Chondrichthyes

The class Chondrichthyes consists of the cartilaginous fishes, including sharks, batoids (rays, skates, guitarfish, and sawfishes), and chimaeras, or ratfishes. A diverse group comprising more than 700 species, Chondrichthyans are found throughout the world's oceans and in some freshwater environments. The group as a whole is characterized not by mineralized bone but by a skeleton of soft, flexible cartilage lined with hard tissue. Chondrichthyans lack the air-filled swim bladder found in most bony fish, and therefore must swim continuously to stay afloat. Buoyancy is assisted by light oils in the liver, which can comprise up to 25 percent of a shark's total body weight.

Chondrichthyan males have a pelvic clasper, a specialized organ used in mating. Unlike most bony fishes, all chondrichthyans have internal fertilization. Reproduction can be oviparous (laying eggs, notably the "mermaid's purses" found on beaches), viviparous (live-bearing), or ovoviviparous (eggs carried within the mother).

The physiology of Chondrichthyan is of interest to cancer researchers because the cartilage of chondrichthyans contains substances known to inhibit the growth of tumors, and cancer is extremely rare in sharks. The group is characterized by placoid scales (also called dermal denticles, or skin teeth), with a structure similar to teeth consisting of an outer enamel layer, dentine, and an inner pulp cavity. As the animal grows, the skin surface is expanded by the addition of more scales rather than by the growth of individual scales. The teeth themselves are simply modified placoid scales; sharks have several rows of teeth, with replacement teeth always developing behind the frontmost rows of functional teeth. In some species, an individual can shed more than 30,000 teeth in its lifetime.

These fishes are often thought of as primitive compared to bony fishes and land-based vertebrates. Many chondrichthyans, however, have evolved sophisticated adaptations that have made them successful predators over a wide range of habitats. The senses of active predators, including many sharks, are especially well developed. A keen sense of hearing allows sharks to locate prey from as far away as 250 meters (800 feet). They are particularly sensitive to low-frequency vibrations such as those emitted by injured animals. Sound is detected through the ears and through the lateral line , a series of fluid-filled canals along the head and sides of the body that contain sensory cells sensitive to vibrations. As in many nocturnal mammals, the inside of a shark's eye is covered with a tapetum , a membrane that reflects light back into the eye, making it easier to see in dim light. Unlike most fishes, sharks can reduce and expand their pupils. In some species, the eyes are protected during feeding by the nictitating membrane, a structure similar to an eyelid. The Elasmobranchi group has external nostrils on the lower side of the body; because of them, sharks can detect tiny concentrations of substances such as blood, which allows them to scent prey from distances of several hundred feet. Finally, sharks can detect electrical signals via the ampullae of Lorenzini, which are specialized organs distributed over a shark's head that detect changes in electrical currents. Sharks use these to sense the electrical fields emitted by the heart and muscles of their prey.

The class Chondrichthyes includes two major groups: the Elasmobranchi (sharks, skates, and rays) and the Holocephali (chimaeras or ratfishes). Elasmobranchs are further divided into selachians (sharks) and batoids (rays and their relatives). The earliest evidence of Chondrichthyes in the fossil record is from the Devonian, the so-called Age of Fishes, from 350 to 400 million years ago.

Selachians (Sharks)

There are approximately 350 species of sharks. They are characterized by a heterocercal tail (the upper half being longer than the lower half), five to seven gill slits for respiration, and a rounded body tapered at both ends. Sharks are among the most misunderstood of all creatures. Popular culture has exaggerated beyond reason the danger posed by sharks to humans. The California coastline, with a high density of both human and great white shark populations, averages only one shark-related fatality every eight years. There is little evidence that sharks prey on humans for food. A disproportionate number of shark attacks occur near seal and sea lion rookeries, and surfers, whose paddling resembles the behavior of a seal on the surface, are more likely to be attacked than scuba divers. These factors indicate that sharks may be mistaking humans for seals or other large marine prey. Spearfishers carrying wounded fish are also at greater risk of shark attack; the vibrations of a thrashing fish may attract sharks, so one's catch should always be immobilized. Over half of total shark attacks appear to be misdirected territorial or courtship displays, with a characteristic motion preceding the attack: the shark shakes its head from side to side and swims back and forth erratically, with its head pointed up. Most sharks that attack humans are mackerel sharks, including the great white sharks (Lamnidae ) and requiem sharks (Carcharhinidae ); sharks in these families feed on large fish or marine mammals. Requiem sharks include the tiger shark and the bull shark, which feed primarily on other sharks. Hammerhead sharks (Sphyrnidae ) specialize in attacking stingrays.

