/kon drik"thee euhn/, n.
any member of the class Chondrichthyes, comprising the cartilaginous fishes.
[ < NL, equiv. to chondr- CHONDR- + Gk ichthýes (pl.) fish]

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▪ fish class
class name  Chondrichthyes,  or  Selachii 
  any member of the group of cartilaginous fishes that includes the sharks (shark), skates (skate), chimaeras, and rays (ray). The class is one of the two great groups of living fishes, the other being the osteichthians, or bony fishes. The term Selachii is also sometimes used as the name for the order of sharks.

      Many structural, physiological, biochemical, and behavioral peculiarities make these fishes of particular interest to scientists. The dissection of a small shark is the biology student's introduction to vertebrate anatomy. These fishes are, in a sense, living fossils, for many of the living sharks and rays are assigned to the same genera as species that swam the Cretaceous seas over 100,000,000 years ago. Although by any reckoning a successful group, the modern chondrichthians number far fewer species than the more advanced bony fishes, or teleosts; 200 to 250 species of sharks and 300 to 340 species of rays are known.

      The danger some sharks and stingrays (stingray) present to humans makes these animals fascinating and, at the same time, abhorrent. Perhaps for this reason, they figure prominently in the folklore and art of many tropical peoples whose living depends on the sea. The danger from shark attack, while very real, is easily sensationalized, and quite frequently little attempt is made to distinguish between dangerous and harmless species.

General features

Problems of taxonomy
      The name Selachii refers to a category of fishlike vertebrates, which are given a variety of treatments by ichthyologists. Some consider the Selachii to be a class or subclass comprising all the modern sharks and rays; others restrict the name to an order limited to the modern sharks and certain extinct ancestral forms. Under the latter system, the rays (including the sawfishes (sawfish), guitarfishes, electric rays, skates, and stingrays) are ranked as a separate order, and the two orders are placed in a class or subclass.

      The chimaeras (chimaera) (Holocephali) bear many similarities to sharks and rays; e.g., in skeletal structure, internal organs, and physiology. Ichthyologists commonly, although not unanimously, emphasize these similarities by grouping the modern and ancient sharks, rays, and chimaeras in the class Chondrichthyes, the cartilaginous fishes. Under this system, which is used in the present article, the sharks, skates, and rays are further grouped into one subclass, Elasmobranchii, and the chimaeras into another, Holocephali. (A classification in which the elasmobranchs constitute one class [Selachii] and the chimaeras another [Helocephali] is found in fish: Classification (fish).) Assigning the two groups class rank implies a degree of distinctness equal to that of the amphibians (Amphibia), reptiles (Reptilia), birds (Aves), and mammals (Mammalia).

Distribution and abundance
      The majority of sharks and rays are marine fishes, but many enter estuaries; some travel far up rivers, and a few are reported to be permanent residents of freshwater. Most species live in the relatively shallow waters of continental margins or around offshore islands; a few roam far out in the vast spaces of the oceans. Some live at great depths, in midwaters or on the bottom; others are surface swimmers or inhabit the bottom in shallow waters.

      Sharks and rays are poorly represented in fish markets of most countries. With limited demand for them, the damage they do to ordinary fishing gear, and the special care required to keep them marketable, fishermen avoid them if possible, or even discard those they happen to catch rather than bring them to port. Consequently, as a source of animal protein sharks and rays are generally underexploited, while the more highly valued bony fishes are generally overexploited. A possible consequence of this may be an increasing prominence of sharks and rays in the marine biota.


Economic uses of elasmobranchs
Sharks as food
      The meat of sharks is marketed for food in all maritime countries. It may be prepared in various ways—fresh, salted, smoked, or pickled—offered in such forms as steaks, fillets, or flakes and under such names as shark, whitefish, grayfish, swordfish, sea bass, and halibut. The flesh is often rather strong tasting; this quality, however, is one that can be removed by cleaning and washing and soaking the flesh in brine.

      Since ancient times, Chinese (China) people have used the dorsal fins of certain sharks and rays as the basis of an epicurean soup. To meet the demand for this product, they have imported fins from far-distant countries. The fins are prepared for market by removing the skin and flesh, leaving only the gelatin-rich cartilaginous rays, which are dried before shipment. Shark liver oil is used in various regions for tanning leather; for preserving wood; as a lubricant; as a folk medicine against rheumatism, burns, and coughs; as a general tonic; as a laxative; and as an ingredient of cosmetics. The liver of a basking shark yields 80 to 600 gallons of oil, which was used in lamps until petroleum products replaced animal oils for illumination. The discovery around 1940 that the liver of the soupfin shark of California is peculiarly rich in vitamin A led to an explosive development of a special fishery in California for this species and a search in other parts of the world for sharks having livers of comparable potency. Within a few years, however, the economic bubble burst, with the invention of a method for manufacturing synthetic vitamin A. The Australian school shark, which was used originally for vitamin A, is now caught for fish fillets.

Other shark products
      The hard scales provide an abrasive surface to the skin of sharks and some rays, giving it a special value, as a leather called shagreen, for polishing hard wood. When heated and polished, shagreen is used for decorating ornaments and, in Japan, for covering sword hilts.

      Shark leather is made in several countries, including the United States, from the skin of certain shark species after removal of the scales by a chemical process. A luxury product, much more durable than cowhide, shark leather is used for footwear, belts, wallets, and other accessories. The most suitable skins for leather are from tiger, dusky, brown, sand, blacktip, and nurse sharks.

      In Greenland some Eskimos make rope from strips of the skin of the sleeper shark. Polynesians once added to the effectiveness of their war clubs with sharks' teeth. Sharks' teeth have some commercial value as curios. The Maori of New Zealand formerly paid high prices for mako sharks' teeth, which they wore as earrings.

Economic value of rays
      About 126,000 tons (roughly 110,000,000 kilograms) of rays are marketed for food in various countries about the world, principally in Europe and Asia. By-products in local demand are skins of scaleless species for drumheads; those of scaly species are used for shagreen. Livers are used for oil, fins for gelatin. People of many tropical regions—Polynesia, Oceania, Malaysia, Central America, and Africa—have used the spines of stingrays for such items as needles and awls, spear tips and daggers, and for the poison they contain. The entire tails of stingrays, complete with spines, have been used as whips in various tropical areas.

      The electric rays (electric ray), or numbfish, have little commercial value. The ancient Greeks and Romans used the electric shock of Torpedo to relieve diseases of the spleen, chronic headaches, and gout. From the Greek word for electric ray, narke, comes the word narcotic. Today these fishes are of interest chiefly as a source of irritation (if not danger) to bathers who step on them and to fishermen who may be shocked when hauling in their wet nets.

Lionel A. Walford

Danger to human life
      Among the known shark species, 27 have been authoritatively implicated in attacks on persons or boats. Hospital and other records attest to many attacks on bathers, divers, and people awash in the sea following sea or air disasters. There are also many documented cases of sharks attacking small boats. A number of surviving victims have been able to identify the attacking animal as a shark; a few even reported the type of shark, such as a hammerhead. In many instances, witnesses have seen the assailant clearly enough to determine the species. Fragments of teeth left in wounds of victims or in the planking of boats have often been large enough to provide ichthyologists with the means for precise identification.

