Which marine mammal uses a rock as a tool to smash open shellfish?

The marine mammal that uses a rock as a tool to smash open shellfish is the sea otter. Floating on its back, it can place a stone on its chest and pound mussels or other hard-shelled prey against it like an anvil.

Sea otters also use rocks as hammers. They may dive down to pry abalones from underwater rocks, striking the shell repeatedly until the animal is freed. Some otters keep a favored stone in a loose pouch of skin beneath the foreleg, allowing them to carry the tool between dives and reuse it during feeding.

This behavior is not universal or equally frequent in every population. It depends partly on local diet and on what young otters learn from their mothers. Californian sea otters commonly use stones on mussels, abalones, sea urchins, and crabs, while populations that eat more fish may use tools less often. Sea otters can also improvise with shells, driftwood, or human-made objects.