The plankton-feeding whale sharks (Rhincodontidae ) are the world's largest fishes, with lengths of up to 18 meters (60 feet). Other huge plankton feeders include the aptly named megamouths (Megachasmidae ) and basking sharks (Cetorhinidae ). These sharks swim with their mouths open, straining plankton through modified structures associated with the gills.

While the larger sharks are better known to the general public, smaller species are far more diverse and abundant. The Squalidae include the spiny dogfish, which feeds mainly on invertebrates; it is a favorite candidate for inclusion in a fish and chips dinner and for classroom dissection. Another member of the Squalidae, the diminutive (four-inch) cookie-cutter shark, apparently uses the light-producing organs arrayed around its body to mimic a school of small fish. This attracts large fish, which the cookie-cutter shark then attacks, cutting out a round portion of flesh with specially modified teeth. Saw sharks (Pristiophoridae ) use a long snout lined with sharp teeth to slash their way through schools of fish; they then return to feed on wounded fish and any detached pieces they may find. The angel shark (Squatinidae ) resembles a ray but can be distinguished by gill openings on the side of the head rather than on its bottom and by the fact that the pectoral fins are not attached to the side of the head as they are in batoids.

Batoids (Rays, Skates, Guitarfishes, and Sawfishes)

The 470 species of this diverse group of fishes have in common a flattened body with expanded pectoral fins fused to the head (the "wings" in rays). Batoids are distributed throughout the world's oceans and in some tropical freshwater environments.

Sawfish (Pristidiformes ) can exceed 7 meters (23 feet) in length; they have a long snout lined with sawlike teeth and capture prey like the saw sharks described above. Guitarfish or shovelnose rays (Rhinobatiformes ) are characterized by a long, thick body and relatively narrow pectoral fins; they feed mainly on invertebrates on the sandy bottom. Skates (Rajiformes ) are found mostly in deep water; they, too, feed on invertebrates and are distinguished by a series of thorns on the tail.

The order Myliobatidiformes contains several types of ray. The large eagle rays (Myliobatidae ) have strong, muscular pectoral fins for rapid swimming. Manta rays (Mobulidae ) are the largest rays, attaining widths of about 7 meters (3 feet); like the largest sharks, they bear specialized mouthparts for feeding on plankton. Stingrays (Dasyatididae ) have a tail ending in a flexible, whiplike section that is equipped with one or more poisonous spines. Persons wading on a sandy bottom, particularly in calm water, often run the risk of stepping on a stingray and being stung as a result; one should always shuffle one's feet while walking on a sandy substrate. These animals almost never sting unless provoked; attack is often preceded by a warning stance where the tail is brought forward over the body.

Torpedo rays (Torpediniformes ), also known as electric rays or numb-fishes, have evolved modified muscles that function as electric organs. The largest electric rays can produce up to 200 volts of electricity in a single discharge. Several species are found exclusively in fresh water.

Holocephalans (Chimaeras, or Ratfishes)

The approximately twenty-five species of ratfishes are mostly bottom dwellers in some of the deeper marine habitats. They feed primarily on mollusks and other invertebrates in the substrate, crushing hard shells with their flat teeth. Ratfishes are characterized by a large head and eyes and by a long, slender tail. They have an operculum , a hard, bony layer of tissue covering the gills, found in many bony fishes but absent in all other chondrichthyans.

see also Phylogenetic Relationships of Major Groups.

Gil G. Rosenthal

Bibliography

Allen, Thomas B. Shadows in the Sea: The Sharks, Skates, and Rays. New York: Lyons and Burford Publishing, 1996.