      In 1958 the American Institute of Biological Sciences established a Shark Research Panel at the Smithsonian Institution and Cornell University to gather historical and current records of shark attacks throughout the world. For the 35 years from 1928 to 1962, inclusive, the panel listed 670 attacks on persons and 102 on boats. Attacks occur most frequently throughout the year in the tropical zone between 21° north and south of the Equator; from midspring to midfall they extend as far north and south as the 42° parallels. For this reason, it was formerly believed that the most dangerous sharks lived in waters warmer than 21° C (70° F) and that the risk of attack was greatest in the tropics and in the summer months. It is now thought that this circumstance simply results from the fact that more people swim in warm water. It is known, for example, that the most dangerous shark, the white shark, or man-eater (Carcharodon carcharias), ranges into the cooler waters of both hemispheres.

Lionel A. Walford Ed.
      In Australia, New Zealand, South Africa, and along other coasts heavily infested with sharks, public beaches have lookout towers, bells or sirens, and nets to protect bathers. Since 1937 Australia has used meshing offshore to catch the sharks. Gill nets suspended between buoys and anchors running parallel to the beach and beyond the breaker line have decreased the danger of attack. The nets enmesh sharks from any direction, and although they touch neither the surface nor the bottom, and are spaced well apart, they provide effective control. South Africa has used a similar protection system and has also conducted experiments with electrical barriers.

      The 27 species implicated by the Shark Research Panel in attacks on persons or boats are mostly large sharks with large, cutting teeth. Size, however, is not a dependable criterion, for man-eaters become dangerous when they are about one metre (three or four feet) long; and the largest ones, the basking shark and the whale shark, which grow to 12 and 18 metres (40 and 60 feet), respectively, subsist on minute planktonic organisms and on small schooling fishes. Although either might attack a boat if provoked, only two records of such occurrences have been reported, both in Scotland and both identified with the basking shark. More than 85 percent of all the shark species are too small, too unsuitably toothed, or too sluggish or live at depths too great to be potentially dangerous. The most dangerous sharks include in addition to the white shark, the hammerheads (Sphyrna), tiger (Galeocerdo), blue (Prionace), and sand sharks (Odontaspis).

      Most stingrays live in shallow coastal waters. Some move with the tides to and from beaches, mud flats, or sand flats. Anyone wading in shallow water where these fishes occur runs some risk of stepping on one and provoking an instant response—the ray lashes back its tail, inflicting an agonizingly painful wound that occasionally leads to fatal complications. Rays can be serious pests to shellfisheries, for they are extremely destructive to oyster and clam beds.

Natural history

Food habits
      All sharks are carnivorous and, with a few exceptions, have broad feeding preferences, governed largely by the size and availability of the prey. The recorded food of the tiger shark (Galeocerdo cuvieri), for example, includes a wide variety of fishes (including other sharks, skates, and stingrays), sea turtles, birds, sea lions, crustaceans, squid, and even carrion such as dead dogs and garbage thrown from ships. Sleeper sharks (Somniosus), which occur mainly in polar and subpolar regions, are known to feed on fishes, small whales, squid, crabs, seals, and carrion from whaling stations. Many bottom-dwelling sharks, such as the smooth dogfishes (Triakis and Mustelus), take crabs, lobsters, and other crustaceans, as well as small fishes.

      The two giant sharks, the whale shark (Rhincodon typus) and basking shark (Cetorhinus maximus), resemble the baleen whales in feeding mode as well as in size. They feed exclusively or chiefly on minute passively drifting organisms (plankton). To remove these from the water and concentrate them, each of these species is equipped with a special straining apparatus analogous to baleen in whales. The basking shark has modified gill rakers, the whale shark elaborate spongy tissue supported by the gill arches. The whale shark also eats small, schooling fishes.

      The saw sharks (saw shark) (Pristiophoridae) and sawfishes (sawfish) (Pristidae) share a specialized mode of feeding that depends on the use of the long, bladelike snout, or “saw.” Equipped with sharp teeth on its sides, the saw is slashed from side to side, impaling, stunning, or cutting the prey fish. Saw sharks live in midwaters; sawfishes, like most other rays, are bottom inhabitants.

      Thresher sharks (thresher shark) (Alopias) feed on open-water schooling fishes, such as mackerel, herring, and bonito, and on squid. The long upper lobe of the tail, which may be half the total length of the shark, is used to frighten the fish (sometimes by flailing the water surface) into a concentrated mass convenient for slaughter.

      Most sharks and probably most rays segregate according to size, a habit that protects smaller individuals from predation by larger ones. Even among sharks of a size category, dominance between species is apparent in feeding competition, suggesting a definite nipping order. Other sharks keep clear of hammerheads (hammerhead shark) (Sphyrna), whose manoeuvrability, enhanced by the rudder effect of the head, gives them an advantage. When potential prey is discovered, sharks circle it, appearing seemingly out of nowhere and frequently approaching from below. Feeding behaviour is stimulated by numbers and rapid swimming, when three or more sharks appear in the presence of food. Activity soon progresses from tight circling to rapid crisscross passes. Biting habits vary with feeding methods and dentition. Sharks with teeth adapted for shearing and sawing are aided in biting by body motions that include rotation of the whole body, twisting movements of the head, and rapid vibrations of the head. As the shark comes into position, the jaws are protruded, erecting and locking the teeth into position. The bite is extremely powerful; a mako shark (Isurus), when attacking a swordfish too large to be swallowed whole, may remove the prey's tail with one bite. Under strong feeding stimuli, the sharks' excitement may intensify into what is termed a feeding frenzy, in which not only the prey but also injured members of the feeding pack are devoured, regardless of size.

      In most cases the initial attraction to the food is by smell. Laboratory studies have shown that sharks do not experience hunger in the normal sense of the word, and they are much more prone to be stimulated to feeding by the olfactory or visual cues announcing the appearance of prey.

      The majority of batoid fishes (members of the order Batoidei; i.e., rays (ray) and allies) are bottom dwellers, preying on other animals on or near the sea floor. Guitarfishes (guitarfish) (Rhynchobatidae and Rhinobatidae), butterfly rays (Gymnuridae), eagle rays (Mylobatidae), and cow-nosed rays (Rhinopteridae) feed on invertebrates, principally mollusks and crustaceans. Whip-tailed rays (Dasyatidae) use their broad pectoral fins to dig shellfish from sand or mud. Skates (Rajidae) lie on the bottom, often partially buried, and rise in pursuit of such active prey as herring, trapping the victims by swimming over and then settling upon them, a practice facilitated by the skates' habit of hunting at night.

      Electric rays (electric ray) (Torpedinidae) are characteristically bottom fishes of sluggish habits. They feed on invertebrates and fish, which may be stunned by shocks produced from the formidable electric organs. With their electricity and widely extensible jaws, these rays are capable of taking very active fishes, such as flounder, eel, salmon, and dogfish. Shallow-water electric rays have been observed to trap fishes by suddenly raising the front of the body disk, while keeping the margins down, thereby forming a cavity into which the prey is drawn by the powerful inrush of water.

      Most of the myliobatoid rays (eagle ray) (seven recognized families of the suborder Myliobatoidea, which includes all of the typical rays) swim gracefully, with undulations of the broad, winglike pectoral fins. Some species, especially the eagle rays, frequently swim near the surface and even jump clear of the water, skimming a short distance through the air.

