Which marine mammal uses tools to crack open shellfish?

The sea otter is the marine mammal famous for using tools to crack open shellfish. It commonly floats on its back, places a stone on its chest, and repeatedly smashes a clam, mussel, or other hard-shelled prey against the stone until the shell breaks.

This behavior is especially useful because sea otters lack the powerful jaws and thick teeth of many shell-crushing predators. Individual otters may carry a favorite stone in a loose pocket of skin beneath the forearm, then reuse it during later meals. They can also use rocks to pry prey from reefs.

Sea otters are unusually dependent on their fur rather than a thick layer of blubber for insulation. Their extremely dense fur traps air and keeps cold water away from the skin, so constant grooming is essential. They eat large amounts of food to fuel their metabolism and often consume roughly a quarter of their body weight daily.

Sea otters are not seals or dolphins. They are members of the weasel family, Mustelidae, and are closely related to river otters, badgers, and martens.