The fisher, tayra, and martens are partially arboreal, while badgers are fossorial. A number of mustelids have aquatic lifestyles, ranging from semiaquatic minks and river otters to the fully aquatic sea otter, which is one of the few nonprimate mammals known to use tools while foraging. It uses "anvil" stones to crack open the shellfish that form a significant part of its diet. It is a "keystone species", keeping its prey populations in balance so some do not outcompete the others and destroy the kelp in which they live.