Otters are carnivores and eat mostly fish and invertebrates. River otters eat mostly fish, frogs, crayfish, crabs, and mollusks, but will occasionally eat a small mammal or bird. Sea otters mostly eat sea urchins, abalone, crabs, fish, octopuses, mussels, and clams — and they’re well-known for their impressive ability to open shellfish by using rocks as tools.