SHARKS

By Tony Corey

The word "shark" might evoke images of the massive, spike-toothed maw of the great white shark immortalized in Jaws. The great white as the archetypal shark emphasizes the dramatic characteristics often associated with sharks—imposing size, formidable jaw and crushing bite, swift and sure attack. But there is such diversity among animals carrying the name "shark" that the Hollywood stereotype can scarcely measure up.

Three hundred fifty species of sharks are identified worldwide, including 73 species known to inhabit U.S. Atlantic coast, Gulf of Mexico, and Caribbean waters. What identifies them all as sharks is the combination of paired fins, including pectoral fins whose rear edges are free rather than attached to the head; five to seven gill slits; and rough, sandpapery skin. Beyond these common characteristics, a wealth of variations distinguishes one shark species from another.

In size, they range from the appropriately named pigmy shark—smaller than 12 inches (30 cm)—to the enormous whale shark—at 40 feet (12 m) the largest fish in the oceans. In shape, they vary their missile-like profile with features such as the exaggerated tail (upper caudal lobe) of the thresher sharks or the wide, flattened head of the hammerhead sharks. One species may have a spined dorsal fin (the spiny dogfish); another may have whiskerlike nasal appendages, or barbels (the nurse shark).

Teeth, often an identifying characteristic, vary in shape and configuration according to diet and feeding habits. Those huge, triangular teeth familiar from Jaws have serrated edges that equip the white shark for cutting into a fare of seals, sea lions, porpoises, even sea turtles. The voracious sand tiger shark has long, slender, smooth-edged teeth with a small cusplet on either side; these teeth grasp small prey such as crabs, lobsters, and various finfish. Interestingly, the largest sharks—whale and basking sharks—are filter feeders, straining small organisms from the water. Both species have teeth featuring a single cusp curved backward. And both species have an exceptional number of teeth: The whale shark, for example, has 300 rows of dentition with hundreds of teeth in each row.

Evolutionary fitness

Fearsome teeth may contribute to sharks' hunting success, but other adaptations also enhance their effectiveness as predators. Sensitive smell receptors, eyes that can adapt to dim light, a receptor system that senses movement in the water, and electroreceptors that can detect prey buried in the sand secure the shark's status as apex predator.

While hunting capabilities have rewarded sharks as predators, reproductive adaptations have protected them as prey. Internal fertilization and production of fully developed young enhance sharks' evolutionary fitness. Although sharks are less prolific than many marine species, their reproductive strategies help ensure survival of the offspring that are produced. Pups are large and fairly developed at birth, so they have fewer potential predators than larval offspring.

Unlike bony fishes, shark young have the advantage of developing within the protection of the mother's body. Most sharks reproduce by ovoviviparity—producing embryos that hatch from eggs and then continue to grow in the uterus until fully developed. A few species, including whale sharks and some nurse sharks, still reproduce by laying eggs externally, a method known as oviparity. The eggs are protected by a tough, fibrous case that usually attaches to plants or rocks on the sea bottom till the young hatch. Hammerheads, smooth dogfishes, and most requiem sharks (the Carcharhinus family) reproduce by viviparity. This is the most advanced mode of reproduction, with the young nourished through the mother's placenta.

The same reproductive factors that have allowed sharks to dominate marine environments make them vulnerable to stock depletion in the face of intense human exploitation. Though born fully developed, sharks grow slowly and mature late—as late as 12 to 18 years of age in some species. They have a long reproductive cycle—generally one year but as long as two years for the spiny dogfish. And they produce few young per brood—usually two to 12, sometimes more depending on the species. In some cases, the small litter results from oophagy—literally "egg-eating": Embryos nourish themselves in the uterus by consuming the unfertilized eggs or smaller embryonic siblings. Consequently, only one embryo in each of two uteri survives to birth. All these factors combine to limit sharks' reproductive potential and hinder recovery from overfishing.

