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A dolphin is an aquatic mammal of the order Cetacea, which also includes the whales and porpoises.
The word is used in a few different ways. It can mean:
- Family Delphinidae (oceanic dolphins),
- Both Delphinidae and Platanistidae (oceanic and river dolphins),
- Or all of suborder Odontoceti (toothed whales).
A porpoise (suborder Odontoceti, family Phocoenidae) is distinct from a dolphin.
There are almost 40 species of true dolphin in 17 genera. They vary in size from 1.2 metres and 40 kilos (Heaviside's Dolphin), up to 7 metres and 4.5 tonnes (the Killer Whale which, despite the name, is a dolphin). Most species weigh between about 50 and about 20o kilos. They are found worldwide, mostly in the shallower seas of the continental shelves, and all are carnivores, mostly taking fish and squid.
The dolphin family is the largest in the Cetacea, and relatively recent: dolphins evolved about 10 million years ago, during the Miocene.
See also military dolphin.
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Dolphin (or more properly, dolphinfish) is also used to describe a species of fish which is unrelated to the mammal. The name is being used more and more infrequently and is has generally been replaced with the name mahi-mahi to avoid confusion with the mammal, especially since the fish is commonly eaten.