Fish | Dolphin | Dolphins are marine mammals that are closely related to whales and porpoises. Dolphins are largely marine cetaceans (order Cetacea) with many teeth that belong to the "toothed whales" suborder Odontoceti, along with whales and porpoises, and they usually have a beaklike snout. (The other cetacean suborder is Mysticeti, the baleen whales.) There are freshwater representatives among these aquatic mammals.
In this article, dolphin will refer to any member of Delphinidae (oceanic dolphins) and Platanistoidea (river dolphins). Porpoises (family Phocoenidae) are thus not dolphins in this sense. Orcas (killer whales) and some closely related species belong to the Delphinidae family and therefore qualify as dolphins, even though they are called whales in common language.
There are almost forty species of dolphin in 17 genera. They vary in size from 1.2 m (4 ft) and 40 kg (90 lb) (Maui's dolphin), up to 9.5 m (30 ft) and 10 tonnes (9.8 long tons; 11 short tons) (the orca or killer whale). They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating fish and squid. The family Delphinidae is the largest in the Cetacean order, and evolved relatively recently, about ten million years ago, during the Miocene. Dolphins are among the most intelligent animals, and their often friendly appearance and seemingly playful attitude have made them popular in human culture.
As with other mammals, cetaceans are warm-blooded, breathe air, have hair at least some stage in their development, and feed their young on milk. Dolphins all give birth to live young.
Dolphins are considered to be among the most intelligent of animals, and their graceful movements, often friendly appearance, seemingly playful attitude, and reputation for often positive interactions with people have made them popular in human culture. On the other hand, the gap between human responsibility to care for nature and actual actions is evident in anthropogenic threats to dolphins, including being caught in fishermen's nets, being hit by propellers, and having their habitats damaged by pollution. All of the river dolphins are endangered.
There is no strict definition of the term dolphin (Grzimek, 2004) and distinguishing whales, dolphins, and porpoises among the Odontoceti is difficult. Body size is useful, but not a definitive distinction, with those cetaceans greater than 9 feet (2.8 meters) generally called whales; however, some "whales" are not that large and some dolphins can grow larger (Grzimek, 2004). Scientifically, the term porpoise should be reserved for members of the family Phocoenidae, but historically has been often applied in common vernacular to any small cetacean (Grzimek, 2004).
Dolphin has been used either to members of the dolphin family or any member of the family Delphinidae and Platanistoidea. It was also used for any member of the odontocetes, including families above and some others. Dolphin is used casually as a synonym for dolphin, the most common species of dolphins and familiar. - FISH