While river otters and sea otters might appear similar at first glance, the two exhibit many striking differences beyond their aquatic habitats. Although the term sea otter refers to only one species—Enhydra lutris—river otter applies to multiple species in the genus Lontra, which includes the well-known North American river otter (L. canadensis), as well as otters of the genera Aonyx, Hydrictis, Lutra, Lutrogale, and Pteronura. Sea otters and the various river otters are all members of the weasel family, Mustelidae.

　　Let’s dive in and explore some of the physical and behavioral differences that distinguish sea otters from the river otters of the genus Lontra.

　　Physical differences

　　river otterNorth American river otter (Lontra canadensis).(more)One major difference between sea otters and river otters is their size. Although they are the smallest of all marine mammals, sea otters are far heftier than their riverine counterparts; males can reach about 41 kg (90 pounds) and females 27 kg (60 pounds). River otters weigh about 4.5–15 kg (10–33 pounds), depending on the species, and females are smaller than the males.

　　Beyond their weight, sea otters also have longer bodies than river otters, though their flat tails are shorter. River otters boast a sleeker build than sea otters, and their long pointed tails can make up about a third of their body length. River otters use their long pointed tails and all four paws to swim, whereas sea otters rely mostly on their short flat tails and two back paws to navigate in water.

　　sea otterSea otter (Enhydra lutris).(more)Fur thickness is another key physiological difference between river and sea otters. Sea otters have a uniquely thick pelt, with about 26,000 to 165,000 hairs per square centimeter (about 170,000 to 1,000,000 per square inch)—the densest coat of any living mammal species. This thick layer of fur likely evolved in order to help sea otters conserve body heat, supplying the insulation that protective blubber provides in other marine mammals. Sea otters were nearly hunted to extinction for their incredible pelts in the 18th and 19th centuries and remain an endangered species.

　　Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now Behavioral differences A number of behavioral elements also distinguish river and sea otters. Exclusively a marine species, sea otters consume food found in their aquatic habitats, such as sea urchins, crabs, and shellfish. They characteristically float on their backs and utilize rocks as tools to crack their hard-shelled prey. To sleep, they float on their backs, sometimes holding onto each other to form “rafts,” which prevent them from drifting away.

　　Eurasian river otterA Eurasian river otter (Lutra lutra) plays underwater.(more)Unlike sea otters, river otters are able to spend significant time on land and are opportunistic, feeding on whatever is most easily obtained in rivers, on land, and even in brackish or saltwater environments in coastal areas. They often hunt for fish, frogs, and crayfish and forage for crabs and mussels under rocks.

　　sea ottersA mother sea otter (Enhydra lutris) with her pup in Prince William Sound, Alaska.(more)River otters reach sexual maturity between two and three years of age and often produce multiple pups in each litter. Sea otters mature more slowly and produce only a single pup at a time. Sea otters frequently position their pups atop their stomachs to guard them while swimming. On hunting trips, mothers sometimes wrap their offspring in a protective layer of kelp and leave them to float along the surface; very young baby sea otters are unable to sink because of their buoyant fur. River otters, in comparison, travel with their pups alongside or shelter them in a burrow along a riverbank.