Most sharks are solitary animals, though a few, such as the spiny dogfish shark, form schools. Sharks may bite when provoked, but fewer than 30 species are considered dangerous, regardless of the situation. The two largest species, the whale and basking sharks, are harmless plankton feeders.
The so-called feeding frenzy, wherein sharks stimulated by the smell of blood feed ravenously and attack any object within reach, is one occasion when different species may be observed together. These feeding frenzies are infrequent. Some authorities doubt whether they occur naturally or only when provoked by humans who supply large quantities of food to attract sharks for studies on feeding behavior.
Sharks will attack humans at any time of day, in warm or cold water. Although most attacks are recorded during daylight hours in shallow warm waters accessible from a public beach, these statistics may simply reflect the fact that these are the conditions in which the greatest numbers of swimmers are found. The waters of coastal North America, South Africa, and the Caribbean and Mediterranean seas are the most frequent sites of shark attacks.
Large sharks, such as the great white, tiger, and bull sharks, that include human-sized prey in their diet are the most dangerous. Hammerhead, gray reef, lemon, dusky, blue, spinner, sand tiger, nurse, and Ganges River sharks will also attack humans.
Sharks will attack when they are hungry, but in most cases the reason for attack is unknown. Possible causes include territorial defense, mistaken identity for some other form of prey (this might explain why a shark often ceases its attack after one bite), chemical attractants such as blood in the water, and simply the movement, noises, and splashing of swimmers.
The sharks, along with their close relatives, the rays, belong to the class Chondrichthyes, or Selachii. The latter name is also used for an order that includes only the sharks.
See detailed statistics as proveded by Florida Museum of Natural History: