The walrus (Odobenus rosmarus) is a large flippered marine mammal with a discontinuous circumpolar distribution in the Arctic Ocean and sub-Arctic seas of the Northern Hemisphere. The walrus is the only living species in the Odobenidae family. It is subdivided into two or three subspecies: the Atlantic walrus (O. rosmarus rosmarus) found in the Atlantic Ocean, the Pacific walrus (O. rosmarus divergens) found in the Pacific Ocean, and a possible third subspecies, O. rosmarus laptevi, found in the Laptev Sea. Walruses are immediately recognizable due to their prominent tusks, whiskers and great bulk. Adult Pacific males can weigh up to 4,500 pounds, and, among pinnipeds, are exceeded in size only by the elephant seals. They reside primarily in shallow oceanic shelf habitat, spending a significant proportion of their lives on sea ice in pursuit of their preferred diet of benthic bivalve mollusks. They are relatively long-lived, social animals and are considered a keystone species in Arctic marine ecosystems. Walruses have played a prominent role in the cultures of many indigenous Arctic peoples, who have hunted walruses for their meat, fat, skin, tusks and bone. In the 19th and early 20th centuries, walruses were the objects of heavy commercial exploitation for blubber and ivory and their numbers declined rapidly. Their global population has since rebounded, though the Atlantic and Laptev sub-population remain fragmented and at historically depressed levels.