Chondrichthyes

A great white shark, an example of the Chondrichthyes.

**Chondrichthyes** is a class that contains the **cartilaginous fishes**: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone, like the Osteichtyes, or bony fishes. The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).

Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

## Anatomy

### Skeletal Anatomy

The skeletons of all chondrichthyes are cartilaginous. The notochord, which is present in the young, is gradually replaced by cartilage. Chondrichthyans also lack ribs, so if they leave water, the larger species' own body weights 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 (special tissue around the gonads, 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, 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, generally backwards, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other. These denticles help smooth the flow of water over the skin of chondrichthyans and helps to reduce drag and aid in achieving lift through water.

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

Most chondrichthyans 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 that 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 (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, distant relatives of the chondricthyans, did not have teeth at all, but had sharp bony plates in their mouths. Thus, it is unknown whether the dermal or oral teeth evolved first. Nor 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 from long ago. However, there is no evidence of this at the moment and speculation on the origin of these denticles continues. Further discoveries may yet uncover the truth of the matter, or yet deepen the mystery.

### Respiration

All chondrichthyans 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 spiracles and out through their gills. However, this is only a general rule and many species differ.

Spiracles are small holes 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), Mantas, and the thresher sharks (Alopiidae), no longer possess them.

### Immune System

All chondrichthyans possess an adaptive immune system, as with all jawed vertebrates living and extinct.

## Reproduction

Fertilization is internal. Development is usually live birth (ovoviviparous species) but can be through eggs (oviparous). Some rare species are viviparous. There is no parental care after birth among any known chondricthyan species; however, some chondrichthyans do guard their eggs.

## Classification

The class Chondrichthyes has two subclasses: the subclass Elasmobranchii (sharks, rays, skates, and sawfish) and the subclass Holocephali (chimaeras).

* Elasmobranchii- Elasmobranchii is a subclass that includes the sharks and the rays and skates. Members of the elasmobranchii have no swim bladders, five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The eyes have a tapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm. These fish are widely distributed in tropical and temperate waters. Some are also present in cold, polar waters as well.

* Holocephali *(complete-heads)* is a subclass of which the order Chimaeriformes is the only surviving group. This group includes the rat fishes (such as the *Chimaera*), rabbit-fishes ( an example being *Hydrolagus*) and elephant-fishes (*Callorhynchus*). Today, they preserve some features of elasmobranch life in Paleaozoic times, though in other respects they are aberrant. They generally live close to the bottom and feed on molluscs and other invertebrates. The tail is long, thin and they move by sweeping movements of the large pectoral fins. There is an erectile spine in front of the dorsal fin, sometimes venomous in certain genera and species. There is no stomach (that is, the gut is simplified and the 'stomach' is merged with the intestine), and the mouth is a small aperture surrounded by lips, giving the head a parrot-like appearance. The fossil record of the Holocephali starts in the Devonian period, and the record is extensive, but most fossils are teeth; the body forms of numerous species are not known, or at best poorly understood.

## Evolution

Cartilaginous fish are considered to have evolved from acanthodians. Originally assumed to be closely related to bony fish or a polyphyletic assemblage leading to both groups, 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 chondrichthyian and acanthodiian traits.

Unequivocal fossils of cartilaginous fishes first appeared in the fossil record by about 395 million years ago, during the middle Devonian. By the start of the Early Devonian, 419 mya (million years ago), jawed fishes had divided into three distinct groups: the now extinct placoderms (a paraphyletic assemblage of ancient armoured fishes), the bony fishes and the clade including spiny sharks and early cartilaginous fish. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. Cartilaginous fishes first appeared about 395 Ma, having evolved from *Doliodus*-like spiny shark ancestors. 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 Elasmobranchii diverged in the Silurian (421 million years ago) and that the sharks and rays/skates split in the Carboniferous (306 million years ago).