Sharks and rays

Chondrichthyes, from the Greek words meaning "cartilaginous fish," is a class of cold-blooded fishlike vertebrates. Like other fish, members of this class, known as chondrichthyans, are characterized by gills, fins, and a dependence on water as a medium in which to live. Unlike the more primitive jawless fish (class Agnatha), chondrichthyans possess well-developed jaws; unlike the bony fish (class Osteichthyes), they possess a skeleton composed entirely of cartilage, although this is often highly calcified (strengthened and hardened by calcium compounds).

Additional features characteristic of chondrichthyans include teeth, lack of a swim bladder (the buoyancy organ present in most bony fishes), a spiral valve intestine, internal fertilization, and osmoregulation (salt balance) by means of the nitrogen compound urea. The fossil record for Chondrichthyes begins in either the Silurian or the early Devonian period—their earliest appearance is debatable, as fossil evidence from the Silurian consists of placoid scales that may or may not have belonged to early chondrichthyans—and is abundant throughout the Devonian. Within the class Chondrichthyes are two distinct subclasses, Elasmobranchii and Holocephali; the separation of the two is generally believed to date back to the early Devonian.

Most living cartilaginous fishes are members of Elasmobranchii. This group includes more than one thousand living species of sharks, rays, and skates in various orders, all contained within the superorder Euselachii. The elasmobranchs are characterized by placoid (toothlike) scales and between five and seven pairs of gill openings, with five being the most common. Sharks and rays are almost exclusively marine, although there are about twenty-eight ray species (the river stingrays, family Potamotrygonidae) that inhabit only freshwater environments and several other rays and at least one shark species (the bull shark) commonly found in rivers.

Sharks

Sharks are a diverse group of carnivorous species, ranging in size from the dwarf lantern shark (Etmopterus perryi), which matures at about twenty centimeters in length or less, to the enormous whale shark (Rhincodon typus), which reaches nearly fifteen meters in length and is the largest known fish species of any kind. Curiously, the whale shark and the nearly-as-large basking shark (Cetorhinus maximus) are plankton feeders. They capture their tiny food organisms by swimming open-mouthed through the water and straining out the plankton with fine comblike structures in their gills, called gill rakers.

Most sharks, however, have sharp, bladelike teeth, suitable for attacking and feeding on more active prey. The white shark (Carcharodon carcharias), also called the great white shark, is a voracious roving predator that may grow to twelve meters in length. It has been implicated in more fatal shark attacks than any other species.

In addition to well-developed eyes, inner ears, and olfactory organs, sharks possess a lateral line system, as do most bony fishes. This is a sense organ consisting of a canal beneath the skin, on each side of the body, connected to the surface by numerous pores. It is sensitive to vibrations in the water, giving sharks a sense of “distant touch” that enables them to navigate and hunt their prey in murky water. Another sensory feature of sharks and other elasmobranchs is an electroreception system, consisting of receptors, called ampullae of Lorenzini, on the surface of the snout. Apparently, this system is useful in hunting, since it allows the weak electric fields produced by the muscle contractions of prey species to be detected. It may also function in intraspecific communication (communication with others of the same species), since many elasmobranchs possess electric organs.

Sharks are typically torpedo-shaped and slightly depressed in form—that is, flattened from top to bottom. They swim by means of rhythmic undulations of the body, which are produced by sequential contraction of the myomeres (body-muscle segments). The tilt of the shark’s pectoral fins (the paired fins toward the front of the body) and heterocercal tail (the upper lobe of the tail fin being larger than the lower lobe) enable it to maintain its relative depth position as it swims forward, despite the fact that the shark lacks a swim bladder. Also improving the buoyancy of sharks are their cartilage skeletons, which are lighter than bone, and their large, oily livers. Some shark livers contain a unique low-density oil called squalene.

Sharks and other cartilaginous marine fishes regulate the concentration of solutes (dissolved substances) in their body in a manner very different from that of the bony fishes, which either retain salt (freshwater bony fishes) or secrete salt (marine bony fishes). Sharks maintain a concentration close to or higher than that of seawater by retaining urea and trimethylamine oxide, two relatively nontoxic nitrogenous waste products.

Reproduction in the sharks and other cartilaginous fishes is characterized by internal fertilization. A pair of intromittent, or copulatory, organs called claspers are located on the pelvic fins (the paired fins nearer the tail region) of the male. These are used to transfer sperm to the female genital opening. Embryos remain in the body or are released in egg cases, for a long gestation, or development, period. A small number of young either are born alive or hatch from an egg case in active, well-developed form.

