How many kinds of sharks are there?
May 15, 2004
More than 400 shark species exist worldwide, and approximately 50 species are found in North Carolina waters. Of those, only 28 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 10 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. Nor do they have swim bladders. 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.
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 body. This line can sense movement in the water without the shark actually seeing the object. Sharks 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 more than 30,000 teeth in a lifetime.
Sharks can’t chew and must swallow food whole or 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. 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. Because of slow growth and reproduction rates, sharks are easily overfished and populations can remain depleted for 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 to make 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 the year 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. 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.