For otters that spend almost all of their time in water, such as the sea otter, their diet consists primarily of fish, crustaceans, and molluscs. Their teeth are particularly suited to eating shellfish, allowing them to crush the shells and scoop out the meat. Sometimes, to eat crustaceans and molluscs, otters hold a rock to their chest and repeatedly strike their prey against the rock until it breaks or splits open. Otters have a high metabolic rate because they need to maintain their body temperature. To generate body heat, they must eat a significant amount of food. River otters eat about 15-20% of their body weight each day, while sea otters eat about 25-30% of their body weight each day. Sea otters require more food to maintain body temperature because they have no fat, unlike most marine animals, and they spend much more time in the water than other otter species, which causes them to lose body heat