Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares,
scales, a heart with its chambers in series, and skeletons made of cartilage rather than
bone.
The class is divided into two subclasses: Elasmobranchii and Holocephali.
Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates,
the extant members of which all fall into Teleostomi.
Anatomy
Skeleton The skeleton is cartilaginous.
The notochord, which is present in the young, is gradually replaced by cartilage.
Chondrichthyes also lack ribs, so if they leave water, the larger species' own body
weight would crush their internal organs long before they would suffocate.
As they do not have bone marrow, red blood cells are produced in the spleen and the epigonal
organ.
They are also produced in the Leydig's organ which is only found in cartilaginous fishes,
although some do not possess it.
The subclass Holocephali, which is a very specialized group, lacks both the Leydig's
and epigonal organ.
Appendages Their tough skin is covered with dermal teeth,
also called placoid scales 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.
Another exception are the electric rays, which have a thick and flabby body, with soft, loose
skin devoid of dermal denticles and thorns.
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 pubioischiadic bars evolved.
In rays, the pectoral fins have 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 Chondrichthyes have toothlike scales called
dermal denticles or placoid scales.
Denticles provide two functions, 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 which migrated into
the mouth, but it could be the other way around as the teleost bony fish Denticeps clupeoides
has most of its head covered by dermal teeth.
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 which of the dermal or oral teeth evolved first.
Neither is it sure how many times it has happened if it turns out to be the case.
It has even been suggested that the original bony plates of all the vertebrates are gone
and that the present scales are just modified teeth, even if both teeth and the body armor
have a common origin a long time ago.
However, there is no evidence of this at the moment.
Respiratory system All Chondrichthyes breathe through five to
seven pairs of gills, 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, to extended and slit-like,
such as found on the wobbegongs.
Many larger, pelagic species such as the mackerel sharks and the thresher sharks no longer possess
them.
Immune system Like all other jawed vertebrates, members
of Chondrichthyes have an adaptive immune system.
Reproduction Fertilization is internal.
Development is usually live birth but can be through eggs.
Some rare species are viviparous.
There is no parental care after birth; however, some Chondrichthyes do guard their eggs.
Classification The class Chondrichthyes has two subclasses:
the subclass Elasmobranchii and the subclass Holocephali.
Evolution
Unequivocal fossils of cartilaginous fishes first appeared in the fossil record by about
395 million years ago, during the middle Devonian.
The radiation of elasmobranches in the chart on the right is divided into the taxa: Cladoselache,
Eugeneodontiformes, Symmoriida, Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphi,
Squaliformes and Batoidea.
By the start of the Early Devonian 419 mya, jawed fishes had divided into four distinct
clades: the placoderms and spiny sharks, both of which are now extinct, and the cartilaginous
and bony fishes, both of which are still extant.
The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian,
about 416 million years ago.
Spiny sharks are not classified as true sharks or as cartilaginous fishes, but as a distinct
group, class Acanthodii.
However, both the cartilaginous and bony fishes may have arisen from either the placoderms
or the spiny sharks.
Cartilaginous fishes first appeared about 395 Ma.
The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian
Period.
A Bayesian analysis of molecular data suggests that the Holocephali and Elasmoblanchii diverged
in the Silurian and that the sharks and rays/skates split in the Carboniferous.
Phylogeny
Subphylum Vertebrata └─Infraphylum Gnathostomata
├─Class Placodermi — extinct └Microphylum Eugnathostomata
├─Class Chondrichthyes └─(unranked) Teleostomi
├─Class Acanthodii — extinct └Superclass Osteichthyes
├─Class Actinopterygii └─Class Sarcopterygii
Note: lines show evolutionary relationships.
See also Cartilaginous versus bony fishes
Largest cartilaginous fishes Threatened rays
Threatened sharks References
Citations
Bibliography Further reading
Taxonomy of Chondrichthyes Images of many sharks, skates and rays on
Morphbank