Protection from excessive harvesting and from practices such as finning, in which the valuable dorsal fin is removed and the carcass discarded, has been only patchy. Being highly migratory species, sharks are not constrained by international boundaries. They move up and down the coast or from estuaries to offshore or deepwater habitats, depending on species, season, and life stage. As they migrate, they slip in and out of regulatory jurisdiction and beyond the reach of national protection or conservation programs.

In 1999, the National Marine Fisheries Service (NMFS) finalized a fishery management plan for domestic management of sharks and for protection of their habitat in U.S. waters. Even without the international cooperation essential for effective management of highly migratory species, the NMFS management plan takes steps to prevent or end overfishing, rebuild depleted stocks, and identify and protect habitat for species included under NMFS management.

Protective management of sharks comes in response to rapid, intensive development of the fisheries. Although fished commercially for food and medicinal applications since the 1930s, sharks were considered an underutilized resource as late as the 1970s. Accelerating commercial catches through the 1980s and an expanding recreational fishery contributed to the stock decline addressed by managers in the 1990s.

Prehistoric to modern roles

Sharks have been adaptable enough to survive for more than 400 million years. Fossils in rocks from the Devonian period indicate that primitive sharks were small creatures less than 3.5 feet (100 cm) long, preyed upon by larger armored fishes that ruled the seas. On their way to becoming the dominant marine predators they are today, sharks evolved skeletons from true bone to cartilage. The lighter, more elastic cartilaginous skeleton helps these fast-moving distance swimmers maintain their position in the water. For identification purposes, it also defines them, along with rays, skates, and deepwater chimeras, as members of the class Chondrichthyes.

As predators near the top of the food web, sharks have few natural enemies other than humans. Their value to humans may vary according to species, geographic location, and management status. Along the Northeast coast, species such as blue sharks, makos, sand tiger sharks, and dogfishes have a market history.

Blue sharks have little value as food fish, but historically were captured for their fins. The practice of finning is now banned, and blue sharks are fished primarily for sport. Growing to 12 feet or more, these sharks are distinguished by their bright blue metallic-hued color and very long, narrow, pointed pectoral fins.

Makos have both commercial and recreational significance. The shortfin mako is the premier species for shark meat. And because it is one of the fastest fishes, and a fighter capable of leaping several times its length from the water, it is highly prized as a gamefish. The less familiar longfin mako is not so actively targeted.

Similar in appearance, the two makos were not recognized as separate species until 1966. About 13 to 15 feet long, both have dark blue coloring that lightens increasingly down the sides and belly; both have crescent-shaped tails, conelike snouts, and long, slender teeth. The primary difference is the length of their pectoral fins. Shortfin makos have shorter fins, about half to three-quarters the length of the head, while longfin makos have fins about equal to head length. Shortfin makos also display snow-white coloring around the mouth and snout.

Sand tiger sharks are popular for aquarium displays because they adapt better to captivity than most species. Their fierce-toothed appearance enhances their "shark" image, while their coloring—gray to light brown with scattered brownish spots—distinguishes them from other species.

Dogfishes are probably the most important commercial species in the Northeast. Spiny dogfish in particular gained prominence in the fishery with the depletion of cod, haddock and other groundfish stocks. Spiny dogfish meat is marketed in England for use in fish and chips. Smooth dogfish are more likely to have nonfood uses: in aquarium displays or as lab animals.

Despite their designation as dogfishes, smooth and spiny dogfishes are different families (Triakidae and Squalidae, respectively). Similar in size at 4 to 5 feet, they differ mainly in fin structure, with the spiny dogfish having dorsal fins each preceded by a single spine. The smooth dogfish has no spines, but has dorsal fins of nearly equal size.

REFERENCES
National Oceanic and Atmospheric Administration. Final Fishery Management Plan for Atlantic Tuna, Swordfish, and Sharks. Prepared by: Highly Migratory Species Management Division, Silver Spring, Maryland.

Castro, J.I. The Sharks of North American Waters. Texas A&M University Press. College Station. 1983.

Stevens, J.D. (consult. ed.). Sharks. Facts on File Publications. International Publishing Corp., Limited, New York, Oxford. 1987.