Octopuses have made themselves at home in most of the world's oceans—from the warmest of tropical seas to the deep, dark reaches around hydrothermal vents. Antarctic species, such as Turquet's octopuses (Pareledone turqueti), even live slow, quiet lives near the South Pole. But these retiring creatures offer a rare opportunity to help understand how this extreme part of the Earth has changed in recent geologic times—and what climate change might bring there in the near future.
Researchers can compare genetic patterns of current animal populations to look back in evolutionary time to estimate when populations of animals might have split off. These fissures are often forced by changing climatic or geographical features, such as giant sheets of ice that come and go with different glacial patterns. The West Antarctic Ice Sheet and some low Antarctic land currently separates the Weddell Sea from the Ross Sea in the Southern Ocean.
Research has suggested that this ice shelf has collapsed a number of times in the past—likely during the Pleistocene interglacial periods, most likely starting some 1.25 million years ago. This melting, along with rising sea levels, would have opened up a seawater thruway between the Weddell and Ross seas for marine life. So scientists have been turning their attention to contemporary species in the two seas to see if they could track their evolutionary history back to a time when these disparate populations might have been connected.
"We wanted to investigate whether there was any genetic information that could tell us what the past environment could have been like," Louise Allcock, of the National University of Ireland Galway's zoology department, said in a prepared statement. And for that, she and her colleagues turned to the benthic Turquet's octopus, which lives as deep as 1,000 meters on the seafloor in the Southern Ocean.
These particular polar octopods, which grow to only about 15 centimeters in length, are found all around the continent, but they don't like to stray too far from home, remaining on the ocean floor most of the time and swimming only in short bursts to escape predators.
"This octopus species, with its large populations around the region and limited movements, was an ideal species to use," Allcock said.
And unlike most octopus species, Turquet's octopus lays relatively few, large eggs (between 22 and 60, each about 20 millimeters long). So when they hatch, instead of floating up into the water column like plankton, as most species of octopus larvae do, these more massive little 'uns start living on the sea floor like their parents. This lifestyle prevents them from dispersing in great numbers with the currents. Plus, both the Weddell and Ross seas have their own circular currents (gyres) that tend to keep any organisms in the area. These factors mean that population pockets of this octopus likely do not mix with others and thus each population would be expected to have developed different genetic signatures across generations if they had been separated for a long period of evolutionary time.
The research team used data collected by the Census of Antarctic Marine Life from more than 450 individual Turquet's octopuses from around the continent's edges. Through that they were able "to examine genetic data on a scale that had not been done before in this area of the world," Phill Watts, of University of Liverpool's Institute of Integrative Biology and co-author of the study, said in a prepared statement. The new findings were published recently online in Molecular Ecology.
And when they did look into the genetic code they found something surprising. "We expected to find a marked difference between Turquet's octopuses living in different regions of the ocean," Watts said. But they found that the genetics of the two populations were actually very similar, "suggesting that at some point in their past these populations would have been in contact with each other, perhaps at a time when the oceans were connected and not separated by the West Antarctic Ice Sheet," he said.
The team found genetic evidence of a large population growth about 1902 near South Georgia and around 1829 at Shag Rocks. And the DNA data also suggests that These tough little critters seem to have been able to ride out even the cold glacial maximum periods in these and other areas.
The new findings shore up "with climate models indicating repeated periods in history when the climate was warmer, which would have released water from the ice and increased the sea levels, allowing dispersal of creatures between the Ross and Weddell seas," Watts said.
Not only does this new genetic picture help support climate patterns of the past, but "it also provides further evidence that scientists should continue to raise awareness about the impact of climate change on Antarctica today," Allcock said. So these octopuses might not be psychic, but they might help us place bets on whether they're likely to be reunited once again in the future near.
Illustration courtesy of Ivan Phillipsen