Hamlett, William C., ed. Sharks, Skates, and Rays: The Biology of Elasmobranch Fishes. Baltimore, MD: Johns Hopkins University Press, 1999.

Moyle, Peter B. Fish: An Enthusiast's Guide. Berkeley: University of California Press, 1993.

Moyle, Peter B., and Joseph J. Cech Jr. Fishes: An Introduction to Ichthyology, 3rd ed. Upper Saddle River, NJ: Prentice-Hall, 1996.

Parker, Steve, and Jane Parker. The Encyclopedia of Sharks. Willowdale, Ontario, Canada: Firefly Books, 1999.

Perrine, Doug. Sharks and Rays of the World. Stillwater, MN: Voyageur Press, 1999.

CHIMAERAS, OR RATFISHES

Chimaeras—also knows as ratfishes—possess unusually large heads with deep, well-developed eyes. They also have a mouth that resembles a rabbit's inside which rest grinding, plate-like teeth.

Cartilaginous fish

Cartilaginous fish such as sharks, skates, and rays are vertebrates whose internal skeleton is made entirely of cartilage and contains no ossified bone. Cartilaginous fish are also known as Chondrichthyes and have one or two dorsal fins, a caudal fin, an anal fin, and ventral fins which are supported by girdles of the internal skeleton.

Placoid scales, or dermal teeth, are characteristic of the skin of both sharks and rays. The touch of shark’s skin is similar to the feel of sandpaper and was used as such for many years. The tiny teeth that protrude from the skin vary in each species of shark. The tooth tip is dentine with an overlay of dental enamel, while the lower part of the tooth is made of bone, which anchors the tooth to the skin. The skin of rays is naked in places, that is, without dermal teeth, but on the back or upper tail surface, the dermal teeth have developed large, strong spines.

The jaw teeth of both sharks and rays are, in fact, modified dermal teeth, which are lost when they become worn, and are replaced by rows of new teeth from the space behind. In some species of sharks, the jaw looks like an assembly line, with new teeth filling spaces immediately.

Both sharks and rays breathe through gills and have an opening called a spiracle on both sides of the head behind the eye. The spiracle enables the rays, which often bury in the sand, and seabed-resting sharks to take in water, pump it through the gill chamber, and release it through the gill slits without taking in large amounts of mud and sand. These fish usually take respiratory water in through the mouth, extract the oxygen from the water in the gills, and pass it out through the gill slits.

Cartilaginous fish are divided into two subclasses on the basis of gill slits and other characteristics. The first is the Elasmobranchs, which have at least five gill slits and gills on each side, one spiracle behind each eye, dermal teeth on the upper body surface, a tooth jaw, and an upper jaw not firmly attached to the skull. Sharks (Selachii), rays, and skates (Rajiformes) belong to this group. The chimeras (Holocephali) have one gill opening on each side, tooth plates, and a skull with a firmly attached upper jaw.

Cartilaginous fish do not have swim bladders, so a swimming motion must be maintained continuously, even when sleeping, or they will sink to the bottom. The caudal fin of the shark provides the propellant force in swimming, the dorsal fin provides balance, and the pectoral fins are used for upward force and depth rudders.

The flattened body and the rear spine of the rays makes their swimming motion unique and completely different than that of sharks. The large flattened body of the rays has become fused with the pectoral fins, which produces vertical waves from front to rear, similar to that of a bird in flight.

The chimeras utilize their pectoral fins when swimming, beating these fins simultaneously for propulsion, or alternately, to change direction. This method is highly effective for this group of cartilaginous fish, but is seen most often in bony fish.

The pectoral fins in the male cartilaginous fish are also used for mating. The rear part of the pectoral fin is modified as a copulatory organ. All cartilaginous fish have internal fertilization. Some species are oviparous, or egg-layers, and some are ovoviviparous, hatching the eggs within the female and giving birth to live young. Still others may be viviparous, with the young developing in utero, similar to mammals, with the yolk sac developing into a yolk placenta providing nutrients to the embryo.

