Sharks are a diverse group of carnivorous species,
ranging in size from the tiny dwarf shark
(Squaliolus laticaudus), which matures at less than
fifteen centimeters in length, to the enormous
whale shark (Rhincodon typus), which reaches fifteen
meters or more in length and represents the
largest 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)
is a voracious roving predator that may grow
to twelve meters in length. It has been implicated
inmore fatal shark attacks than any other species.
In addition to well-developed eyes, inner ears,
and olfactory (smell) 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. Itmayalso
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
Sharks and other cartilaginous marine fishes
regulate the concentration of solutes (dissolved
substances) in the 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 man-eaters, the tiger shark (Galeocerdo cuvieri) 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.
Sharks and people
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 fromsharks 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 U.S. 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.
cartilage: a gristlelike supporting connective
tissue that forms the entire skeleton
of cartilaginous fishes
Chondrichthyes: the scientific name for the taxonomic group, or class, in which the jaw-bearing cartilaginous fishes are placed; it includes sharks, rays, skates, and ratfishes
Devonian: a geological period from about 400 million years ago to about 350 million years ago; during this period, 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 from about 440 million years ago to about 400 million years ago; the first jawed fishes appeared during this period
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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