Sharks can find prey by following the electrical impulses that animals emit, and some species of shark can smell a drop of blood in one million drops of seawater.
Sharks have an unlimited supply of teeth that are set in layered rows in the gums. If one tooth falls out, a tooth from another layer takes its place.
A shark may shed as many as 50,000 teeth in its lifetime. This is one reason why prehistoric shark teeth are the most commonly found fossils.
Unknown, but in general, species are declining.
The worldwide population of each species of shark is unknown.
Shark populations in many areas have declined from excessive fishing and unintentional catching in fishnets.
Scientists often examine animal teeth to determine age, but this technique isn't possible with sharks because they go through so many teeth during their lives. This makes determining the age of a shark difficult, so the lifespan of sharks isn't known.
There are at least 350 species of shark ranging from the smallest, the pygmy ribbontail catshark (8 to 10 inches long) to the largest, the whale shark (upto 45 feet long).
Sharks are equipped with a special set of sensors, arranged in clusters over their heads, that can pick up electromagnetic currents emitted from other animals. These sensors help detect prey from 1 to 3 yards away. Sharks can also detect the electromagnetic fields of the Earth and may use this ability to guide them during migration.
Sharks are often thought of as cold-blooded killers that prey on humans. However, only 32 species have been known to attack humans, and most attacks are accidents. Often, sharks inhabit the same shallow, warm-water areas as humans. The shark may mistake a person standing or floating in water for natural prey.
Many shark attacks involve people trying to free sharks from fishing nets. Sharks also are territorial animals and may attack if they feel threatened. The chances of being attacked by a shark are very small, and the chances of dying from a shark attack have greatly decreased over the years. In fact, in the United States, a person is 30 times more likely to be killed by lightning than by a shark. Dog bites are 1,000 times more common than shark bites.
Sharks are fished for their fins and cartilage. After the fin is cut off, the rest of the shark is discarded. Shark finning and fishing for cartilage have caused a decline in shark populations in some areas of the world. Sharks mature and reproduce slowly, so it is difficult for them to rebound from a decline.
Great white shark (Carcharodon carcharias):
Best known from the movie Jaws, the great white is a large, heavy-bodied shark, about 20 feet long, with large bladelike teeth. Widely the most-feared of sharks, great white attacks are rare, and most scientists agree that its reputation is undeserved. Many scientists believe it is endangered due to sport fishing and shrinking food supplies.
Whale shark (Rhincodon typus):
Reaching lengths of up to 50 feet, the whale shark is the largest fish on earth. This gentle giant has small teeth like whale baleen, through which it strains small fish and crustaceans.
Tiger shark (Galeocerdo cuvier):
The tiger shark is considered one of the most dangerous sharks. It is about 18 feet long and inhabits shallower water, often where people swim. The diet of tiger sharks varies widely and includes all types of sea life.
Blue shark (Prionace glauca):
The graceful blue shark is well known to scuba divers and commercial fishers - they have been seen circling divers and have followed fishing boats for days, eating stray fish.
Mako shark (Isurus oxyrinchus):
Short-fin and long-fin makos are close cousins with the great white shark. They are very fast swimmers and can reach speeds of up to 60 miles per hour.
Hammerhead shark (Sphyrna mokarran):
Hammerheads are best known for their distinctive mallet-shaped heads and widely spaced eyes, which they swing back and forth while swimming to detect prey. They are the only species of shark known to travel in schools.
Bull shark (Carcharhinus leucas):
The bull shark gets its name from its snout, which is wider than it is long. It is possibly more dangerous to humans than the great white shark because it lives in shallow, murky water in areas where people swim. The real shark attacks on which the movie and book Jaws were based were done by a bull shark.
Sharks can be found in all the world's oceans. In the National Park System, sharks can be found in Channel Islands National Park, CA; Point Reyes National Seashore, CA; Cape Cod National Seashore, MA; Assateague Island National Seashore, MD; Cape Hatteras National Seashore, NC; Cape Lookout National Seashore, NC; Cumberland Island National Seashore, GA; Canaveral National Seashore, FL; Virgin Islands National Park, VI; Padre Island National Seashore, TX; Gulf Islands National Seashore, FL and MS; Haleakala National Park, HI; Dry Tortugas National Park, Everglades National Park, and Biscayne National Park, FL.