      Manta, or devil, rays (Mobulidae (manta ray)) swim mostly at or near the surface, progressing by flapping motions of the pectoral fins. Even the largest often leap clear of the water. In feeding, a manta moves through masses of macroplankton or schools of small fish, turning slowly from side to side and using the prominent cephalic fins, which project forward on each side of the mouth, to fan the prey into the broad mouth.

      Chimaeras and ghost sharks (Chimaeridae) dwell near the bottom in coastal and deep waters, to depths of at least 2,500 metres (about 8,000 feet). They are active at night, feeding almost exclusively on small invertebrates and fishes.

Reproductive behaviour
      Mature individuals of some species of sharks segregate by sex, coming together only during the mating season, when the males, at least those of the larger, more aggressive species, stop feeding. Segregation is a behavioral adaptation to protect the females, one principal courting activity used by the male to induce cooperation of the female in mating being that of slashing her with teeth especially developed for that purpose. After mating, the sexes again separate. The pregnant females also tend to keep apart from the other females of like size. As the time of parturition approaches, the pregnant females move to particular areas, which presumably have properties of environment especially suitable as nursery grounds. When giving birth to their young, they stop feeding, and, soon after parturition is completed, they depart.

      Nursery areas vary with species. Some sharks—e.g., the bull and sandbar sharks—use shallow waters of bays and estuaries; the silky shark uses the bottom far out on oceanic banks such as the Serrana Bank in the western Caribbean. The Atlantic spiny dogfish (Squalus acanthias) bears its young mostly during the winter far out on the continental shelf of northeastern America almost two years after mating.

      A few skates that have been observed mating may be characteristic of other rays. The male seizes the female by biting the pectoral fin and presses his ventral surface against hers while inserting one, or in some species, both claspers into her cloaca. Male skates have one to five rows of clawlike spines on the dorsal side of each pectoral fin. These are retractile in grooves of the skin and are used to hold the female during mating.

      The eggs of skates in aquaria have been observed to be extruded in series, usually of two but sometimes one, with rests of one to five days between extrusions. A female of a European skate, Raja brachyura, laid 25 eggs over a 49-day period in the aquarium located at Plymouth, England.

      Although the mating of chimaeroids has not been observed, it is generally presumed that the mode of copulation is similar to that of sharks and that the male's frontal spine and anterior appendage of the pelvic fins are probably used in securing the female. Two eggs are laid simultaneously, one from each oviduct. They are often carried for a relatively long period before being laid, several hours or even days, each protruding for the greater part of its length.

Form and function

Distinguishing features
      The elasmobranchs are fishlike vertebrates differing from bony fishes in many respects. The skeleton is composed of cartilage and, although partly calcified (especially in the vertebrae), lacks true bone. There are five to seven fully developed gill clefts, opening separately to the exterior. Most sharks and all rays have an opening behind each eye, called a spiracle, which is a modified first gill cleft. The dorsal fin or fins and fin spines are rigid, not erectile. Scales (scale), if present, are structurally minute teeth, called dermal denticles, each consisting of a hollow cone of dentine surrounding a pulp cavity and covered externally by a layer of hard enamel-like substances called vitrodentine. The scales covering the skin do not grow throughout life as they do in bony fishes, but have a limited size; new scales form between existing ones as the body grows. Certain other structures, such as the teeth edging the rostrum (beak) of sawfishes and saw sharks, the stinging spines of sting rays, and the teeth in the mouth, are structurally modified scales. The teeth, arranged in rows in the mouth, are not firmly attached to the jaws but are imbedded in a fibrous membrane lying over the jaws. When a tooth becomes broken, worn, or lost, it is replaced by one moving forward from the next row behind; at the base of the innermost row are rudimentary teeth and tooth buds that develop and move forward as needed. A spiral membranous fold (spiral valve) extends through the intestine of all sharks, rays, and chimaeras.

      The rays differ externally from sharks in having the gill openings confined to the lower surface; the eyes of the rays are on the dorsal surface, and the edges of the pectoral fins are attached to the sides of the head in front of the gill openings. Some rays lack scales, and others are variously armed with thorns, tubercles, or prickles, all of which are modified scales; the tails of some have long, saw-toothed spines equipped with poison glands. In the sawfishes the snout is prolonged into a long, flat blade armed on either side with teeth. Some skates and a few rays have electric organs by which they can administer electric shocks to enemies or prey.

      The chimaeras have only one external gill opening. In the adult the skin on each side of the head is smooth and lacks scales; the teeth consist of six pairs of grinding plates. The dorsal fin and spine are erectile. Like male sharks and rays, male chimaeras have claspers that serve to transfer sperm to the female, but, in addition, they have an erectile clasping device, the tantaculum, in front of each pelvic fin; most species have another such organ on top of the head.

      Although sharks are often said to have a low order of intelligence, they, as well as rays and chimaeras, have survived successfully over a long period of geologic time. They are well equipped to locate prey and their own kind; to direct the course of their seasonal migrations; to discriminate specific localities; to respond to variations of temperature; to react to attractive or repelling substances in the water; and perhaps even to feel objects some distance away from them. They can see, hear, smell, taste, feel, and maintain their equilibrium. The roles of the sense organs have been studied in only a few species, principally sharks, and consequently remain imperfectly understood.

      The sense of smell is highly developed and probably the principal means of locating prey and guiding the predator toward it. Given a favorable direction of current, sharks can detect incredibly minute concentrations—fractions of a part per million (i.e., less than 1 × 10-6 parts)—of certain substances in the water, such as blood.

      Although their eyes are structurally and functionally adapted for seeing, it is believed that their visual acuity in discerning the form and colour of an object is limited. The importance of sight relative to smell increases as a shark approaches its target.

      The hearing (sound reception) apparatus, located in the auditory capsule of the cranium, includes a system of semicircular canals, which are responsible for maintaining equilibrium. Sharks seem to be remarkably sensitive to sounds of low frequency and to possess extraordinary faculty for directional hearing. Whether or not hearing is more sensitive than smell has not yet been established.

      Sensory organs identified as taste buds are located on the floor, sides, and roof of the mouth and on the throat, as well as on the tongue. Experiments on several species of large sharks indicate that they do discriminate food types, preferring tunas, for example, to other fish species. Under some conditions, however, they become less fastidious, going into a feeding frenzy in which they attack anything, including others of their own kind.

      Sensory organs located in the skin of all sharks, rays, and chimaeras receive a variety of information—vibrations of low frequencies, temperature, salinity, pressure, and minute electrical stimuli, such as are produced by another fish in the vicinity. These organs are located in the lateral line system (a series of sensory pores along the side), in groups of pores on the head (ampullar organs), and in pit organs distributed on the back, flanks, and about the jaws.

Salt and water balance
      Most marine vertbrates maintain lower concentrations of salts and other chemicals in their blood than are found in seawater, and so face a continuous problem of water loss to the environment, because of the tendency of water to move through membranes from regions of low salt concentration to regions of higher concentration. The marine cartilaginous fishes differ from almost all of the bony fishes (except the coelacanths and aestivating lungfishes) in being able to reabsorb in the renal (kidney) tubules most of their nitrogenous waste products (urea and trimethylamine oxide) and to accumulate these products in their tissues and blood, an ability termed the urea retention habitus. The concentration within the body thus exceeds that of the surrounding seawater, and water moves into the body with no expenditure of energy. When any of these fishes moves into freshwater, as many do, the urine flow to the outside increases; hence, the concentration of urea in the blood decreases. In the sawfish, for example, the increase of urine output is more than twentyfold; the blood urea concentration decreases to less than one-third the amount observed in marine forms. Purely freshwater elasmobranchs, such as the stingrays of the Orinoco and Amazon drainage systems, seem to lack the urea retention habitus.

