The sea otter is famous for using stones as tools to crack open shellfish.
A sea otter may place a stone on its chest and repeatedly strike a clam, mussel or sea urchin against it. Some individuals carry a preferred stone in a loose pouch of skin beneath the forearm. This behaviour is one of the clearest examples of tool use among marine mammals.
Sea otters have powerful jaws and broad molars, but hard shells can still be difficult to open. A stone provides a stable anvil or hammer, allowing the otter to reach energy-rich prey. Their thick fur, rather than a heavy layer of blubber, supplies most of their insulation.
Sea otters are not the same as river otters. They spend nearly all their lives in the ocean and are important predators of sea urchins, helping protect kelp forests from overgrazing.