SHARKS
P1607
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.
posted 3/01
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