There are 32 species of dolphins, members of the cetacean family Delphinidae, that are distributed in all of the oceans of the world. These marine mammals are usually found in relatively shallow waters of coastal zones, but some may be found in open ocean. Dolphins are a relatively modern group; they evolved about 10 million years ago during the late Miocene. The Delphinidae represents the most diverse group, as well as the most abundant, of all cetaceans. Among the delphinids are the bottlenose dolphins (Tursiops truncatus), best known for their performances in oceanaria; the spinner dolphin (Stenella longirostris), which have had their numbers decline due to tuna fishermen's nets; and the orca or the killer whale (Orcinus orca), the largest of the dolphins. Dolphins are distinguished from their close relatives, the porpoises, by the presence of a beak.

A playful bottlenosed dolphin. (Photograph by Stephen Frink. Corbis-Bettmann. Reproduced by permission...