Only true rays, species of sharks which live near the sea bed, and the chimeras lay eggs. The eggs are often encased in a leathery shell with twisted tendrils which anchor the egg case to rocks or weeds. These leathery shells are known as the “mermaid’s purse.”

Cartilaginous fish are predatory, meaning that they feed on other animals, from zooplankton to shellfish to whales. Humans fish for cartilaginous fish as a food source. Shark meat, once marketed under the pseudonyms of “flake” and “steakfish” is now popular worldwide. Shark fins have long been popular in the Asia. Rays are considered delicacies in Great Britain and France, and thornback rays and flapper skates are often sold as sea trout.

F. C. Nicholson

Fishes, Cartilaginous

Sharks, rays, and chimaeras or ratfishes make up the cartilaginous fishes (Chondrichthyes), one of the two major groups of fishes. The other major group is the ray-finned fishes (Actinopterygii). Unlike the more familiar rayfinned fishes, the cartilaginous fishes have skeletons composed entirely of cartilage. Moreover, cartilaginous fishes possess teeth that are replaced on a regular basis: a single shark may produce over ten thousand teeth in its lifetime. Cartilaginous fishes have four to seven pairs of gill slits that open separately to the outside (except chimaeras have a flap-like covering over the gill slits), lack a swim bladder, and practice internal fertilization (union of sperm and egg).

Compared to ray-finned fishes, cartilaginous fishes are relatively large, produce few young, mature slowly, and include a relatively low number of species . Chondrichthyes produce fewer than two hundred young per year and many reproduce every other year. Maturity in Chondrichthyes is attained from after several years of life to over 20 years of life. There are an estimated 934 species of cartilaginous fishes compared to about 22,000 species of ray-finned fishes. The low reproductive rates and slow rates of maturity make these fishes vulnerable to overfishing, and some species have declined to the point of concern.

Scientists estimate that there are about 383 species of sharks, 516 species of rays, and 35 species of rarely seen chimaeras. With the exception of the fresh-water family of stingrays (Potamotrygonidae), a few marine stingrays, and several sharks, Chondrichthyes are limited to salt water. Within the marine environment, they are found between the shoreline and 3,000 meters (nearly 10,000 feet or nearly 2 miles) in depth. They are most common on or over continental shelves (to depths of 200 meters or about 656 feet).

Rays and Skates

Rays and skates are distinguished from sharks in having their pectoral fins connected to the sides of their heads to form a disc, and gill slits are located on the ventral (underneath) side of the body. They are moderately to extremely flat and have rather stout to very slender tails, and are classified according to specializations. Electric rays are flattened but rather thick-bodied and have stout to moderately stout tails, with their gill arch muscles modified into electric organs (from which they can generate a shock of up to 200 volts). The electric organs are used to stun prey and defend against predators

Sawfishes and guitarfishes are shark-like, with rounded, wedge-shaped, or blade-rostrums. Sawfishes are distinguished from guitarfishes by the teeth on each side of their blade-rostrum.

Skates have guitarfish-rostrums and a broad disc, with bi-lobed pelvic fins and electric organs on each side of their slender tail. The skates use their electric organs for communication.

Stingrays have a broad disc, with single-lobed pelvic fins, and generally a serrated spine on the base of the tail. The tail spines are used in defense against predators.

Skates and rays with stout tails (electric rays, guitarfishes, and sawfishes) swim by undulating their tails from side to side. Skates and more generalized stingrays swim by undulating the margin of their disc. The more specialized stingrays swim by flapping their disc like birds in flight.

Sharks

Sharks are elongate, stout-bodied, and more uniform is shape than the skates and rays. They lack a connection between the pectoral fins and the sides of the head, and the gill slits are laterally located. Benthic species are moderately flattened, whereas pelagic species are streamlined. They are also divided into a number of subgroups, but the subgroups are not clearly distinguished by obvious specializations as in the case of the rays.

One group of sharks, termed the dogfish (Squalomorphii) generally possesses a spine preceding each dorsal fin and lives at high latitudes or in deep water. The remainder of sharks (Galeomorphii) are variable is shape but generally live at low latitudes. Most species live in water on or over the continental shelves, but some live at great depths.

