Which marine mammal uses a rock as an anvil to open shellfish?

The sea otter is the marine mammal known for using a rock as an anvil to open shellfish.

Floating on its back, a sea otter places a stone on its chest and repeatedly smashes mussels, clams, crabs, or other hard-shelled prey against it. It may also use a rock as a hammer to pry abalones from the seafloor or surrounding rocks. A loose pouch of skin beneath the foreleg can store both food and a favored tool.

This behavior is especially well documented in California sea otters, although tool use varies among populations and individuals. Some otters use stones frequently, while others rely on their teeth and paws or target softer prey. Mothers can pass useful foraging techniques to their pups, contributing to local traditions.

The rock is not the only tool in a sea otter's repertoire. Observations include the use of shells, driftwood, discarded containers, and kelp. Kelp can wrap or anchor prey, while stones provide a hard surface for cracking shells and extracting edible flesh.