Marine mammals are a diverse group of roughly 120 species of mammal that are primarily ocean-dwelling or depend on the ocean for food. They include cetaceans(whales, dolphins, and porpoises), sirenians (manatees and dugongs), pinnipeds (true seals, eared seals and walrus), otters (sea ottera and marine otters) and polar bears.The order Cetacea contains ninety species, of which five are truly freshwater dolphins. The order is divided into two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins and porpoises). In India we find 31 species of cetaceans and one species in the Order Sirenia.
Marine mammals evolved from land dwelling ancestors and share several adaptive features for life at sea such as a generally large size, hydrodynamic body shapes, modified appendages and various thermoregulatory adaptations. Their body is fusiform (spindle-shaped). The forelimbs are modified into flippers. The tiny hindlimbs are vestigial; they do not attach to the backbone and are hidden within the body. The tail has horizontal flukes. Different species are, however, adapted to marine life to varying degrees. The most fully adapted are the cetaceans and the sirenians, whereas the other groups spend at least some time on land.Pinnipeds, Marine and Sea otters and Polar bears are temperate species and not found in our waters. Estuarine and freshwater otters are found in India.