Common Shark Species

Among the more familiar shark species are members of the family Lamnidae. This family includes the dreaded white shark and other “mackerel sharks,” such as the shortfin mako (Isurus oxyrinchus)—a popular game fish and food fish, but a dangerous species as well. Another family, the Carcharhinidae (requiem sharks), with dozens of species, includes two species known to prey on humans, the tiger shark (Galeocerdo cuvier) and the bull shark (Carcharhinus leucas). Bull sharks have been found in rivers and lakes in Central and South America; they have penetrated the Amazon River as far as Peru.

Yet another group of dangerous sharks is the family Sphyrnidae, the hammerheads. These species are distinguished by a laterally expanded head, having the eyes and nasal openings at the ends of the hammerlike extensions. The function of this arrangement is unclear, but it probably aids in detecting and homing in on prey organisms. To students of comparative anatomy, the spiny dogfish (Squalus acanthias) is perhaps the most familiar shark, since it is often dissected in the classroom as a typical representative of the lower vertebrates. This worldwide species, inhabiting temperate coastal areas, is also an important food fish in many parts of the world. It commonly appears in England, for example, in fish and chips.

Rays, Skates, and Sawfish

Most of the more than six hundred living species of the order Rajiformes, the rays and skates, are adapted for bottom living. In body form they are strongly depressed (flattened), with enlarged pectoral fins extending forward to the head region. Their teeth are usually pavementlike, for crushing their hard-shelled invertebrate prey. Most species give birth to live young, except the skates (family Rajidae), in which the eggs develop in a leathery egg case (the “mermaid’s purse” that beach visitors often find in the sand). Other orders of rays include the Myliobatiformes (stingrays), which have a venomous spine on the tail, and the Torpediniformes (electric rays), which can generate an electric charge.

The largest species among the rays and skates is the giant manta ray (Manta birostris), of the order Myliobatiformes, which may attain a width of over six meters between the tips of its pectoral fins and a weight in excess of 1,300 kilograms. Like the whale shark and basking shark, the giant manta ray is a plankton feeder. It directs plankton into its mouth as it swims by means of large scooplike extensions on its head, then filters out the plankton with its comblike gill rakers.

Rays and skates swim by means of flapping movements of their winglike pectoral fins. Some species, including eagle rays and manta rays, can make spectacular leaps from the water. Among the more remarkable rays are the electric rays. These sluggish fishes use electrical discharges of up to two hundred volts, produced by a pair of disk-shaped electric organs on the sides of the head, to stun their prey and perhaps to repel predators.

Another order of elasmobranchs is the Pristiformes (sawfish), which consists of only one living family, Pristidae. A sawfish resembles a somewhat flattened shark in body form but has a long, flat, toothed extension—the “saw”—on the end of its snout. This is used to slash through a school of prey fish.

Chimaeras

The other major subgroup of cartilaginous fishes, the subclass Holocephali, comprises about forty living marine species, all of which belong to a single order, the Chimaeriformes (chimaeras). They have a single gill opening on each side, like the bony fishes, but unlike them have a soft (rather than bony) gill cover. These fishes, sometimes called ratfish because of their long, slender tails, live and feed on the ocean bottom, usually in deep water. They have pavementlike teeth for crushing their mollusk and crustacean food, and they have a venomous spine on the leading edge of the first dorsal fin (the forwardmost of the unpaired fins on the upper surface of the body) for defensive purposes. Male chimaeras have a fingerlike barbed clasper on the top of the head, and two pairs of claspers on the ventral (belly) side of the body. At least one pair of these ventral claspers is involved in mating, and it is believed that the forehead clasper may be as well. The female lays eggs in leathery capsules somewhat like skate egg cases.

Ichthyology

The study of sharks, rays, and related species is part of the larger discipline known as ichthyology, which has its origins in the writings of Aristotle. He was the first to report, for example, that the sex of sharks can be determined by the structure of the pelvic fins, that is, by the presence of claspers in the male. Aristotle also contrived some rather fanciful interpretations of shark anatomy and behavior, as in his explanation for the fact that the shark mouth is on the under side of the head, far back from the tip of the snout, unlike the mouths of most other fish. In his view, this made it difficult for the shark to feed on its prey, requiring it to turn on its back, and thus nature allowed some chance for the poor animals to escape the jaws of this ravenous predator.

