Lost history of Antarctica revealed in octopus DNA
Genes show signatures of an ice sheet collapse and warn of a precarious future
- 21 DEC 2023
- 2:40 PM ET
- BY ERIK STOKSTAD
The genomes of Turquet’s octopus, like this one, hold clues to the history of an Antarctic ice sheet.DAVE BARNES; BAS
Some 100,000 years ago, scientists believe Antarctica’s massive western ice sheet collapsed, temporarily opening waterways between a trio of seas surrounding the continent. New evidence for that scenario comes from a surprising source: octopus DNA.
The ice sheet’s collapse allowed long-separated populations of Turquet’s octopus (Pareledone turqueti) to interbreed for thousands of years; when the sheet reformed, the animals were isolated once more, a story that has been recorded in the sea creatures’ genes, researchers report today in Science. The work also bolsters concerns that a large rise in sea level may be in our planet’s near future.
“It’s a really creative approach” to uncovering Antarctica’s lost history, says Andrea Dutton, a geologist at the University of Wisconsin-Madison who studies ancient sea levels but was not involved in the study.
About 129,000 to 116,000 years ago, a warm spell called the last interglacial gave our planet a brief break in between several million years of ice ages. The average temperature of the planet was about 0.5°C warmer than it is today—and climate projections predict it will be again within decades. The global sea level was also 5 meters to 10 meters higher than current levels. Many scientists believe the collapse of the West Antarctic Ice Sheet and consequent melting could have been a primary reason.
Geological evidence suggests this collapse happened at least once in the past several million years—perhaps during the last interglacial. A team of biologists believed it could find additional evidence by analyzing DNA from creatures usually separated by the ice sheet today. Enter Turquet’s octopus, a cephalopod with a body about the length of a pencil, not including its arms. The animal lives all around Antarctica in water down to 1 kilometer deep, eating bristle worms, amphipod crustaceans, and other small invertebrates.
Researchers behind the new study took small tissue samples from 96 octopuses collected over 33 years, many of which were accidental bycatch from fishing vessels. From these, the team extracted and sequenced the animals’ DNA.
Sally Lau, an evolutionary geneticist at James Cook University, looked at broad patterns of genetic markers called single-nucleotide polymorphisms (SNPs) that could be used to subdivide the octopuses into separate distinct populations. Next, she built demographic models to test different interbreeding scenarios between ancient octopuses that would give rise to their genetic patterns today. The model assumed that several million years ago, before the ice ages, four populations of octopus—those living in the Weddell Sea, the Amundsen Sea, the Ross Sea, and in East Antarctica—were all connected by the ocean currents that encircle Antarctica, keeping the populations genetically similar to each other. But after the West Antarctic Ice Sheet grew, these populations became isolated from one another and began to accumulate genetic differences. The ice sheet expands when global temperature drops, creating more ice, which lowers sea level.
Lau used her models to predict how different ice sheet conditions during the last interglacial would have affected the octopuses’ genomes. Under one scenario the collapse would have allowed three of the populations to come together once again via seaways, leaving the East Antarctica octopuses isolated. Another scenario assumed the ice sheet only partially collapsed, which would join only the populations in the Weddell and Amundsen seas. A third final scenario posited that the ice sheet stayed intact and populations remained isolated from each other until the present day.
After running the model simulations many thousands of times, Lau found that the ice sheet total collapse scenario best matched to the SNP patterns seen in octopuses’ genomes today. Based on the animals’ average generation times and mutation rates, the researchers calculated that these different populations of octopuses resumed interbreeding sometime between 139,000 and 54,000 years ago.
The findings are consistent with growing geological evidence supporting the ice sheet collapse. In 2019, for example, a scientific drill ship pulled up sediment cores from Iceberg Alley, a primary escape route for Antarctic icebergs heading northward. The cores show a huge increase in iceberg-carried debris during the last interglacial, supporting the likelihood that there was a massive collapse during the last interglacial, says Claire Jasper, a graduate student at Columbia University, who presented the work last week at a meeting of the American Geophysical Union. The new octopus genome data, she adds, “is pretty convincing evidence that a full collapse happened.”
The findings reinforce the importance of understanding how modern climate conditions are affecting the West Antarctic Ice Sheet, Dutton says. “This is telling us that we need to take this bigger picture seriously.” Continued ocean warming—driven by greenhouse gas emissions—could destabilize the submerged portion of the ice sheet. To lower the chance of another collapse, she says, “We can’t just kick the can down the road and wait to make emissions cuts for another 5 years, another 10 years. It really demands that we do it now.”
With reporting by Paul Voosen.