      Sharks with spiracles take in some water through them, but they breathe chiefly by opening the mouth while expanding the mouth-throat (bucco-pharyngeal) cavity and contracting the gill pouches to close the gill slits. With the mouth closed, they contract the bucco-pharyngeal cavity while dilating the gill pouches, thus drawing the water over the gills where the exchange of oxygen and carbon dioxide takes place. Then, with the mouth still closed, they contract the bucco-pharyngeal cavity and gill pouches and open the gill slits to expel the water. Most of the rays, on the other hand, take in water chiefly through the spiracles; these then close by contraction at their anterior margins, which bear rudimentary gill filaments and a spiracular valve. Folds of membrane on the roof and floor of the mouth prevent the water from passing down the throat and direct it to the gill openings. Skates, which usually hold the lower surface of the head slightly above the bottom, inhale some water through the mouth; mantas, which have small spiracles and live near the surface, respire chiefly through the mouth. Skates, stingrays, guitarfishes, and angel sharks frequently reverse the direction of flow through the spiracles, apparently to clear them of foreign matter. Chimaeras take in water chiefly through the nostrils, keeping the mouth closed for the most part. The water reaches the mouth primarily through grooves leading there from the nostrils.

Reproduction and development
      All species of sharks, rays, and chimaeras produce large, yolk-rich eggs (egg). These are fertilized internally, for which the males are equipped with two copulatory organs called claspers along the inner edges of the pelvic fins. Each clasper has a groove for guidance of sperm. The few published descriptions of mating sharks and rays are probably characteristic of the entire group. The male grasps one of the female's pectoral fins with his teeth to hold her in position as he inserts a clasper through a cavity (cloaca) and into a tube (oviduct). Males of most species probably use only one clasper at a time. The sperm travel to the anterior end of the oviduct, where they fertilize the eggs. The eggs then move down the oviduct past the shell gland, where they are covered by a shell or capsule.

      In oviparous (egg-laying) species, which include some of the sharks, probably all the skates, possibly some of the guitarfishes, and all of the chimaeras, the eggs are enveloped in a horny shell, usually equipped with tendrils for coiling around solid objects or with spikelike projections for anchoring in mud or sand. The egg cases of most species are more or less pillow-shaped; those of the horned sharks (Heterodontidae) are screw-shaped with a spiral flange. The eggs of chimaeras are elliptic, spindle-shaped, or tadpole-shaped and open to the exterior through pores and slits that permit entrance of water during incubation. An egg of the whale shark found in the Gulf of Mexico measured 30 centimetres (12 inches) long by about 14 centimetres (51/2 inches) wide and was eight centimetres (three inches) thick. Protected by the shell and nourished by the abundant yolk, the embryo of an oviparous species develops for 41/2 to 143/4 months before hatching.

      The majority of sharks and most, possibly all, rays other than the skates are ovoviviparous (i.e., the egg hatches within the mother). In this case, the egg is first coated in the shell gland with a temporary membranous capsule that lasts only during early development. After emerging from its capsule, the embryo remains in the oviduct of the mother, nourished by the yolk sac to which it remains attached. Embryos of some ovoviviparous sharks, notably the porbeagle (Lamna nasus), mako (Isurus oxyrinchus), and sand shark (Odontaspis taurus), ingest yolks of other eggs and even other embryos within the oviduct of the mother after the contents of their own yolk sacs are exhausted. In the majority of ovoviviparous sharks and rays, organically rich uterine secretions provide supplemental nourishment, which is absorbed by the yolk sac and in many cases by appendages borne on its stalk. In some genera of rays, vascular filaments producing these secretions extend through the spiracles and into the digestive tract of the embryos.

      Several shark species are viviparous—i.e., the yolk sac develops folds and projections that interdigitate with corresponding folds of the uterine wall, thus forming a yolk placenta through which nutrient material is passed from the mother.

      Growth of a few shark species has been measured or estimated by the differences in length at the times of tagging and recapturing specimens, by statistical analysis of length in systematically collected samples, by the space between concentric circles on the centra of the vertebrae, and by periodic measurements of specimens kept in aquariums. All studies indicate a slow growth rate. During the 10 years between birth and maturity, male Atlantic spiny dogfish grow an average of 47 centimetres (19 inches) and females 67 centimetres (26 inches). The Greenland shark (Somniosus microcephalus), which attains 61/2 metres (21 feet) or more (although rarely taken larger than about four metres [13 feet]), grows only about 71/2 millimetres (a little more than 1/4 inch) per year. The annual growth increments of tagged juvenile whitetip reef and Galápagos sharks, both species that become at least 21/2 metres (eight feet) long, were found to be 31 to 54 millimetres (just over one to two inches) and 41 millimetres (about 11/2 inches), respectively. The Australian school shark (Galeorhinus australis) grows about 80 millimetres (three inches) in its first year and 30 millimetres (one inch) in its 12th year. By its 22nd year, it is estimated to be approaching a maximum length of 160 centimetres (just over five feet).

      The disk of the eastern Pacific round stingray (Urolophus halleri) increases in width on the average from 75 millimetres (three inches) at birth to 150 millimetres (six inches) when mature, when 2.6 years old. In the next five years it grows about 60 millimetres (about 23/8 inches) more toward its maximum recorded width of 25 centimetres (10 inches) in males or 31 centimetres (121/4 inches) in females. The males of European thornback rays (Raja clavata) are about 50 centimetres (20 inches) wide when they reach first maturity, about seven years after birth; females are 60 to 70 centimetres (24 to 28 inches) at first maturity, nine years after birth.

Evolution and classification

      The earliest fossil remains of fishlike vertebrates are too fragmentary to permit tracing the modern fishes precisely to their origins. It is believed that the ancestral forms evolved during the Silurian Period (from about 430,000,000 to 395,000,000 years ago) in the upper reaches of streams. During the end of the Silurian and the beginning of the Devonian that followed, there appeared an exceedingly diverse group of armour-plated fishes with jawlike structures, paired fins, and bony skeletal tissue. Paleontologists refer to these extinct forms as a distinct class, Placodermi. Between the beginning and end of the Devonian (the latter about 350,000,000 years ago), the placoderms reached their peak in diversity and numbers and almost completely died out; only a few lingered another 10,000,000 years into the Early Mississippian (roughly, the Lower Carboniferous). During their flowering, the placoderms evidently gave rise to the Osteichthyes (the bony fishes) and the Chondrichthyes (the cartilaginous fishes). Even though the lines of evolution remain to be discovered, it seems quite clear that the two groups evolved independently, the Chondrichthyes appearing much later than the Osteichthyes.

      Although a few sharklike forms remained in fresh water, the vast majority soon invaded the sea, perhaps in response to the arid Devonian climate. There they adapted to life in salt water by evolving the urea retention habitus (see above Salt and water balance (chondrichthian)). Their cartilaginous skeleton, far from representing an evolutionary stage antecedent to the Osteichthyes, as was once believed, is more than likely degenerate rather than primitive. Possibly their precursors were the petalichthyids, a group of Devonian sharklike placoderm fishes that had ossified skeletons and well-developed fins.

      The phyletic relationship of the chimaeras and the sharks and rays is a subject capable of varying interpretation. Although both groups have many characteristics in common, such as the possession of a cartilaginous skeleton, placoid scales, teeth simply embedded in gums, a spiral valve in the intestine, urea retention habitus, internal fertilization (for which the males have claspers), and the absence of a swim bladder, the two groups may have evolved independently along parallel lines, the chimaeras from the pyctodonts, an order of Devonian placoderms with body form and tooth structure very suggestive of modern chimaeras.

      The first fishes clearly identified with the Chondrichthyes were sharklike in form. One order, the Pleurocanthodii, consisting of one family of freshwater, sharklike fishes, appeared in the Late Devonian, was abundant in the Carboniferous and Early Permian (until about 250,000,000 years ago), and disappeared during the Triassic Period, which followed. These fishes were characterized by the following features: the skeletal structure of both pectoral and pelvic fins had an axis with side branches (called the archipterygial type); the tail was almost symmetrical, being only slightly tilted upward; a long movable spine projected backward from the back of the head; the teeth had two divergent prongs and a central cusp set on a buttonlike base; the anal fin was two-lobed; and the males had claspers.

      The other order, Cladoselachii, consisted of marine fishes known only from fossils of the late Middle Devonian, Carboniferous, and Early Permian periods. Their distinguishing characteristics were that each tooth had a long base composed of a bonelike tissue, from which rose three conical cusps, a tall central one and two smaller ones on either side; the body scales had several lobes or cusps; the jaws had double articulation, extending forward to the snout; claspers were lacking; the outline of the caudal (tail) fin was almost symmetrical but with differing internal structure of the upper and the lower lobes.

      The cladoselachians were probably ancestral to a group closer to modern sharks, the order Hybodontii. They probably represent an intermediate state in selachian evolution and are classified by some authorities in the order Selachii. Although the jaws had the primitive double articulation, the skeletal support of the pectoral and pelvic fins was close to that of modern selachians, with basal elements projecting outward into the fins. The teeth near the front of the mouth were generally sharp-cusped; the cusps of those further back were sometimes reduced to a rounded crown. The front teeth were suitable for seizing prey; those in the back were suitable for crushing mollusks. The hybodonts appeared toward the end of the Devonian, flourished in the Late Paleozoic, and died out during the latter half of the Mesozoic, a few lasting into the Late Cretaceous (about 80,000,000 years ago).

      The great period of radiation (diversification) in marine vertebrates characterizing the Mesozoic ended in the Permian, and the chondrichthyed fishes, which had reached their greatest flowering during the Carboniferous, became greatly reduced, remaining so until the Jurassic (about 190,000,000 years ago), when the areas of the seas expanded and those of the land diminished. Then the six-gilled shark (Hexanchus), horned shark (Heterodontus), and guitarfishes appeared. By the end of the Cretaceous, most of the families and many genera of modern sharks, as well as those of skates and rays, were represented. The evolution of elasmobranch fishes, much as they are known today, had been accomplished.

Annotated classification
      The most recent approaches to a comprehensive review of the chondrichthyeds are that of the American ichthyologists H.B. Bigelow and W.C. Schroeder and that by the American paleontologist Alfred S. Romer. The following synopsis, based on their work, provides principal identifying characteristics of all major Recent groups.

Class Chondrichthyes (or Selachii)
      Subclass Elasmobranchii (sharks and rays)
 Chondrichthyeds with 5–7 pairs of gill clefts not covered by a fold of skin, opening separately to the exterior.

      Order Selachii (sharks)
 Elasmobranchs with gill clefts opening at least partly on the side of the body.

      Suborder Notidanoidei
 Sharks having 6 or 7 gill openings. Anal fin present.

      Family Hexanchidae (cow shark and 7-gilled sharks)
 Lower Jurassic to present; marine. The cow shark (Hexanchus griseus), in deep water, down to 1,875 m (about 6,000 ft). Distinguished by presence of 6 gill slits; teeth of lower jaw strikingly unlike those of upper, the 5 or 6 on either side of the central tooth being about twice as broad as high, their inner edges saw-toothed with 5–8 pointed cusps. Size up to at least 5 m (about 161/2 ft), estimated length at maturity about 2 m (about 61/2 ft). Ovoviviparous; 4.5-m (15-ft) specimen contained 108 embryos. The 7-gilled sharks (Heptranchias and Notorhynchus) are widely distributed in warm and temperate continental waters.

      Suborder Chlamydoselachoidei

      Family Chlamydoselachidae (frilled shark)
 Miocene to present. One modern species known, rather rare. Distinguished by 6 gill slits, the margins of the first being continuous across the throat. Size to about 2 m (about 61/2 ft). Moderately deep water of the eastern North Atlantic from Portugal to Norway and in the North Pacific off California and Japan.

      Suborder Heterodontoidei
 Upper Devonian to present. Five gill openings on each side of body; anal fin present; 2 dorsal fins, each preceded by a spine. Marine.

      Family Heterodontidae (Horned sharks, bullhead, Port Jackson shark)
 With 1 Recent genus and about 10 species. Oviparous; egg case screw-shaped, a double spiral flange extending from apex to large end. Teeth in upper and lower jaws alike, those in front incisor-like, those on sides much larger and molar-like. Bottom dwellers out to about 180 m (about 590 ft) depth. Australia, New Zealand, East Africa, East Indies, China, Japan, eastern Pacific, north as well as south. Not known in Atlantic or Mediterranean. Size up to about 1.4 m (about 41/2 ft).

      Suborder Galeoidei (typical sharks)
 Five gill openings on each side of body; anal fin present; dorsal fin or fins not preceded by spines.

      Family Odontaspididae (sand sharks)
 Formerly Carchariidae. Upper Jurassic to present. Marine. Caudal peduncle (narrow “stalk” of the tail) without lateral keels; with a distinct pit on its upper surface but none on its lower. Teeth large, slender, smooth-edged, lower eyelid without a nictitating membrane (a transparent extra eyelid). Development is ovoviviparous; maximum size varies with species, from around 2.8 to 6 m (about 9 to 20 ft). One recent genus (Odontaspis) recognized, with some 6 species, found in warm temperate and tropical coastal waters of all oceans. Frequent shallow water near shore; sluggish except when feeding.

      Family Scapanorhynchidae (goblin sharks)
 Lower Cretaceous to present. Marine. One genus, known from Japan, Portugal, and India, perhaps from Australia. Prominent elongation of the snout; protruding jaws. Maximum size to about 3.4 m (about 11 ft). Probably ovoviviparous. A deepwater shark, fished commercially in Japan for its liver and flesh.

      Family Isuridae
 Upper Cretaceous to present. Three genera, marine, although at least 1 species (the white shark) occasionally strays into estuaries. Distinguished by 2 dorsal fins, of which the first is much larger than the second and the rear end of its base situated well in advance of the pelvic fins; caudal fin lunate (crescent-shaped), its axis steeply raised. Teeth large. Ovoviviparous or viviparous. Circumglobal, occurring in boreal to warm temperate belts of all oceans in both hemispheres. Size in the great white shark (Carcharodon carcharias) varies from 1.4 to 6 m (4.6 to 19.7 ft) in length, but individuals may possibly exceed 8 m (26.2 ft). Three genera, Lamna, Isurus, and Carcharodon, the last 2 dangerous to man, the great white shark unquestionably the most dangerous of all fishes.

