Porpoise a group of purely aquatic marine mammals related to the dolphins and whales. They are actually small toothed whales included in the family Phoeconidae under the order Cetacea. Porpoises have streamlined body with strong forelimbs modified as flippers and horizontally set tail flukes; generally possess well-developed centrally located dorsal fin with the exception of the Finless or Indian Porpoise (Pakhnahin Shushum Mach), Neophocaena phocaenoides that has no dorsal fin. All porpoises are beakless and have similar looking spatula-shaped teeth on both the jaws. They have no hind limbs and hairs excepting occasional bristles. Two nasal cavities have become fused to form a single blowhole. All cetaceans breathe fresh air.
Porpoises usually eat fish; use some kinds of ultrasonic sonar to locate their prey. A high-frequency clicking sound released by them hit objects that bounces the echoes to the porpoise, which can at once estimate the size, distance and speed of movement of the objects. Porpoises are social animals, often living in groups or ‘pods’ and communicate with each other through the emission of clicks and whistles that are not audible to human ears
Porpoises produce single baby after a gestation period of one year or so. The baby does not suck milk from the mother's teat but receives squirts of it into its mouth. Pakhnahin Shushum Mach is the only porpoise species that is found in Bangladesh which lives in the Sundarbans, Hatiya, Maheshkhali and Kutubdia estuaries and the River Naaf in small number. Finless Porpoise is the smallest cetacean and measures about 1.5 m. It has 15-21 teeth; forehead melon-shaped; body colour varies from grey to black and whitish underside. The species is rare and is in Appendix-I of CITES (Convention on International Trade on Endangered Species of Fauna and Flora); also protected under Bangladesh Wildlife Preservation Act (1974). [Ali Reza Khan]