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Trevor Day
1978

Shark

There are about 400 living species of shark, yet they comprise less than 2 percent of all fish species. Nevertheless, as predators and scavengers, sharks play an important role in the ecology of the sea. Fierce in appearance, assaulting fish for food, and occasionally attacking people, sharks are perceived as one of humankind's greatest enemies. Together with skates, rays and chimaeras, sharks are members of the vertebrate class Chondrichthyes (cartilaginous fishes; fishes with proper jaws and a skeleton of cartilage), as distinct from the other major group of jawed fishes, the class Osteichthyes (bony fishes; jawed fishes with a skeleton of bone). Skates and rays have large, flattened pectoral fins that extend forward and are attached on either side of the head, and their gill openings are located below the pectoral fins. In sharks, they are not. In sharks, gill openings are on either side of the head.

Diversity

Sharks are found in most marine environments, from the greatest depths to the shallowest surf, and from tropical to polar seas. Only a few species venture into fresh water, most notably the bull shark, Carcharinus leucas, found in Lake Nicaragua and in rivers ranging from the Mississippi and Amazon to the Ganges and Tigris. During the Silurian period, some 450 million years ago, early sharks evolved from jawless fish ancestors that possessed primitive bony skeletons. A cartilaginous skeleton replaced the bone probably to increase buoyancy, a useful adaptation for a fast-swimming predator.

Today, sharks retain a notorious reputation as aggressive predators. However, the two largest sharks–the basking shark, Cetorhinus maximus, and the whale shark, Rhincodon typus–are gentle filter feeders, straining plankton from the water by using modified gill structures. The whale shark reaches about 18 meters (60 feet) long, and can weigh more than 15 metric tons. It is the largest fish in the sea. A feeding adult filters more than 1 million liters (260,000 US gallons) of water through its gills every hour. The smallest shark is the dwarf lantern shark, Etmopterus perryi, which reaches a diminutive 20 centimeters (8 inches) and weighs only 15 grams (0.5 ounces).

Many sharks feed almost exclusively on fish and squid. The great white shark, Carcharodon carcharias, thrives on a high-fat diet that includes seals, elephant seals, and sea lions. Bottom-living sharks often feed on mollusks and crustaceans as well as fish. Perhaps the shark with the strangest feeding habits is the cookiecutter shark, Isistius brasiliensis. It grows to only 50 centimeters (20 inches) long but attacks dolphins, whales, and large fish. The cookiecutter uses its suckerlike lips to anchor on the target animal's body. It then plunges its serrated lower teeth into the skin and twists, neatly removing an oval plug of flesh and leaving behind a craterlike wound. In saw sharks, order Pristiophoriformes, the head is elongated into a blade armed with toothlike projections along its edges. The “saw” is swept back and forth to stun or injure fish prey or to stir up sediment and reveal bottom-living invertebrates. Hammerhead sharks, Sphyrnidae, have dorsoventrally flattened heads. This adaptation may increase the fish's hydrodynamic efficiency and generate more lift. The arrangement also increases the distance between paired sensory organs, enhancing the “stereo” effect and enabling the shark to better sense the direction of incoming stimuli, particularly smell.

General Features

Cartilaginous fish have a skeleton of cartilage rather than bone. Bony remnants, however, are found in the form of dentine in the teeth and skin covering (denticles). Most sharks have a fusiform (torpedolike) shape and are fast, elegant swimmers. Many swim by sinuous side-to-side movement of the body. Unlike bony fishes (most of), which have air-filled swim bladders, sharks are denser than water and must swim forward to counteract sinking. They also counteract their density by the laying down of low-density lipid in the liver. The wedge-shaped head and flattened pectoral fins act like airfoils to generate lift. In nearly all shark species, the tail has a larger dorsal (upper) lobe. While swimming, this drives the tail downward and the head upward. A few sharks, among them the angel sharks and wobbegongs, are sluggish and bottom-living and have dorsoventrally flattened bodies reminiscent of skate.

Sharks extract oxygen from water using their gills, which are located in the pharynx (interior throat region). When swimming, water enters through the mouth, passes across the gills, and exits through gill slits. Water flow across the gills can be generated or enhanced by the expansion and contraction of the mouth and pharyngeal cavities, which together act as a waterpump. The shark's skin is tough and sandpapery. Embedded in the skin are numerous toothlike placoid scales (denticles). These scales are similar in structure to the shark's teeth and are highly abrasive.