      Family Cetorhinidae (basking shark)
 Oligocene to present. Marine. Two dorsal fins, the first well in advance of pelvics; lunate caudal fin; gill openings extending around sides almost meeting at throat. Hundreds of minute teeth. Ovoviviparous. Embryonic development undescribed. Size at birth probably 1.5–1.8 m (5–6 ft); maximum size to 13–14 m (421/2–46 ft). Single genus (Cetorhinus) inhabiting temperate and boreal zones around the world. Whether basking sharks of the Northern and Southern hemispheres belong to a single species (C. maximus) is undetermined. Sluggish, inoffensive sharks, living at or near the surface, feeding wholly on plankton, which they sieve out of the water with their gill rakers.

      Family Alopiidae (thresher sharks)
 Eocene to present. One genus, 5 species. Distinguished by the elongated upper lobe of the tail fin, which is almost as long as the rest of the body. Teeth small, bladelike. Ovoviviparous. Total length to about 6 m (20 ft). Cosmopolitan at low and middle latitudes of all oceans. Harmless to man. Occasionally sold for food.

      Family Orectolobidae (carpet and nurse sharks, wobbegongs)
 Upper Jurassic to present. Marine. Distinguished by the presence of 2 dorsal fins, the origin of the first over or behind the pelvic fins; nostril connected with mouth by a deep groove, its anterior margin with a well-developed fleshy barbel (tentacle). Teeth small, with several cusps; development ovoviviparous in some, oviparous in others. Some species (carpet sharks) live on the bottom and are ornamented with fleshy flaps along the sides of the head. Large family of many genera and species occurring mostly in western Pacific, Australasia, Indian Ocean, Red Sea. Only 1 species, the nurse shark, in Atlantic.

      Family Rhincodontidae (whale shark)
 Distinguished from all other sharks by large, lunate tail, mouth at end of snout, 3 prominent ridges extending the length of body along the sides, back marked with round white or yellow spots and a number of white or yellow transverse stripes. Oviparous. Size said to reach over 18 m (59 ft), the largest of modern fishlike lower vertebrates. One species only (Rhincodon typus); open waters of all oceans, mostly in tropics, but north to 42° N latitude (near New York) and south to 33°55′ S (Table Bay, South Africa). Sluggish and inoffensive.

      Family Scyliorhinidae (cat sharks, European dog shark, swell sharks)
 Upper Jurassic to present. Most with 2 dorsal fins (1 genus with 1); first dorsal fin situated far back on body, at least half of it behind the origin of the pelvic fins. Furrows are more or less developed at the angle of the jaws; teeth small, numerous, with several cusps. A large group of small sharks comprising many genera, occurring in temperate to tropical latitudes. The swell sharks (Cephaloscyllium) can inflate the belly with air or water, presumably a defense mechanism. Of little, if any, commercial value; harmless to man.

      Family Pseudotriakidae (false cat sharks)
 Distinguished by the base of the first dorsal fin being at least as long as the caudal fin. Teeth minute, numerous. One genus, Pseudotriakis; 2 species, 1 on both sides of the North Atlantic, the other in the western Pacific. Size to nearly 3 m (about 10 ft). Deepwater sharks (taken down to 1,477 metres [4,850 ft]) rarely straying near shore and known only from a few specimens.

      Family Triakidae (smooth dogfishes)
 Upper Cretaceous to present. The principal distinguishing feature is small, closely crowded teeth in series, rounded or somewhat compressed and with 3 or 4 cusps. True nictitating membrane lacking in eye. Development ovoviviparous or viviparous. Small sharks of coastal waters in tropical to temperate zones of all oceans. The family comprises at least 7 genera and numerous species. Smallest species, Triakis barbour, reaches only about 40 cm (16 in.); maximum size for others of family 150–175 cm (59 to 69 in.). Although sharks of this family are generally considered harmless, there is one authenticated case of a California leopard shark (Triakis semifasciata) attacking a man in northern California.

      Family Carcharhinidae
 The largest family of sharks, with 13 genera and numerous species, including the tiger shark, the great blue, whalers, and many with various local common names. Upper Cretaceous to present. Two dorsal fins, the first in front of the pelvics. All species except 1 with well-developed nictitating membrane. Teeth bladelike, with only 1 cusp, only 1 or 2 rows functional along sides of jaws. Development either ovoviviparous or viviparous. The species range in maximum size from about 1.4–5.5 m (about 41/2 to 18 ft). Members of this family occur from tropical to temperate zones in all oceans. Although most species are marine, several frequent brackish water or freshwater, and some occur in lakes that connect with the sea. The Carcharinus leucas–gangeticus group, a collection of several closely related species or subspecies, has a bad reputation; several cases of unprovoked attacks on persons are on record in both salt water and freshwater.

      Family Sphyrinidae (hammerhead sharks)
 Upper Cretaceous to present. The most obvious distinguishing feature is the lateral expansion of the head in a hammer or bonnet form, with the eyes at the outer edges. Teeth large, triangular, smooth edged in some species, serrate in others. Viviparous or ovoviviparous; size varies with species, the largest (Sphyrna mokarran) is said to reach 6 m (about 20 ft). Predacious. Marine, but occasionally straying into estuaries. Occur in tropical and temperate zones of all seas. Hammerheads have a sinister reputation of initiating unprovoked attacks, documented by authoritative cases on record.

      Suborder Squaloidei (spiny dogfishes, bramble sharks, sleeper sharks, pygmy sharks)
 Upper Cretaceous to present. Widely distributed, found in all of the oceans from tropical to both Arctic and sub-Antarctic latitudes; from shallow to deep depths. Anal fin lacking; snout not elongated into a beak; body subcylindrical (nearly round in section); not flattened dorsoventrally; margins of pectoral fin not expanded forward past first pair of gill openings.

      Family Squalidae (spiny dogfishes, sleeper sharks and several others lacking common names)
 Upper Cretaceous to present. Distinguished by having about as many upper teeth in anterior row as in succeeding rows. Diverse forms, habits, and sizes. Spiny dogfishes (Squalus) grow to about 120 cm (471/4 in.); the Greenland sleeper shark to over 6 m (about 20 ft); a pygmy shark (Euprotomicrus) to about 26 cm (101/4 in.). Sleeper sharks (Somniosus) taken for food in waters around Iceland and west Greenland, but the fish must be dried before eating; otherwise it produces a mild poison.

      Family Oxynotidae (prickly dogfish)
 Miocene to present. Distinguished by number of functional upper teeth increasing in each row from front to rear; dermal denticles large and prominent. Taken from depths of 60–530 m (about 200 to 1,740 ft); 2 species known in eastern North Atlantic, Tasmania, and New Zealand.

      Suborder Pristiophoridei

      Family Pristiophoridae (saw sharks)
 Cretaceous to present. Anal fin lacking, snout greatly elongated, each edge studded with sharp toothlike structures; upper eyelid is free; gill slits at the side of the head, not underneath as in the sawfish. Ovoviviparous. Marine. Indo-Pacific, South Africa, Tasmania, Australia, Philippines, Korea, Japan. The order comprises 1 family, 2 genera, Pristiophorous, with 5 gill openings, and Pliotrema, with 6. Good food fish, harmless to man.

