Sharks are among the world's most successful and most diverse groups of animals. The earliest evidence of sharks is isolated spines, teeth and scales that appeared about 430 million years ago in the Silurian Period, known as the "Age of Fishes." For the past 280 to 345 million years, sharks have remained relatively unchanged. Altogether, over 350 species of sharks have been identified in today's oceans.

Sharks have been named after other creatures, such as the crocodile, goblin, zebra, bull, tiger, cow, cat, and dogfish sharks. Some are named for their colors -- the great white, blue, blacktip, whitetip, copper, silvertip, grey reef, and lemon. The angel, silky, bramble, carpet, spinner, hammerhead, thresher, basking, megamouth, nurse, and cookiecutter shark are named for other resemblances. For example, the "cookiecutter" shark is dubbed for its round mouth with wedge-shaped teeth on the lower jaw, used to carve out cookie-shaped plugs of flesh from large fish and marine mammals.

DISTRIBUTION

Sharks inhabit tropical and temperate seas as well as some cold and polar seas and even some freshwater lakes. Many juvenile sharks spend early portions of their lives in bays or estuaries which serve as nursery grounds. Shark migration may be short or long. Food availability, environmental cycles or reproductive cycles probably determine most migrations. Females of many species migrate to specific locations to lay eggs or give live birth to pups.

Shallow-water sharks spend most of their lives on the continental shelves, in water less than 650 feet (200m) deep. Examples of shallow-water sharks are the grey reef, hound sharks, nurse, wobbegong, reef whitetip, blacktip, hammerhead, zebra, angel, sandbar, sand tiger, horn, Port Jackson, lemon, copper, bull, bonnethead, dusky, sharpnose, and blacknose sharks.

Pelagic or open-sea sharks stay in the upper few hundred meters of the deep oceans, occasionally coming nearshore. These include the oceanic whitetip, crocodile, shortfin mako, longfin mako, whale, basking, silky, blue, porbeagle, and thresher sharks.

Deep-dwelling sharks, such as the megamouth, goblin, frilled, cookiecutter, and pygmy sharks inhabit the deeper parts of the open oceans, usually not swimming close to the surface.

DIVERSITY

Sharks show great diversity in size. The largest of the sharks--and the largest fish that has ever existed that we know of--is the whale shark, Rhincodon typus, which reaches a maximum length of about 45 feet (13.7 m). The basking shark, Cetorhinus maximus, is the second largest fish in the sea, reaching lengths of about 32 feet (9.8 m). Among the smallest sharks are the 8-10 inch (20-25 cm) spined pygmy or dwarf shark, Squaliolus laticaudus, and pygmy ribbontail catshark, Eridacnis radcliffei.

In addition to size, an array of shapes, colors, behaviors and abilities exists among sharks. The deep sea holds many species in surprising forms about which relatively little is known. Let's examine some of this diversity. The great white shark, Carcharodon carcharias, is found in both warm and cool temperate seas, especially along the continents and around islands. Large, serrated teeth and powerful jaws allow it to eat almost any animal, including sea turtles and marine mammals. Great whites have a slaty brown or charcoal topside and white underside. The great white is stouter than most sharks and recognized by its large jet-black eyes and crescent-shaped tail fin.

Bull sharks, Carcharhinus leucas, with teeth resembling the great white's, cruise along the continental shelves to eat almost anything but prefer feeding in shallow tropical waters. This grey shark with an off-white underside has a wide, blunt heard. The bull shark is unique among shark species for its adaptability to freshwater. These sharks inhabit Lake Nicaragua and have been found 2,000 miles inland in freshwater rivers.

The blue shark, Prionace glauca, has extremely long pectoral fins and a long pointed snout. Blues are brilliant dark blue with white undersides. They are found worldwide in tropical and temperate seas, often on the surface of deep water. Blues are among the most traveled species of shark. This species wanders throughout the North Atlantic Ocean. Migrations of 1,200 to 1,700 nautical miles (2,220 to 3,145 km) are common. The record distance so far by a shark is held by a blue which swam 3,740 nautical miles (6,919km) from New York state, where it was released, to Brazil. They sometimes travel in groups and have been observed rounding up schools of fish for prey.

Hammerhead sharks can be recognized by their flattened, rectangular-shaped heads suggesting a hammer. The nostrils, as well as the eyes, are located on the outer edges of the head. As the hammerhead swims, it swings its head back and forth through the water, possibly increasing the chances of detecting food. These sharks are found in tropical and suptropical waters of all oceans, and the favorite food of some species includes stingrays. The dark olive great hammerhead (Sphyrna mokarran), olive-brown scalloped hammerhead (Sphyrna lewini), olive gray smooth hammerhead (Sphyrna zygaena) and mouse-gray bonnethead (Sphyrna tiburo) are all types of hammerhead sharks.

The common thresher shark, Alopias vulpinus, is easily recognized by the elongated top lobe of its tail fin. All thresher sharks have tail fins nearly as long as the rest of the body. They have gray topsides and white undersides. Threshers are found in the warm and subtropical waters of the Atlantic and Pacific Oceans, often feeding in groups on schools of small fish. These sharks thresh their prey, that is, they use their tail fins to stun and scare fish into a group that can be fed upon more easily.

The whale shark and basking shark, Cetorhinus maximus, have an unusual feeding method. These two species, along with the recently discovered megamouth, Megachasma pelagios, are specially adapted for filtering large numbers of small organisms out of the water. These filter feeders engulf small fish, crustaceans, and drifting invertebrates as they swim slowly along. The small prey are engulfed in the mouth then filtered near the surface of the shark's gills as water passes out through the large gill slits. Whale sharks, dark brown with yellowish spots topside and white underside, are typically found worldwide in tropical seas. Occasionally, they may range into temperate seas. The basking shark is deep blue to charcoal with a pale underside. Basking sharks can be found traveling in schools of up to 100 individuals in temperate waters of the Atlantic and Pacific Oceans. The megamouth shark, known only from less than ten specimens, was first discovered in 1976. The dusky brown to black megamouth has an enormous shiny mouth with 100+ rows of teeth, possibly attracting plankton, jellyfish, and squids that the animal sieves from deeper waters.

The blackish-brown spined pygmy shark is rarely seen because it lives near the ocean floor during the day and travels up to depths of about 650 feet (200m) at night to feed. Spined pygmy sharks have numerous luminescent organs on their undersides which makes them more difficult to see from beneath, therefore less vulnerable to predators.