Sea otter problems predate the fur trade

Sea otters (Enhydra lutris) were in a genetically perilous position long before the fur trade pushed the species towards extinction, genetic research reveals.

They represent an evolutionary line of land-dwelling mammals that returned to the sea sometime around 30 million years ago. This is a comparatively late re-entry, with other groups of mammals – notably the ancestors of whales and sea cow species such as dugongs (Dugong dugon) – taking the plunge (so to speak) up to 20 million years earlier.

Despite its comparatively short time in the ocean, the sea otter has evolved several specific adaptations to its environment, including changes to limb shape and a dense insulating fur. 

It was this last development that nearly resulted in its demise, because of heavy hunting from the eighteenth to the twentieth centuries to service the demands of the fur trade. The surviving otters – low in numbers and restricted in distribution – represent an extreme evolutionary bottleneck, a brake on the sort of genetic diversity generally regarded as a key component of species robustness.

To better assess the implications of the species recovery since protections were put in place, by gaining a more thorough understanding of its evolutionary past, researchers led by Annabel Beichman of the University of California, Los Angeles, US, analysed the genome of a subspecies, the southern sea otter (E. l. nereis).

The results were surprising. By comparing the genome to that of a related species, the freshwater giant otter (Pteronura brasiliensis), the researchers were able to identify helpful mutations that resulted in limbs well adapted to ocean life, and the thick coat.

Interestingly, they also found evidence of “extensive pseudogenisation”, a term that indicates loss of functionality in some genes – in this instance, primarily in those associated with the sense of smell. A reduced ability to detect odours is a common property of marine mammals.

As expected, the sea otter genome showed “extremely low genomic diversity”. However, Beichman and colleagues dated the start of the reduction to millions of years before the human hinting began. It seems that the sea otter has long been a species subject to population declines, consequent bottlenecks and the loss of genetic options.

The investigation also found evidence of recent inbreeding, the result of the fur trade numbers crash, adding further concerns to an already fragile base.

The findings, published in the journal Molecular Biology and Evolution, carry significant implications for the management and protection of the animals.

“These declines pre-date the fur trade and appear to have resulted in an increase in putatively deleterious variants that could impact the future recovery of the sea otter,” the researchers conclude.