A shark's teeth are arranged in rows that move forward to the front of the jaw to replace old teeth that become lost or damaged. The shape of the teeth is a useful diagnostic tool in identifying shark species and sheds light on the shark's dietary habits and lifestyle. For example, large sharks such as the great white and many requiem sharks (Carcharhinus spp.) have triangular teeth with serrated edges for seizing, cutting, and tearing sizable creatures such as large fishes, seals, and turtles. Some bottom-living sharks, including smooth hounds (Mustelus spp.) and zebra sharks (Stegostoma sp.), have broad, flat teeth used for crushing shellfish.

Sharks have a remarkable array of sensory equipment. The shark's legendary sense of smell can detect chemicals, such as those in blood and body secretions, at concentrations well below 1 part in a billion. The shark's well-developed lateral line system is highly sensitive to vibrations in the water, such as those produced by distressed fish. On the head are located blind-ended, jelly-filled tubes that open onto the skin as pores. These structures, called ampullae of Lorenzini, detect electrical fields. They can pinpoint the weak bioelectrical activity of prey animals; even fish buried in sand. The eyesight of sharks is poor to moderately good (depending on species). When homing in on potential prey items, pelagic sharks may track smell and vibration at a distance, use eyesight at moderate to close quarters, and depend on their electrical sense at close range.

The reproductive strategy of sharks, like that of skates and rays, involves the production of few offspring, but with great energy invested in each. In all species, copulation occurs and eggs are fertilized internally. Sperm is delivered through the male's modified pelvic fins, called claspers. Reproductive strategies range from simple oviparity (egg-laying) to advanced viviparity (live-bearing) with embryos nourished via a placentalike structure.

Reputation

In 1998, the U.S.-based International Shark Attack File (ISAF) recorded 58 shark attacks on humans worldwide, resulting in six fatalities. Of the 400 or so species of shark, about 40 are implicated in attacks on people. In some cases, assaults have followed provocation, for example, supposedly docile species such as the nurse shark, Ginglymostoma cirratum, being harassed by divers. Top of the shark attack list is the great white. Evidence suggests that it sometimes attacks people through mistaken identity. A great white's preferred prey is a blubber-rich seal or sea lion, and a surfer paddling on a surfboard produces a silhouette that can be mistaken for these creatures.

Humans have a low fat content and great whites rarely consume the person they bite. Tiger sharks, Galeocerdo cuvier, and bull sharks, Carcharhinus leucas, do, however, eat human victims.

Conservation

Most sharks are predators at or near the apex of food webs and, as such, are naturally low in abundance. Because sharks grow slowly, mature late, and produce few young, they are particularly vulnerable to overexploitation. Many shark species are now caught for their meat, skin, fins and for other body tissues that reputedly have medicinal qualities. Sharks are also caught inadvertently in nets and longlines set for other fish.

The preservation of the more charismatic sharks--great whites, whale sharks, and basking sharks--has become a major conservation issue in some parts of the world. The great white shark, for example, receives varying degrees of protection in the waters of Australia, Israel, South Africa, and the United States. Most shark species, however, are completely unprotected, or their protection may not be adequately enforced.

Australia, Canada, New Zealand, South Africa, and the United States are among the few nations that have integrated research and management plans for some of their shark fisheries. Throughout the world, most sharks are caught in mixed fisheries, where sharks are an incidental and unregulated catch. Until agencies monitor the catching of sharks more closely and begin to implement regulatory measures, the stocks of many of the larger sized shark species are likely to fall dramatically. Habitat deterioration and loss of species are contributory factors to shark population declines. At present, there is the tangible risk that some shark species will be driven to near or real extinction before we have even had the opportunity to study them.

At present, we poorly understand the effects of removing apex predators from a marine community. We do know that once-abundant cartilaginous fish can be fished to near-extinction within a matter of years. In the late 1940s, the porbeagle shark became economically extinct in Norwegian waters seven years after a local shark fishery was established. In the northeast Atlantic, dogfishes and some skate species have increased in numbers as bony fishes have become fished out. Meanwhile, the once-abundant barndoor skate, Raja laevis, has all but disappeared.