What are the differences between a dolphin, a porpoise and a whale? As it turns out, what appears to be a simple question is not easily answered. Certain dolphins are called whales, including the largest dolphin - the killer whale, or orca. Some fishermen tend to use the word porpoise for all dolphins and porpoises, while others use the word dolphin for both. To confuse matters further, there happens to be a white saltwater fish that is referred to as dolphin; it is a popular restaurant fish that tastes like snapper or cod. Also called mahi-mahi, the dolphinfish has nothing to do with the air- breathing mammals we know as dolphins and porpoises.

True dolphins and porpoises, as well as whale, belong to the order of mammals known as Cetacea, which comes from the Greek word for sea monster. Cetaceans are either baleen or toothed whales, a division that is based primarily on feeding habits.

Baleen whales strain huge mouthfuls of small fish, shrimp-like crustaceans and other tiny prey from the water. the strainers are plates that grow from the roof of the mouth. The large whales, such as the blue and the bowhead, are baleen whales.

Toothed whales hunt individual prey and swallow it whole. Dolphins and porpoises are all toothed whales. With the exception of the sperm whale and certain beaked whales, toothed whales are smaller in size than baleen whales.

There are only six porpoise species, sometimes called "true porpoises," four of which live in the northern hemisphere. Porpoises are smaller in size than whales and dolphins and have a low dorsal fin (one species has no dorsal fin at all) and, most distinctively, no beak. Most are shy, live close to shore and are less likely to ride alongside the bows of ships than are dolphins.

The two main groups of dolphins are the river dolphin, a group that includes 4 species in the northern hemisphere, and the oceanic dolphin, with 22 species. The oceanic dolphin is the largest family of toothed whales and contains most of the species presented in this database. The basic difference between the two is that the river dolphin lives in rivers and the oceanic dolphin lives mostly in the open ocean, but there are some exceptions.

There are other important differences as well. Compared with the oceanic dolphin, the river dolphin generally has smaller teeth and more of them, a longer beak and poorer vision. The oceanic dolphin is the creature most of us picture when we hear the word dolphin. The bottlenose and common dolphins are two of the best-known oceanic species. Also included, however, are six dolphins that have the word whale in their names: melon-headed whale, pygmy killer whale, false killer whale, killer whale, long-finned pilot whale and short-finned pilot whale. Indeed, all of these species are toothed whales - as are all other dolphins and porpoises, but they are also members of the family of oceanic dolphins.

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