Taxonomy and Evolution of Sea Otters

Sea otters (Enhydra lutris) are members of the Mustelidae family, a monophyletic basal group of arctoid carnivores. They are the only member of the otter clade (subfamily Lutrinae; 13 extant species) that is fully aquatic (i.e., foraging, giving birth, raising offspring) in the marine environment, although they may occasionally rest on land. Lutrinae diverged from other mustelid lineages ~8 Mya in Eurasia. The earliest lineage to diverge from Eurasian otters ~5 Mya were the ancestors of sea otters (Enhydra). From the late Miocene to early Pliocene, a sister group of modern sea otters, Enhydritherium, was present in Eurasia. One hypothesis is that this group dispersed around the northern rim of the North Atlantic Ocean into the Gulf of Mexico. From there, E. terraenovae disseminated into the Pacific Ocean through the Central American Seaway. There it presumably gave rise to Enhydra (3–1 Mya), which is found only in the North Pacific Ocean. An alternative hypothesis is that Enhydra evolved in the North Atlantic and entered the North Pacific through the Arctic Ocean and Bering Straits. Today there exists one species of sea otter with three subspecies based on geographical and morphological differences.