sea otters most commonly use stones as anvils. they do not express a preference for the shape of the stone or rock, but they do choose fairly large stones, between , to rest on their chests. while all sub-species of otters use the forelimbs to rip open urchins, the californian otters were observed to have used rocks as a surface to pound urchins and crabs, as well as mussels (observed most frequently). otters seem to rise from a dive and immediately lie on the surface of the water with their chests up and place a stone on the chest to function as an anvil. the average length of a dive is 55 seconds. otters hold mussels so as to orient the flat sides of the mussels against the "palms" of the paws and the seam of the two shells contacts the stone in a pounding instance. mussels are pounded against rock or stones at a rate of two pounds per second. it takes approximately 35 blows to crack open a mussel.