Sharks are large fish that live mostly in salt water. They vary in size. The dogfish only 3 feet long but the checkered and spotted whale shark is the largest fish known. It may grow to 50 feet (15.2 meters) long and weigh 15 tons or more. Among water animals, only the true whale, which is not a fish, grows larger. In addition to size, the whale shark resembles the true whale on the way it feeds. Small sea animals are strained from the water through many fine teeth.
The giant white or man-eating shark reaches 30 feet or more. The hammerhead of the Pacific Ocean has long, flat skull extensions on either side of its head. Eyes are on the ends of these extensions. The thresher sharks are a large with a very long projection from the top or dorsal side of its tail. It uses this to stir up prey by swishing it back and forth among a school smaller fish.
Most sharks are flesh eaters, feeding upon fish of various sizes. The larger ones will kill and eat sea mammals such as seals and porpoises. A few, as the nurse shark in Florida waters, are scavengers feeding upon animal and plant remains.
Sharks belong to the older of two classes of fish with jaws. As far as is known from the study of fossil fish, sharks have changed very little from the other group of jawed fish, the so-called true or modern fish.