Shark
Appearance: Sharks (superorder Selachimorpha) are a type of
fish with a full cartilaginous skeleton and a highly streamlined
body. They respire with the use of five to seven gill slits. Sharks
have a covering of dermal denticles that protect their skin from
damage and parasites and improve
Skeleton: The skeleton of a shark is very different from that of
bony fish and terrestrial vertebrates. Sharks and other cartilaginous
fish (skates and rays) have skeletons made from cartilage but they
are still considered bones. They function in the same way as
human bones do. Like its relatives, rays and skates, the shark's jaw
is not attached to the cranium. The jaw's surface, like its vertebrae
and gill arches, is a skeletal element that needs extra support due to
its heavier exposure to physical stress and its need for extra
strength. It has therefore a layer of unique and tiny hexagonal
plates called "tesserae", crystal blocks of calcium salts arranged as
a mosaic. This gives these areas much of the same strength found
in real and much heavier bony tissue.
Resperation: Like other fish, sharks extract oxygen from seawater
as it passes over their gills. Shark gill slits are not covered like
other fish, but are in a row behind its head. A modified slit called a
spiracle is located just behind the eye; the spiracle assists the water
intake during respiration and even plays a major role in bottom
dwelling sharks, but is also reduced or missing in active pelagic
sharks. While moving, water passes through the mouth of the shark
and over the gills — this process is known as "ram ventilation".
While at rest, most sharks pump water over their gills to ensure a
constant supply of oxygenated water. A small subset of shark
species that spend their life constantly swimming,
Teeth: The teeth of carnivorous sharks are not attached to the jaw,
but embedded in the flesh, and in many species are constantly
replaced throughout the shark's life; some sharks can lose 30,000
teeth in a lifetime. All sharks have multiple rows of teeth along the
edges of their upper and lower jaws. They stick out of their mouth
at angles of up to thirty degrees. New teeth grow continuously in a
groove just inside the mouth and move forward from inside the
mouth on a "conveyor belt" formed by the skin in which they are
anchored. In some sharks rows of teeth are replaced every 8–10
days, while in other species they could last several months. The
lower teeth are primarily used for holding prey, while the upper
ones are used for cutting into it The teeth range from thin, needlelike teeth for gripping fish to large, flat teeth adapted for crushing
shellfish.
Tails: The tails (caudal fins) of sharks vary considerably between
species and are adapted to the lifestyle of the shark. The tail
provides thrust and so speed and acceleration are dependent on tail
shape. Different tail shapes have evolved in sharks adapted for
different environments. Sharks possess a heterocercal caudal fin in
which the dorsal portion is usually noticeably larger than the
ventral portion. This is due to the fact that the shark's vertebral
column extends into that dorsal portion, allowing for a greater
surface area for muscle attachment which would then be used for
more efficient locomotion among the negatively buoyant
cartilaginous fishes. This is in contrast to the bony fishes, class
osteichthyes, which possess a homocercal caudal fin.
Life span: The maximum lifespan of a shark varies from species to
species. Most sharks live for 20 to 30 years, while the spiny
dogfish lives a record lifespan of more than 100 years. Whale
sharks (Rhincodon typus) have been hypothesized to also live over
100 years.
Evolution: Evidence for the existence of sharks extends back over
450–420 million years, into the Ordovician period, before land
vertebrates existed and before many plants had colonised the
continents. All that has been recovered from the first sharks are
some scales. The oldest shark teeth are from 400 million years ago.
The first sharks looked very different from modern sharks. The
majority of the modern sharks can be traced back to around 100
million years ago.
Reproduction: The sex of a shark can be easily determined. The
males have modified pelvic fins which have become a pair of
claspers. The name is somewhat misleading as they are not used to
hold on to the female, but fulfill the role of the mammalian penis.
Diet: All sharks are carnivorous. Some, including the whale shark,
are filter feeders. They sieve plankton and small animals. The only
larger animals that filter feed are baleen whales. Other sharks, like
the great white shark, eat fish and even marine mammals. Many
bottom living shark like the zebra shark eat mollusks and
crustaceans.
Habitat: Sharks are found all around the globe from the north to the
south in all seas, they generally do not live in freshwater except for
a few exceptions like the bull shark and the river sharks which can
swim both in seawater and freshwater. Sharks are common down
to depths of 2,000 metres (7,000 ft), and some live even deeper,
but they are almost entirely absent below 3,000 metres (10,000 ft).
The deepest confirmed report of a shark is a Portuguese dogfish
that was found at 3,700 metres (12,000 ft).