii (shark and rays)
├─
Selachii (true sharks)
└─
Batoidea
Batomorphi is a Division (taxonomy), division of Chondrichthyes, cartilaginous fishes, commonly known as rays, this taxon is also known as the Order (biology), superorder Batoidea, but the 5th edition of ''Fishes of the World'' classifies it as ...
(rays and relatives)
*Note: Lines show evolutionary relationships.
See also
*
List of cartilaginous fish
*
Cartilaginous versus bony fishes
*
Largest cartilaginous fishes
*
Important Shark and Ray Areas (ISRA)
*
Threatened rays
*
Threatened sharks
*
Placodermi
References
Further reading
Taxonomy of ChondrichthyesImages of many sharks, skates and rays on Morphbank
{{Authority control
Fish classes
Pridoli first appearances
Extant Silurian first appearances