# Chondrichthyes

Chondrichthyes (/kɒnˈdrɪkθiiːz/; from Ancient Greek χόνδρος (khóndros) 'cartilage' and ἰχθύς (ikhthús) 'fish') 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 skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, 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 (sharks, rays, skates and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Extant chondrichthyans range in size from the 10 cm (3.9 in) finless sleeper ray to the over 10 m (33 ft) whale shark.

## Anatomy
### Skeleton

The skeleton is cartilaginous. The notochord is gradually replaced by a vertebral column during development, except in Holocephali, 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 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, 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 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 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 (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.

## 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; 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 (sharks, rays, skates, and sawfish) and the subclass Holocephali (chimaeras). To see the full list of the species, click here.
Subclasses of cartilaginous fishes :
| Elasmobranchii | Sharks and rays, skates, and sawfish | 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. |
| Holocephali | Chimaeras | Holocephali (complete-heads) is a subclass of which the order Chimaeriformes is the only surviving group. This group includes the rat fishes (e.g., Chimaera), rabbit-fishes (e.g., Hydrolagus) and elephant-fishes (Callorhynchus). Today, they preserve some features of elasmobranch life in Paleaozoic times, though in other respects they are aberrant. They live close to the bottom and feed on molluscs and other invertebrates. The tail is long and thin and they move by sweeping movements of the large pectoral fins. There is an erectile spine in front of the dorsal fin, sometimes poisonous. 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. The record is extensive, but most fossils are teeth, and the body forms of numerous species are not known, or at best poorly understood. |
Extant orders of cartilaginous fishes :
| Group | Order | Image | Common name | Authority | Families | Genera | Species | Species | Species | Species | Note |
| Group | Order | Image | Common name | Authority | Families | Genera | Total | | | | Note |
| Galean sharks | Carcharhiniformes | | ground sharks | Compagno, 1977 | 8 | 51 | >270 | 7 | 10 | 21 | |
| Galean sharks | Heterodontiformes | | bullhead sharks | L. S. Berg, 1940 | 1 | 1 | 9 | | | | |
| Galean sharks | Lamniformes | | mackerel sharks | L. S. Berg, 1958 | 7 +2 extinct | 10 | 16 | | | 10 | |
| Galean sharks | Orectolobiformes | | carpet sharks | Applegate, 1972 | 7 | 13 | 43 | | | 7 | |
| Squalomorph sharks | Hexanchiformes | | frilled and cow sharks | de Buen, 1926 | 2 +3 extinct | 4 +11 extinct | 7 +33 extinct | | | | |
| Squalomorph sharks | Pristiophoriformes | | sawsharks | L. S. Berg, 1958 | 1 | 2 | 6 | | | | |
| Squalomorph sharks | Squaliformes | | dogfish sharks | Goodrich, 1909 | 7 | 23 | 126 | 1 | | 6 | |
| Squalomorph sharks | Squatiniformes | | angel sharks | Buen, 1926 | 1 | 1 | 24 | 3 | 4 | 5 | |
| Rays | Myliobatiformes | | stingrays and relatives | Compagno, 1973 | 10 | 29 | 223 | 1 | 16 | 33 | |
| Rays | Rhinopristiformes | | sawfishes | | 1 | 2 | 5–7 | 5–7 | | | |
| Rays | Rajiformes | | skates and guitarfishes | L. S. Berg, 1940 | 5 | 36 | >270 | 4 | 12 | 26 | |
| Rays | Torpediniformes | | electric rays | de Buen, 1926 | 2 | 12 | 69 | 2 | | 9 | |
| Holocephali | Chimaeriformes | | chimaera | Obruchev, 1953 | 3 +2 extinct | 6 +3 extinct | 39 +17 extinct | | | | |
* Taxonomy according to Leonard Compagno, 2005 with additions from
* †Order Mongolepidiformes Karatajüte-Talimaa & Novitskaya, 1990
* †Order Omalodontiformes Turner, 1997
* †Order Coronodontiformes Zangerl, 1981
* †Order Symmoriiformes Zangerl, 1981
* Subclass Holocephali
* †Superorder Paraselachimorpha
* †Order Desmiodontiformes Zangerl, 1981
* †Order Polysentoriformes Cappetta, 1993
* †Order Orodontiformes Zangerl, 1981
* †Order Petalodontiformes Zangerl, 1981
* †Order Helodontiformes Patterson, 1965
* †Order Iniopterygiformes Zanger, 1973
* †Order Debeeriiformes Grogan & Lund, 2000
* †Order Eugeneodontiformes Zangerl, 1981
* Superorder Holocephalimorpha
* †Order Psammodontiformes* Obruchev, 1953
* †Order Copodontiformes Obručhev, 1953
* †Order Squalorajiformes
* †Order Chondrenchelyiformes Moy-Thomas, 1939
* †Order Menaspiformes
* †Order Cochliodontiformes Obručhev, 1953
* Order Chimaeriformes Berg, 1940 sensu Obručhev, 1953 (chimaeras)
* Subclass Elasmobranchii
* †Plesioselachus
* †Order Antarctilamniformes Ginter, Liao & Valenzuela-Rios, 2008
* †Order Elegestolepidiformes Andreev et al., 2016
* †Order Lugalepidida Karatajute-Talimaa, 1997
* †Order Squatinactiformes Zangerl, 1981
* †Order Protacrodontiformes Zangerl, 1981
* †Infraclass Cladoselachimorpha
* †Order Cladoselachiformes Dean, 1909
* †Infraclass Xenacanthimorpha Berg, 1940
* †Order Bransonelliformes Hampe & Ivanov, 2007
* †Order Xenacanthiformes Berg, 1940
* Infraclass Euselachii (sharks and rays)
* †Order Altholepidiformes Andreev et al., 2015
* †Order Polymerolepidiformes
* †Order Ptychodontiformes
* †Order Ctenacanthiformes Zangerl, 1981
* †Division Hybodonta
* †Order Hybodontiformes Owen, 1846
* Division Neoselachii Compagno, 1977
* Subdivision Selachii (modern sharks)
* Superorder Galeomorphi Compagno, 1977
* Order Heterodontiformes (bullhead sharks)
* Order Orectolobiformes (carpet sharks)
* Order Lamniformes (mackerel sharks)
* Order Carcharhiniformes (ground sharks)
* Superorder Squalomorphi
* Order Chlamydoselachiformes
* Order Hexanchiformes (frilled and cow sharks)
* Order Squaliformes (dogfish sharks)
* †Order Protospinaciformes
* †Order Synechodontiformes
* Order Squatiniformes (angel sharks)
* Order Pristiophoriformes (sawsharks)
* Subdivision Batoidea
* Order Torpediniformes (electric rays)
* Order Pristiformes (sawfishes)
* Order Rajiformes (skates and guitarfishes)
* Order Myliobatiformes (stingrays and relatives)
* * position uncertain