The galeomorphs include the sluggish nurse sharks that occur around coral reefs; the large basking and whale sharks that feed by filtering zoo-plankton; and the white, mako, requiem, and hammerhead sharks that are pelagic predators.

The smallest shark, Etmopterus perryi, is 16 to 20 centimeters (6 to 8 inches) in length, and the largest, the whale shark (Rhincodon typus ), is 12 meters (about 40 feet), making it the largest of all fishes.

see also Coastal Ocean; Corals and Coral Reefs; Fish; Life in Water.

John D. McEachran

Bibliography

Moyers, Peter B., and Joseph J. Cech, Jr. Fishes: An Introduction to Ichthyology, 4th ed. Englewood Cliffs, NJ: Prentice Hall, 1999.

Stevens, John, and Peter R. Last. "Sharks, Rays, and Chimaeras." In Encyclopedia of Fishes, eds. J. R. Paxton and W. N. Eschmeyer. New York: Academic Press, 1994.

OVERRATED SHARK ATTACKS

Sharks can and do attack humans, although the risk of dying from a shark bite is miniscule: about 1 percent of the risk of being killed by lightning. Worldwide, only about one to two dozen deaths by shark attack occur annually.

In contrast, an estimated 30,000 human illnesses occur each year when people eat poisonous fish, shellfish, and other marine animals. In Japan alone, about 100 people die each year from pufferfish poisoning, even though the dangers of consuming this fish are well known.

Cartilaginous fish

Cartilaginous fish such as sharks , skates , and rays are vertebrates whose internal skeleton is made entirely of cartilage and contains no ossified bone. Cartilaginous fish are also known as Chondrichthyes and have one or two dorsal fins, a caudal fin, an anal fin, and ventral fins which are supported by girdles of the internal skeleton.

Placoid scales, or dermal teeth, are characteristic of the skin of both sharks and rays. The touch of shark's skin is similar to the feel of sandpaper and was used as such for many years. The tiny teeth that protrude from the skin vary in each species of shark. The tooth tip is dentine with an overlay of dental enamel, while the lower part of the tooth is made of bone, which anchors the tooth to the skin.

The skin of rays is naked in places, that is, without dermal teeth, but on the back of or upper tail surface, the dermal teeth have developed large, strong spines.

The jaw teeth of both sharks and rays are in fact modified dermal teeth, which are lost when they become worn, and are replaced by rows of new teeth from the space behind. In some species of sharks, the jaw looks like an assembly line, with new teeth filling spaces immediately.

Both sharks and rays breathe through gills and have an opening called a spiracle on both sides of the head behind the eye . The spiracle enables the rays, which often bury in the sand , and seabed-resting sharks to take in water , pump it through the gill chamber, and release it through the gill slits without taking in large amounts of mud and sand. These fish usually take respiratory water in through the mouth, extract the oxygen from the water in the gills, and pass it out through the gill slits.

Cartilaginous fish are divided into two subclasses on the basis of gill slits and other characteristics. The first is the Elasmobranchs, which have at least five gill slits and gills on each side, one spiracle behind each eye, dermal teeth on the upper body surface, a tooth jaw, and an upper jaw not firmly attached to the skull. Sharks (Selachii), rays, and skates (Rajiformes) belong to this group. The chimeras (Holocephali) have one gill opening on each side, tooth plates, and a skull with a firmly attached upper jaw.

Cartilaginous fish do not have swim bladders, so a swimming motion must be maintained continuously, even when sleeping, or they will sink to the bottom. The caudal fin of the shark provides the propellant force in swimming, the dorsal fin provides balance, and the pectoral fins are used for upward force and depth rudders.

The flattened body and the rear spine of the rays makes their swimming motion unique and completely different than that of sharks. The large flattened body of the rays has become fused with the pectoral fins, which produces vertical waves from front to rear, similar to that of a bird in flight.

The chimeras utilize their pectoral fins when swimming, beating these fins simultaneously for propulsion, or alternately, to change direction. This method is highly effective for this group of cartilaginous fish, but is seen most often in bony fish .

