Whales are much larger than most dolphins and porpoises. Scientists apply the term dolphin to two families of cetaceans, marine dolphins and river dolphins. There are 32 species of marine dolphins. They are found in nearly all oceans, and most of them live only in salt water. Many species of marine dolphins remain near land for most of their lives, but some live in the open sea.
River dolphins live in fresh or slightly salty water.
This article focuses on marine dolphins. For information about river dolphins, see River dolphin . The most familiar types are the bottle-nosed dolphin and the common dolphin. Its short beak gives this dolphin an expression that looks like a smile. Most performing dolphins in aquariums and zoos are bottle-nosed dolphins.