Sea otters commonly use rocks as tools to crack open shellfish. Floating on their backs, they place a stone on their chest and pound mussels, clams, crabs, or other hard-shelled prey against it until the shell breaks.
A rock can serve as an anvil, but otters also use stones as hammers. They may pick up a large stone from the seafloor and strike an abalone to dislodge it from rock. Some otters carry a favored stone in a loose pouch of skin beneath a foreleg, keeping the tool available during later feeding.
Rocks are the classic answer, although sea otters are flexible tool users. Researchers have recorded them using empty shells, driftwood, pieces of crab shell, and even human-made objects. Tool use also varies among populations and individuals, partly because young otters learn foraging techniques from their mothers.
This behavior is especially useful because sea otters have no blubber and must eat frequently to fuel their metabolism. Their tool-assisted feeding helps them exploit prey that would otherwise be difficult to open with their paws or teeth alone.