The pectoral fins in the male cartilaginous fish are also used for mating. The rear part of the pectoral fin is modified as a copulatory organ . All cartilaginous fish have internal fertilization . Some species are oviparous , or egglayers, and some are ovoviviparous , hatching the eggs within the female and giving birth to live young. Still others may be viviparous, with the young developing in utero, similar to mammals , with the yolk sac developing into a yolk placenta providing nutrients to the embryo.

Only true rays, species of sharks which live near the sea bed, and the chimeras lay eggs. The eggs are often encased in a leathery shell with twisted tendrils which anchor the egg case to rocks or weeds. These leathery shells are known as the "mermaid's purse."

Cartilaginous fish are predatory, meaning that they feed on other animals, from zoo plankton to shellfish to whales. Cartilaginous fish themselves are sought after by humans as a food source. Shark meat, once marketed under the pseudonyms of "flake" and "steakfish" is now popular worldwide. Shark fins have long been popular in the Orient. Rays are considered delicacies in Great Britain and France, and thornback rays and flapper skates are often sold as sea trout.

F. C. Nicholson

Cartilaginous Fish

The cartilaginous fish, or Chondricthyes, include the sharks, rays, skates, and chimaeras. There are over eight hundred living species of sharks and rays, and about thirty species of chimaeras. Cartilaginous fish are true fish. They have fins and breathe with gills. Unlike the more familiar bony fish, the Osteichythes, the skeletons of the cartilaginous fish are made of cartilage. Other features that distinguish the cartilaginous fish from the bony fish are multiple gill slits, tiny toothlike scales, nostrils on the side of the head, teeth that are not fused to the jaw, and internal fertilization . Internal fertilization also occurs in some bony fish such as sea horses, guppies, and mollies. The ancestors of cartilaginous fish and bony fish diverged in the late Silurian, more than 400 million years ago.

Sharks are large, long-lived, slow-growing ocean predators. The whale shark (Rhincodon typhus ) is the world's largest fish; adults can be as long as 18 meters (59 feet). The spiny dogfish shark (Squalus acanthias ) is the most studied shark, and while it rarely grows longer than 1.2 meters (almost 4 feet), it matures at 35 years and lives to be 70 or 80 years old. Sharks have internal fertilization and many shark species bear live young after a gestation of six or more months. The number of sharks in a clutch is often low, but can range from one or two to hundreds, depending on the species. The combination of slow maturity, long gestation, and small clutches means that shark populations cannot increase very rapidly. As a result, shark populations are very vulnerable to overfishing.

Sharks are tremendous predators, with their mouths full of ever-sharp teeth and jaw strength capable of exerting over 2,500 kg/cm2 (30,000 psi) of pressure at the tooth tips. (A single shark may produce over ten thousand teeth in its lifetime, and as a result, the most common fossils of the cartilaginous fish are their teeth.) Sharks also have excellent senses of smell, waterborne vibrations, and the ability to sense the faint magnetic fields generated by the muscles of their prey. The large white shark (Carcharodon carcharias ) preys on seals, sea lions, and large fish, and has been known to attack swimmers and boats.

Rays are bottom-dwelling fishes that are able to "fly" through the water with their enlarged and flattened pectoral fins. Stingrays can cause excruciating pain using a venomous stinger at the base of their tail. Electric rays can generate a shock of 200 volts. The manta ray has a wing span of up to 7 meters (almost 23 feet) and is sometimes seen following ships in the open ocean.

Chimearas, also known as ratfishes, are a small group of rarely seen bottom-dwelling cartilaginous fish with large platelike teeth, no scales, and long skinny tails.

see also Bony Fish; Ocean Ecosystems

Virginia Card

Bibliography

Carl, E. Biology of Fishes, 2nd ed. Philadelphia, PA: W. B. Saunders, Co., 1997.

Moyers, Peter B., and Joseph J. Cech, Jr. Fishes: An Introduction to Ichthyology, 4th ed.Englewood Cliffs, NJ: Prentice Hall, 1999.