      Suborder Squatinoidei

      Family Squatinidae (angel sharks)
 Upper Jurassic to present. Marine, widely distributed in continental temperate and warm waters of Atlantic and Pacific oceans, on or close to the sea bottom. Characterized by flattened body, eyes on upper surface; anterior margin of pectoral fins far overlapping gill openings, which are partly on side of body; no anal fin. Largest up to about 2.4 m (about 8 ft). Ovoviviparous. One genus; possibly as many as 11 species.

      Order Batoidei (rays, sawfishes, guitarfishes, skates, and stingrays)
 Jurassic to present. Five gill openings, wholly on ventral surface; pectoral fins united with sides of head forward past the gill opening. Differ from all sharks in lacking upper free eyelid.

      Suborder Pristoidei

      Family Pristidae (sawfishes)
 Jurassic to present. Distinguished by extension of snout into long, narrow, flattened blade armed on either side with teeth but without barbels; gills on lower side of body, as in other batoids. Ovoviviparous. Size varies with species; common Atlantic sawfish to at least 5.5 m (18 ft); species in Indian and Australian waters to over 7 m (23 ft). Widely distributed in tropical and subtropical zones of all oceans; occur in estuaries and run far up large rivers into freshwater; but whether they remain resident and reproduce in freshwater lakes is not clearly established. Six species are known.

      Suborder Rhinobatoidei (guitarfishes)
 Lower Jurassic to present. Electric organs are lacking; well-developed dorsal and caudal fins are present; base of tail is stout, not sharply marked off from rest of body. Most species are ovoviviparous, some perhaps oviparous.

      Family Rhynchobatidae
 Cretaceous to Recent. Distinguished by caudal fin being conspicuously bilobed and somewhat lunate; posterior edge of pectorals does not reach foremargin of pelvics. Two genera, widely distributed in tropical and subtropical shallow waters of Indo-Pacific. Maximum size over 2 m (61/2 ft).

      Family Rhinobatidae
 Caudal fin not bilobed; posterior edges of pectoral fins extending rearward at least as far as the origin of the pelvics. Small, rounded, closely set teeth. About 7 genera and 26 species; tropical and warm temperate shallow coastal waters of all oceans, in some localities entering freshwater and perhaps even permanently residing and breeding there. Size to about 1.8 m (about 6 ft). Harmless to bathers.

      Suborder Torpedinoidei (electric rays, numbfishes, torpedoes)
 Eocene to present. Distinguished principally by highly developed electric organs on either side of the head and gill chambers; the outlines of these organs visible externally in most species. Pectoral fins with the head form a circular or ovate disk. Skin of most species soft and entirely scaleless. Eyes small, functional in most species but rudimentary or obsolete in deepwater forms. Mostly sluggish bottom dwellers in all the oceans from tropical to temperate latitudes and from the intertidal zone to depths of at least 1,100 m (3,600 ft). Three families, Torpedinidae, Narkidae, and Temeridae, distinguished by whether 1, 2, or no dorsal fins are present. Numerous genera and species. The largest electric rays of the genus Torpedo reach a length of about 180 cm (71 in.); the smallest, of the genus Narke, less than 30 cm (about 12 in.).

      Suborder Rajoidei (skates)
 Lower Cretaceous to present. Moderately slender tail, on which the caudal fin is reduced to a membranous fold, though sometimes the caudal fin is entirely lacking; outer margins of the pelvic fin are more or less concave or notched. It is probable that all of the species are oviparous. Three families are distinguished by whether 1, 2, or no dorsal fins are present.

      Family Rajidae (the great majority of skates)
 Two dorsal fins. Upper surface of the body disk more or less rough with spines, thornlike denticles, or both. Some species with electric organs along the sides of the tail, which, as far as known, produce very weak shocks. Six genera, widely distributed from tropical to subarctic belts of both hemispheres but with curious gaps in distribution; scarce, if present, in the Micronesian, Polynesian, and Hawaiian islands in the Pacific, in the western Atlantic between Yucatán and mid-Brazil, and in West Africa between Cape Verde and Walvis Bay. They occur from estuaries seaward, several species down to depths of over 500 m (1,640 ft). Several species inhabit deep water, at last one being found at over 2,700 m (almost 9,000 ft). They live mostly on the bottom, often partially buried.

      Family Arhynchobatidae
 Distinguished from other skates by having a single dorsal fin. Single genus and species, Arynchobatis asperimus, known only from New Zealand.

      Family Anacanthobatidae
 No dorsal fin; completely smooth skin; the pelvic fins so deeply notched as to form leglike structures anteriorly. Two genera, Anacanthobatis from KwaZulu/Natal coast, South Africa, and Springeria from Gulf of Mexico.

      Suborder Myliobatoidei
 Upper Cretaceous to present. Distinguished by a slender tail, usually whiplike toward the tip; outer margin of the pelvic fins being straight or convex. Most with 1 or more saw-toothed, poisonous spines on upper surface of tail. Seven families are recognized. Tropical to warm temperate waters of all oceans, most abundant in shallow depths, entering brackish water and freshwater freely. One family is confined to freshwater.

      Family Dasyatidae (whip-tailed rays)
 Lower Cretaceous to present. Caudal fin lacking; no distinct dorsal fin; tail, measured from the anus to the tip, longer than the breadth of the disk. Ovoviviparous. Tropical to warm temperate latitudes in all oceans. Generally in depths less than about 100 m (328 ft), most abundant close to shore, including tidal embayments. The largest reaches at least 2 m (61/2 ft) in breadth. Five genera, 2 in tropical and subtropical rivers of South America. A peculiarity in the structure of the pelvis has been used to differentiate a separate family, Potamotrygonidae.

      Family Gymnuridae (butterfly rays)
 Miocene to present. Distinguished by the body being more than 1.5 times as broad as long and the tail considerably shorter than the body. Saw-toothed spine on the back of the tail in some species but not all. Maximum breadth about 2 m (61/2 ft). Shallow coastal waters of tidal embayments and river mouths in tropical to warm-temperature latitudes of all oceans.

      Family Urolophidae (stingrays)
 Eocene to present. Distinguished by having well-developed tail fin supported by cartilaginous rays; tail with at least one large saw-toothed spine. Ovoviviparous. The numerous species look very much alike; the largest does not exceed about 70 cm (271/2 in.) in breadth. Tropical to warm temperate coastal waters less than about 70 m (230 ft) deep in western Atlantic and both sides of the Pacific from Japan to Tasmania, including the East Indies; they are unreported from eastern Atlantic or the Indian or African coasts of the Indian Ocean.

      Family Myliobatidae (eagle rays)
 Upper Cretaceous to present. Distinguished from other myliobatoids by the forepart of the head projecting conspicuously beyond the rest of the body; eyes and spiracles on the sides of the head; tail as long as the disk or much longer and in most species bears a serrate venomous spine. Ovoviviparous. Some attain a width of about 2.5 m (about 8 ft). Cosmopolitan, occurring in continental waters and around islands and island groups from tropical to temperate latitudes; 4 genera.

