The sea otter is famous for using rocks as tools to open hard-shelled prey such as sea urchins, crabs, and mussels.
A sea otter may float on its back, place a stone on its chest, and strike prey against the stone until the shell breaks. Some individuals keep a favorite rock in a loose pocket of skin beneath the forearm. Tool use is not the species’ only specialized behavior: sea otters also use their forepaws to manipulate food and groom their dense fur.
Sea otters have the densest fur of any mammal, with up to roughly a million hairs per square inch in commonly cited estimates. Because they lack the thick blubber layer used by many marine mammals, they rely on that fur and a high metabolic rate for insulation.
By eating sea urchins, sea otters can help maintain kelp forests. Their historical range was greatly reduced by the fur trade, and many populations remain protected or threatened. They are mustelids, related to weasels and river otters, not seals.