The tiger shark (Galeocerdo cuvier) is one of the largest and most recognizable sharks. It is covered with dark spots as a juvenile, which merge into stripes as it grows older and then eventually fade. Its powerful jaws and teeth are especially useful in hunting large animals such as loggerhead turtles (Caretta caretta), but it will eat almost anything and has the most varied diet of any shark.

Like many other sharks and rays, tiger sharks do not lay eggs but give birth to live young. With up to 80 pups per litter, tiger sharks may be more resilient than most sharks to overfishing, but their numbers are nevertheless in decline.

An Evolutionary Marvel
Sharks have roamed the oceans since well before the days of the dinosaurs and remain among the world's most effective hunters.

Sharks originated some 450 million years ago, and many species have changed little in the past 100 million years. Unlike most fishes, they have no gas bladder to keep them afloat, so many species must move constantly to keep from sinking. Their skeletons are made of light, tough cartilage instead of bone, and many have large, oil-filled livers that make them more buoyant.

	A Sixth Sense For Hunting Sharks' mouths bristle with rows of teeth which are constantly replaced by new ones as they become broken or worn. The first vertebrates to develop an immune system, sharks may have greater immunity to cancer than humans, and they are being studied for potential new cancer medications and antibiotics. © Marty Snyderman / Visuals Unlimited, Inc.

CONSERVATION: Sharks Under Attack
Most sharks are no more dangerous than other fishes. Humans, however, have proved extremely dangerous to sharks. Of the roughly 340 species of sharks, most never attack people. Fewer than 100 people per year are bitten by sharks, and less than 15 percent of these attacks are fatal. Yet humans kill 50 to 100 million sharks each year, placing entire populations and some species at serious risk.

As top ocean predators, sharks maintain the balance of other populations, and they keep other species healthy by culling the weakest members, as wolves do on land. But growing consumption of shark meat and fins, and the popularity of shark fishing for sport, have caused some shark species to decline by more than 90 percent, threatening the sharks' survival and disrupting ocean ecosystems.

"Flying" sea turtle Sea turtles swim in a completely different way from aquatic land turtles, which paddle through rivers and ponds with webbed feet. Sea turtles flap their long front flippers up and down, not forward and back, to fly through the water like birds.

Sea Turtles
Turtles are the only animals whose shoulders and hips are inside their rib cages. Fused to their ribs and backbone are several bony plates that form a protective shell.

Turtles have been around for at least 210 million years; the ancestors of modern sea turtles lived before many dinosaurs. Sea turtles are flat and streamlined, and some, called leatherback turtles, have leathery skin over their shells. Unlike many land turtles, sea turtles cannot retract their head and legs into their shells, but their shells still provide protection; many sea turtle shells have scars from shark bites.

Sea turtles spend their entire lives in the ocean—except when females lay eggs in holes they dig in the sand after migrating thousands of miles to particular beaches. Mothers, eggs and hatchlings are at great risk onshore, and all sea turtles are now endangered due to hunting, egg-collecting, loss of nesting beaches, fishing gear, plastic trash, pollution and other threats.