A baleen whale (order Cetartiodactyla, parvorder Mysticeti) that grows to 30 m in length and weighs more than 170 tonnes, making it the largest extant animal. It has a long, tapering body with a small dorsal fin. It is usually alone or with one companion. Blue whales feed almost entirely on krill, of which an adult can eat up to 3.5 tonnes a day, and they occur in all oceans. There are three or four subspecies. Having been hunted almost to extinction, they were afforded protection which has led to their numbers increasing.