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Dolphin aquatic mammal, also called smaller toothed whales, of the family Delphinidae (marine true dolphin) and Platanistidae (freshwater dolphin), order Cetacea. The order also includes whales and porpoises. Dolphins are often mistakenly called porpoises. Dolphins, meaning beak, have a beak-like snout and slender body, whereas the porpoise (family Phoceonidae) is smaller with a blunt snout and stocky body. The name dolphin is also applied to a species of fish, called Dolphin Fish, which is a large and swift game fish, Coryphaena hipparus, and is distributed nearly worldwide in warm waters.

Ganges River Dolpin

There are about 32 species of true marine dolphins, and 5 freshwater species found in rivers of South America and southern Asia. The river dolphins resemble their ancestor whales, they have long beaks with whiskers, and the neck vertebrae are not fused, and grow up to a length of about 2.7 metres. They live in turbid waters. Their sense of sight is limited, and the animals are reliant on echolocation. The eyes of the Ganges River Dolphin, Platanista gangetica (also found in Bangladesh) lack lenses; it swims on its side and feels for the bottom with one flipper, and picks up bottom-dwelling crabs and fish. The Indus River Dolphin hunts for catfish while swimming on its back.

Bangladesh has 10 species of marine and six species of freshwater dolphins. Of the four marine species, two are critically endangered, and two are data deficient (status could not be evaluated due to paucity of data); the only freshwater species, Platanista gangetica, is endangered.

Most species of dolphins are highly gregarious, and feed chiefly on fish. They breathe air through a single, dorsal blowhole. They are fishlike in form, and have streamlined, hairless bodies. They have 160-200 sharp teeth. The eyes are of moderate size, the ear aperture is minute, and the blowhole crescentic. Most dolphins are black above and white below. The female gives birth to a single young and is a devoted mother. 

Dolphins have long been famous for riding the bows of ships, and it is now known that they also ride the bows of large whales. Today they are valued and exploited as entertainers in more than 40 water shows around the world and have thus become available for extensive scientific study. The best known are the common dolphin (Delphinus delphis) of worldwide distribution (Bangladesh has also this species in its sea-waters), and the bottle-nosed dolphin (Tursiops truncatus), found in the coastal waters of the North Atlantic Ocean and the Mediterranean Sea. The common dolphin averages 2.4 m in length and 75 kg in weight, and has a life span of about 50 years.

Dolphins produce an enormous variety of sounds, up to frequencies ten times those heard by human beings. Each dolphin has a signature whistle with which it identifies itself; a calf soon learns to recognize its mother's whistle. Clicking and rapid creaking sounds are the basis of the echolocation mechanism (sonar) with which the dolphin gathers extremely precise information about the size, location, and nature of surrounding objects. They can convey instructions among each other. The US navy has trained dolphins to act as messengers to underwater stations, to rescue wounded frogmen and protect them from sharks, and to seek and destroy submarines.

Dolphins are edible, and until recently dolphins formed the basis of a widespread fishing industry; only the Japanese continue to hunt them for food on a large scale. However, not every society has exploited cetaceans for food. In the Mediterranean, the Ancient Greeks had obviously incorporated dolphins into their culture. Nevertheless they are accidentally caught and killed in large numbers in fishing operations. Smaller ones are caught in gill nets, trawls, purse seines, set nets, and long-line fisheries throughout the world. [Md Anwarul Islam] See also Aquatic Mammal; Cetacean; Mammal; Porpoise.