Modern study of cartilaginous fishes, like fish biology in general, involves several disciplines. Ichthyology, or systematic ichthyology, is particularly concerned with the naming and classifying of species and higher taxa (taxonomic categories) and determining their interrelationships. Living cartilaginous fishes are probably better known (that is, more of the extant species have been discovered) than living bony fishes, simply because they tend to be larger, more conspicuous, and less secretive. Yet, it was not until 1976 that one of the largest shark species, a deep-water filter-feeding species called the megamouth shark (Megachasma pelagios), was discovered near Hawaii. There may exist many additional Chondrichthyes species in deep ocean waters and remote coral reef areas.

Chondrichthyes systematics (that is, the Classification of the fishes) has undergone many changes and revisions as more has become known about fossil representatives and about the characteristics of the anatomy, biochemistry, and the like, of the living species. Studies of fossil cartilaginous fishes are limited almost entirely to samples of teeth, since these are virtually the only body parts durable enough to be preserved in the fossil record. Nevertheless, there is enough information in the characteristics of the teeth so that knowledge of the interrelationships of fossil species, both among one another and with living species, is quite advanced.

Systematic study of modern species requires collection of specimens, generally by means of nets, traps, hooks, and lines, or spearing. Specimens are then preserved in some way and maintained in a museum collection. Entire specimens, if they are relatively small, can be maintained in diluted alcohol after fixation in formalin (formaldehyde solution). Other specimens, especially large ones, are dissected, and only certain parts are preserved, particularly the head skeleton with jaws and teeth. Certain new techniques provide taxonomic information from samples of living tissue. Karyotyping (analysis of the chromosomes), protein analysis (determination of the amino acid sequence), and DNA hybridization (estimation of genetic similarities) are all techniques that can elucidate interrelationships among the Chondrichthyes. Other disciplines concerned with the study of cartilaginous fishes include fisheries biology (the science of management and exploitation of commercially important fish species) and comparative physiology.

The Place of Sharks and Rays in Evolution

Cartilaginous fish represent an early line in the evolution of vertebrates. Understanding their interrelationships is crucial to an understanding of the ancestry of other fishes and of tetrapods (amphibians, reptiles, birds, and mammals) and thus, ultimately, of humans. Even though humankind’s ancestors split from the ancestors of sharks and rays more than four hundred million years ago, many anatomical and physiological features are shared. A prime example is the eye, which is extraordinarily similar in all vertebrates. The same system of eye movement, involving six muscles innervated by the same three cranial nerves, has remained unchanged throughout vertebrate evolutionary history. Thus, the study of shark eyes, or any other aspect of shark biology, deepens the understanding of the evolution of higher animals.

Sharks and their relatives are important and interesting in other ways as well. Many species have importance as food, especially in Asia and the South Pacific. Other products derived from sharks include shark liver oil (which was an important vitamin A source before the development of synthetic vitamin A), shark skin (for leather products), and shark cartilage derivatives (used in medicine).

Even though the real risk of shark attack anywhere in the world is statistically very small, sharks have been known to be such brutal killers that interest in preventing shark attacks is widespread. Various chemical shark repellants such as “shark chaser” have been tried. This water-soluble mixture of dye and copper acetate was given to US military personnel during World War II for use if they were stranded in the sea after their ships were sunk or planes downed. It was, however, later shown to have little or no effect on sharks. Other techniques have included the cartridge-loaded “bang-stick,” which is probably more dangerous to the untrained user than to a shark. A more promising device is the “shark screen,” a floating plastic bag that can be filled with water and entered—masking the odors, sounds, and movements that might attract sharks.

Much remains to be learned about sharks, rays, and other cartilaginous fishes. Studying their ecology, behavior, and evolutionary relationships is important for further understanding of their basic biological nature. It is also essential for maximizing the benefit of commercially important species and minimizing the risk posed by dangerous species.

Principal Terms

cartilage: a gristle-like supporting connective tissue that forms the skeletons of chondrichthyans

Chondrichthyes: the scientific name for the taxonomic class that contains cartilaginous fish, including sharks, rays, skates, and ratfishes

Devonian: a geological period lasting from about 416 million years ago to about 359 million years ago, during which ancestral sharks were abundant and diverse

placoid scales: hard, toothlike scales, sometimes called denticles, that are embedded in the skin of most sharks, rays, and skates

Silurian: a geological period lasting from about 444 million years ago to about 416 million years ago, during which the first jawed fishes appeared

vertebrate: a member of the chordate subphylum Vertebrata, characterized by the possession of a vertebral column made of cartilage or bone

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