## 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 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, 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 fishes had divided into three distinct groups: the now extinct placoderms (a paraphyletic assemblage of ancient armoured fishes), the bony fishes, and the clade that includes 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. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period. The first cartilaginous fishes evolved from Doliodus-like spiny shark ancestors.
Extinct orders of cartilaginous fishes :
| Group | Order | Common name | Authority | Families | Genera | Note |
| Holocephali | †Orodontiformes | | | | | |
| Holocephali | †Petalodontiformes | Petalodonts | Zangerl, 1981 | 4 | | Members of the holocephali, some genera resembled parrot fish, but some members of the Janassidae resembled skates. |
| Holocephali | †Helodontiformes | | | | | |
| Holocephali | †Iniopterygiformes | | | | | Members of the holocephali that resembled flying fish, are often characterized by large eyes, large upturned pectoral fins, and club-like tails. |
| Holocephali | †Debeeriiformes | | | | | |
| Holocephali | †Symmoriida | Symmoriids | Zangerl, 1981 (sensu Maisey, 2007) | 4 | | Members of the holocephali, they were heavily sexually dimorphic. |
| Holocephali | †Eugeneodontida | Eugeneodonts | Eugeneodontida Zangerl, 1981 | 4 | | Members of the holocephali, they are characterized by large tooth whorls in their jaws. |
| Holocephali | †Psammodonti- formes | | | | | Position uncertain |
| Holocephali | †Copodontiformes | | | | | |
| Holocephali | †Squalorajiformes | | | | | |
| Holocephali | †Chondrenchelyi- formes | | | | | |
| Holocephali | †Menaspiformes | | | | | |
| Holocephali | †Cochliodontiformes | | | | | |
| Squalomorph sharks | †Protospinaci- formes | | | | | |
| Other | †Squatinactiformes | | Cappetta et al., 1993 | 1 | 1 | |
| Other | †Protacrodonti- formes | | | | | |
| Other | †Cladoselachi- formes | | Dean, 1894 | 1 | 2 | Holocephalans, and potential members of the symmoriida. |
| Other | †Xenacanthiformes | Xenacanths | Glikman, 1964 | 4 | | Eel-like elasmobranchs that were some of the top freshwater predators of the late Paleozoic. |
| Other | †Ctenacanthi- formes | Ctenacanths | Glikman, 1964 | 2 | | Shark-like elasmobranchs characterized by their robust heads and large dorsal fin spines. |
| Other | †Hybodontiformes | Hybodonts | Patterson, 1966 | 5 | | Shark-like elasmobranchs distinguished by their conical tooth shape, and the presence of a spine on each of their two dorsal fins. |

## Taxonomy

* Note: Lines show evolutionary relationships.