Sharks are a group of carnivorous ocean going fish with 368 different species in a total of 30 families. Some species of shark, such as the great white, are infamous among humans because they have been involved in attacks resulting in severe injury and sometimes death. In general, however, sharks are shy fish, who will not attack unless provoked.
Sharks have been around since prehistoric times, with fossils indicating that sharks have maintained their basic shape and evolutionary adaptations for millions of years. The fish is simple, efficient, and relatively intelligent, with evidence suggesting that sharks can acquire skills on par with many land animals. Some sharks are endangered due to heavy human trade in their fins, including many of the large species of shark.
Sharks range in size from the pygmy shark, which is seven inches (18 centimeters) long, to the whale shark, which can reach 50 feet (15 meters) in length. Most sharks have gray upper bodies and pale lower bodies for camouflage purposes. Sharks have several other characteristics which distinguish them from most fish, beginning with their skeleton, which is formed from cartilage rather than bone. This makes sharks stronger and more flexible that many fish species.
Sharks also have rough, sand-papery skin which can be quite painful to the touch. In addition, shark teeth will grow back if dislodged, guaranteeing the fish a formidable life-long dentition. Many sharks have slightly serrated teeth as well. Sharks have multiple gills on each side, with some species having as many as eight, while most range between five and six.
Most sharks have streamlined bodies designed for moving efficiently through the water. Many species have a projecting dorsal fin, and all have split caudal or rear fins. The shark can move very rapidly in search of prey, and depending on the size of the fish, sharks eat small food, plankton, shellfish, and crustaceans.
Sharks can be found all over the world, inhabiting temperate waters as well as tropical, with some species favoring shallow water. Shallow water species can pose a risk to humans, as the sharks will feel threatened by swimmers, surfers, and the like, and may attack if they feel cornered, or if they mistake the human for prey. Swimmers should avoid areas known to harbor sharks, as well as being aware of their environment. If attacked, biologists recommend that victims fight back in every way possible.
Shark meat is consumed by many cultures, although the rising levels of mercury found in shark meat make this consumption inadvisable. The fins of the shark are highly prized, leading to a global threat to shark species.