Unlike whales and dolphins that have lived in the marine world for more than 50 million years, sea otters have only had about 5 million years to develop the suite of adaptations necessary to survive the harsh conditions of life in the sea. What happened genetically to assist in that incredible transition?
Researchers from the Smithsonian Conservation Biology Institute along with the University of California, Los Angeles, and additional partners are studying both sea otters and comparing them to a freshwater otter species, the South American giant otter, to better understand what happens at the genetic level when a terrestrial species evolves rapidly from land to freshwater to sea life.
“We are interested in the rapid evolution of otters as it gives us a snapshot into an ongoing transition of a mammal to aquatic environments,” says Klaus-Peter Koepfli, SCBI research scientist and co-author on a study published June 18 in the scientific journal, Molecular Biology and Evolution. “It is exciting to see how quickly adaptations can occur during an evolutionary transition from a terrestrial weasel-like ancestor to the marine and freshwater otters we see today."
The study, which is among the first genomic studies on otters, found that both the sea otter and giant otter have lost a considerable number of genes related to the sense of smell — a faculty that is not essential underwater. Instead, over time sea otter evolution has selected for genes that enable their marine life — such as genes to increase the bone density of their limbs to help with diving by acting as ballast (like a scuba diver’s weight belt).
And unlike seals and whale species that have a layer of fat to insulate them from the cold temperatures associated with a marine lifestyle, sea otters have instead evolved a thick layer of fur — the densest of any living mammal, with as many as a million hairs per square inch. The study found that the unique fur of sea otters may be due to a shared set of very small changes across many genes, rather than substantial changes to only a few genes.
Unfortunately, sea otters were hunted almost to extinction for their dense fur in the 18th and 20th centuries, creating a genetic bottleneck that the species is still trying to recover from today. A lack of genetic diversity can cause health and reproductive issues in a species. Interestingly, while the study confirmed that genetic bottleneck, the findings suggest that the sea otter may have experienced earlier genetic bottlenecks, perhaps linked to changes in climate.
“Understanding the history of these fluctuations in population size and their impact on genetic diversity can give us insights into what sea otter populations have gone through in the past, and what they may be able to withstand in the future,” says Annabel Beichman, a Ph.D. candidate at the University of California, Los Angeles, and lead author on the study. “The combination of these declines can impact the amount of harmful genetic variation in the population, which can be an important factor in species survival.”
While the sea otter is almost entirely aquatic and lives in cold coastal marine environments of the North Pacific Ocean, the giant otter is a semi-aquatic species that lives along freshwater streams, rivers and lakes of South America. Habitat destruction, pollution, predation, disease and lack of genetic diversity are the biggest threats to the two species today, with both populations continuing to decline in the wild.
The researchers are now collaborating with the Monterey Bay Aquarium and U.S. Fish and Wildlife Service to communicate their results to sea otter managers. They’re also using the genomic resources from this study to assess the genetic diversity of more than 130 individual sea otters across the species’ range, information that will be incorporated into sea otter recovery goals.
“The sea otter is a remarkable case study for understanding the impact of extreme, recent bottlenecks on a species that is critical to the health of its ecosystem,” Koepfli says. “Sea otters are essential for healthy kelp forests due to their predation of sea urchins. Without sea otters, coastal ecosystems can become barren. This history of sea otter populations, and their future recovery, is therefore of critical importance for coastal ecosystems.”
The paper’s other authors are Gang Li, Shaanxi Normal University; William Murphy, Texas A&M University; Pasha Dobrynin, SCBI; Sergei Kliver, Saint Petersburg State University; M. Tim Tinker, University of California, Santa Cruz; Michael Murray, Monterey Bay Aquarium; Jeremy Johnson, Kerstin Lindblad-Toh and Elinor Karlsson, Broad Institute of MIT and Harvard; Kirk Lohmueller and Robert Wayne, University of California, Los Angeles.