How many kinds of sharks
May 15, 2004
More than 400 shark species exist worldwide, and approximately
50 species are found in North Carolina waters. Of those,
only 26 species are found within North Carolina’s
continental shelf to nearshore waters. Their presence is
not year-round. Some move north to south, others move inshore
to offshore with the seasons.
Of the 400 known species, less than ten percent are considered
dangerous or are known to have been involved in attacks.
Some of the more common species are not considered dangerous,
such as the sand bar, nurse, silky and dogfish.
Sharks were around when dinosaurs walked the Earth. They are related to skates,
rays and sawfish, and their skeletons are made of
cartilage, the same material that makes up our nose
and ears. They don’t have scales like fish,
but instead have skin made up of minute teeth-shaped
denticles, giving it a sandpaper-like texture. Sharks
don’t have swim bladders either. Many have
oil-rich livers, and because liver oil is less dense
than water, the liver works like a swim bladder
to help keep the shark buoyant.
A sand tiger shark, (Odontaspis
taurus) cruises amid a school of bait fish near
the offshore wreck, Caribsea. (photo credit: Vlad
The eyes of sharks are well suited for seeing in dim light.
Their ears are positioned deep in their heads, and are attracted
to low-frequency sounds. Sharks also have a sensor system
– known as a lateral line – that runs the length
of their bodies. This line can sense movement in the water
without the shark actually seeing the object. Sharks also
have a very acute sense of smell. It’s estimated they
can detect blood in water up to a quarter-mile away!
Different sharks have different shaped teeth; some are
long and curved for holding slippery fish, others are serrated
for cutting. Some, like the smooth dogfish, have teeth designed
for crushing. Surprisingly, sharks lose or break teeth easily,
but the teeth are quickly replaced from a warehouse of spares
continuously produced and stored in parallel rows in the
shark’s jaws. Some sharks can produce over 10,000
teeth in a lifetime.
Sharks can’t chew food and must swallow it in chunks.
They eat at one- to two-day intervals and are more active
during twilight and at night. Most sharks feed on sick or
injured animals, with diets varying from fish to stingrays
to seals. However, the examination of the contents of one
tiger shark’s stomach revealed half a crocodile, a
sheep’s hind leg, three gulls, two cans of unopened
peas and assorted bike parts. The basking shark, one of
the largest sharks reaching 45 feet in length, has tiny
teeth and feeds exclusively on tiny plankton.
Sharks grow slowly and take many years to mature –
some species as long as 12 to 18 years. Many sharks reproduce
only every other year. Because of slow growth and reproduction
rates, sharks are easily overfished and populations can
remain depleted for years. Some sharks give live birth and
others lay eggs. The number of young varies from species
to species. The average natural life span of most sharks
is 25 years.
Sharks are used to produce a number of commercial products
around the world. Their flesh is used for fish and chips;
the oil from their liver for heating; squalene, a chemical
in the liver, for cosmetics, skin creams and medicines;
their skin for sandpaper; and chemicals from shark cartilage
have been used to stop cancer growth in lab animals and
for making artificial skin for burn patients.
Most encounters between sharks and people are cases of mistaken
identity. In 2001, there were 76 recorded unprovoked shark
attacks in the United States, versus 86 in 2000. According
to the International Shark Attack File, the numbers of shark
bites from year-to-year seem directly associated with increased
numbers of humans swimming, diving and surfing in the ocean.
The shark mistakes thrashing arms or dangling feet for prey,
darts in, bites, and lets go when it realizes it hasn’t
captured a fish.
Sharks are native to the ocean, just as bears are native
to the forest. Anyone entering marine waters must acknowledge
that an encounter with a shark is possible. However, the
possibility should be kept in perspective. Bees, wasps and
snakes are responsible for more fatalities each year than
sharks, and in the United States the annual risk of death
from lightning is 30 times greater than from a shark attack.
Many experts agree that by taking certain precautions, swimmers
and surfers can avoid shark attacks.
Sharks are fascinating creatures. Their biology has remained
virtually unchanged for millions of years. And, as top predators,
they play an important role and provide a valuable balance
to the marine ecosystem.