      Family Rhinopteridae (cow-nosed rays)
 Upper Cretaceous to present. Similar to eagle rays except that the projecting head is deeply incised at the midline, forming two distinct lobes. Ovoviviparous. Maximum breadth about 2 m (about 61/2 ft). Coastal waters of tropical and warm temperate latitudes of all oceans.

      Family Mobulidae (devil rays, or mantas)
 Pliocene to present. Continental waters and around offshore island groups of tropical to warm temperate belts of all oceans. Distinguished by a pair of armlike structures (cephalic fins) projecting forward, one on each side of the head. Tail whiplike; with or without a serrate edged spine. Teeth minute, arranged in many rows. Maximum size (breadth) of smallest species about 60 cm (about 24 in.); largest species at least 7 m (23 ft).

      Subclass Holocephali (chimaeras (chimaera), ghost sharks)
 Upper Devonian to present. Cartilaginous skeleton, 4 pairs of gills, covered on each side of the body by an opercular fold of skin leading to a single external gill opening. First dorsal fin and spine erectile. Skin with small denticles along midline of back in some species and on tentacula and claspers of males. Teeth united to form grinding plates. Claspers of males are supplemented by an erectile organ, called tentaculum, in front of the pelvic fins, and all except one genus (Harriotta) have another club-shaped tentaculum on the forehead. Oviparous, laying elliptical, spindle-shaped, or tadpole-shaped eggs enclosed in brown horny capsules, remarkably large in proportion to the size of the parent. In breathing, chimaeroids take in water chiefly through the nostrils and thence through grooves leading to the mouth, which is generally kept closed. Variously distributed in temperate and boreal zones of all oceans, in coastal waters, and river estuaries and seaward down to over 2,500 m (8,200 ft).

      Order Chimaerae

      Family Chimaeridae (ghost sharks, ratfishes, chimaeras)
 Lower Jurassic to present. Rounded short or conical snout. Claspers of males bifid or trifid. Size to about 1.5 m (about 5 ft). Warm temperate and boreal latitudes of all oceans. Two genera, each with several species.

      Family Callorhinchidae (elephant fish)
 Hoe-shaped proboscis. One genus (Callorhinchus) with a few species, which may eventually prove to be identical. Size to about 1.3 m (4 ft 3 in.). Restricted to cool temperate and boreal latitudes of Southern Hemisphere, generally taken in rather shallow water, sometimes entering estuaries and rivers.

      Family Rhinochimaeridae (long-nosed chimaeras)
 Snout projecting into a long, straight point. Lateral line an open groove. Size to about 1.3 m (4 ft 3 in.). Probably cosmopolitan in middle latitudes of both hemispheres, taken in depths of 685–2,000 m (2,250 to 6,560 ft).

Critical appraisal
      Many of the elasmobranchs are difficult subjects for taxonomic study. Differences between species are often subtle and hard to measure. Lacking skeletal support such as that possessed by the bony fishes, captured sharks collapse along the soft undersides of the body when taken out of the water, thus reducing the accuracy of measurements. A satisfactory taxonomic study of any species requires adequate samples over a full range of sizes, representing the full geographical distribution of the species. The sampling allow for rather large variations in body proportions between individuals of like size and different size groups and between populations inhabiting different regions of the total distribution. Hence, the identity of many species remains unsettled. The number of living species of sharks, now estimated at 200 to 250, tends to diminish as ichthyologists in different parts of the world accumulate and exchange careful anatomical measurements of fresh specimens, discovering that fishes from widely separated areas, formerly thought to be distinct, are actually of the same species.

      The rays, except the larger ones, are somewhat easier to work with. About 300 to 340 species have been described. Here again, however, the number tends to diminish as comparative studies in different parts of the world show many of them to be cosmopolitan.

      The classification of chondrichthyed fishes is a somewhat controversial subject. An authoritative opinion as to how sharks, rays, and chimaeras should be grouped can be reached only from a comprehensive critical review of all available pertinent living and fossil material. Students continuously add to the accumulation of field measurements and museum specimens, and so such a classification needs to be revised from time to time. Because this revision involves a vast amount of work, it is not undertaken often.

Lionel A. Walford

Additional Reading
J.S. Babel, “Reproduction, Life History, and Ecology of the Round Stingray, Urolophus halleri Cooper,” Fish Bull. Calif. 127 (1967), an excellent natural history study of a ray; H.B. Bigelow and W.C. Schroeder, “Sharks,” Fishes of the Western North Atlantic, Mem. Sears Fdn. Mar. Res., no. 1, pt. 1, pp. 59–546 (1948); “Sawfishes, Guitarfishes, Skates and Rays,” and “Chimaeroids,” ibid., no. 1, pt. 2 (1953), two cosmopolitan syntheses in spite of emphasis on the western North Atlantic; “A Study of the Sharks of the Suborder Squaloidea,” Bull. Mus. Comp. Zool. Harv., vol. 117, no. 1 (1957), a revision of the group; P. Budker, La Vie des requins (1947; rev. Eng. trans., The Life of Sharks, 1971), a nontechnical, authoritative work; J.F. Daniel, The Elasmobranch Fishes, 3rd rev. ed. (1934), a classic treatise on the anatomy of sharks and rays; D.H. Davies, About Sharks and Shark Attack (1964), an authoritative, nontechnical work about sharks, principally South African species; P.W. Gilbert (ed.), Sharks and Survival (1963), a collection of 22 papers about sharks, with emphasis on dangerous species, including a list of documented attacks throughout the world; P.W. Gilbert, R.F. Mathewson, and D.P. Rall (eds.), Sharks, Skates and Rays (1967), a collection of technical papers covering a wide range of subjects; E.W. Gudger (ed.), Archaic Fishes, Bashford Dean Memorial Volume (1937), contains articles on the anatomy, reproduction, and development of the frilled shark (Chlamydoselachus anguineus), and on natural history and development of Heterodontid sharks; H.W. McCormick and T. Allen, with W.E. Young, Shadows in the Sea: The Sharks, Skates, and Rays (1963, reissued 1978), an authoritative, well-illustrated, nontechnical work; L.H. Matthews and H.W. Parker, “Notes on the Anatomy and Biology of the Basking Shark (Cetorhinus maximus),” Proc. Zool. Soc. Lond., 120:535–576 (1950); P.M. Roedel and W.E. Ripley, “California Sharks and Rays,” Fish Bull. Calif. 75 (1950), a handbook for identification of California species (of general use on the Pacific coast), illustrated with excellent photographs; A.S. Romer, Vertebrate Paleontology, 3rd ed. (1966), a comprehensive review, with a classification of archaic and recent fishes; H.W. Smith, From Fish to Philosopher (1961), a chapter on the elasmobranchs discusses their evolution and physiological adaptations to marine environment; D.W. Strasburg, “Distribution, Abundance, and Habits of Pelagic Sharks in the Central Pacific Ocean,” Fish Bull. U.S. Dep. Interior, 58:335–416 (1958), contains field observations and quantitative data on 12 species; G.P. Whitley, The Fishes of Australia, Part I, The Sharks, Rays, Devil-Fish, and Other Primitive Fishes of Australia and New Zealand (1940), a nontechnical book about Australian species; Carl Gaus and T.S. Parsons, A Photographic Atlas of Shark Anatomy: The Gross Morphology of ‘Squalus Acanthias' (1964, reprinted 1981).

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