But new research, led by Richard Aronson, a professor of biological sciences at the Florida Institute of Technology, finds that rising water temperatures off the coast of Antarctica, brought about by global climate change, could be pitting the delicate marine ecosystem against an unexpected threat: an invasion of shell-crushing crabs.
A study published Monday in Proceedings of the National Academy of Sciences finds that conditions are ripe for an invasion of king crabs on the continental shelf off the western Antarctic peninsula. The crabs, which are known for their habit of breaking open the outer skeletons of animals like starfish, urchins and mollusks, feed on the kinds of soft-bodied organisms living on the shelf and have the potential to wreak havoc on the local ecology.
King crabs are found on the bottom of the ocean in deep waters around the world and are also known to turn up in shallower waters in subpolar regions. A few species, such as red king crabs, are famous for growing to enormous sizes, although many other species are much more modest in size. But while they’re widespread across the globe, for millions of years they’ve been unable to venture too close to the Antarctic land mass because conditions there are too cold for them.
In general, king crabs can’t cope with temperatures much colder than 1 degree Celsius (34 degrees Fahrenheit). Shallow waters over the Antarctic continental shelf tend to be very cold, because they’re so close to the icy continent. A little farther offshore, over the continental slope, the waters warm up a bit — and then, as the waters deepen, they start to get colder again.
In recent years, scientists have been observing king crabs in the waters over the Antarctic continental slope off the western Antarctic peninsula. Temperatures there are pretty frigid but still within livable range for the crabs. So while scientists aren’t sure how long the crabs have been there — they might have moved into the area just a few years ago, or they might have been there for decades, Aronson said — it’s not wholly surprising that they’ve shown up in the area.
However, scientists weren’t quite sure how big a population existed there. So in 2010, Aronson and his colleagues conducted a photographic survey off the Marguerite Bay on the western Antarctic peninsula to find out how many crabs were present.
It turns out there are more than just a few crabs crawling around down there. The researchers found a large population, thousands strong, mostly consisting of a species of king crab called Paralomis birsteini, which grows to an average of a few inches long — and the population appears to be stable and reproducing. This in and of itself wasn’t that surprising, bcause it was already common knowledge that the crabs existed at these depths. But the researchers were also interested in finding out whether there were any barriers to the crabs moving farther up onto the continental shelf.
To find out whether the crabs could survive on the shelf, they measured environmental conditions, including water temperature, salinity and pressure. Their conclusion was that conditions are tolerable for the crabs on the continental shelf, meaning there are no barriers to the crabs moving into the shallower waters.
During the survey, crabs were observed at depths of 2,759 feet to 7,434 feet — but Aronson said they could move up to about 1,312 feet deep, which is on the lower end of the Antarctic continental shelf. This is thanks to rapid warming in the waters off the western Antarctic peninsula, which have warmed by about 1.5 degrees Celsius in the past 50 years — a warming rate that’s about double the global average.
“I think it’s really an important study,” said Craig Smith, professor of oceanography at the University of Hawaii, who was not involved with the study. In 2012, Smith published a paper describing another colony of king crabs in a different part of the western Antarctic peninsula. “It’s kind of substantiating some of the speculation and some of the other finds about king crabs and their potential to invade the continental shelf.”
If the crabs continue colonizing the area, there could be big consequences for the soft-bodied organisms living there. “These prey haven’t experienced this kind of predation in tens of millions of years,” Aronson said. So they haven’t developed adequate defenses, such as harder shells, to protect themselves from the hungry crabs. In other parts of the world, the kinds of organisms that king crabs feed on have developed much stronger skeletons that are better able to stand up to the crab’s crushing claws.
And, in fact, the crabs already seem to be making a difference in the lower depths where they’re living. During their survey, Aronson and his colleagues observed that where the crabs are most abundant, their prey animals are relatively scarce. But in shallower waters, where the crabs haven’t established themselves yet, the animals are much more abundant.
“You might think that maybe it’s the crabs that are having the impact,” Aronson said. “So this gives us at least some preliminary corroboration of the idea that these crabs, if they move up, will devastate these invertebrate populations in shallow water.”
The thing that scientists can’t explain yet is why the crabs haven’t moved into the shallower area, because it appears to be hospitable to them.
“There are a couple possibilities,” Smith said. “One is that there are barriers that we don’t quite understand.” In other words, some unidentified factor could still be preventing the crabs from moving upward. The other possibility is that conditions have only very recently become habitable for the crabs on the continental shelf, and they haven’t had time to move into the area yet.
If they do start moving forward, though, Aronson said he expects they could become a fixture on the shelf within the next five years or so. And as the region continues to warm, the authors say, the crabs could move into even shallower depths — perhaps above 656 feet — within the next few decades.
Smith says similar research in other parts of West Antarctica would be warranted, as it’s possible that the crabs are creeping up on other parts of the continent as well. West Antarctica is a particular concern, more so than the eastern side of the vast continent, because it has experienced so much warming in recent decades.
And he says experimental laboratory studies could also help scientists get a better grip on the kinds of conditions crabs can tolerate and how quickly they might be able to colonize shallower waters in Antarctica. “There’s an urgent need to bring these [crabs] into the lab and reproduce the conditions on the Antarctic shelf and see whether they can reproduce, what their physiological tolerances are,” Smith said.
Making accurate predictions about the crabs’ behavior over the next few years will be crucial to understanding how the Antarctic ecosystem might change in the future as a result of climate change.
“This is about the diversity of marine communities on the planet,” Aronson said. Even if there are no obvious economic effects associated with changing Antarctic ecology, there are still ethical issues to consider, he added. “This is about what you want this planet to look like — what this planet ought to look like — and how we are doing as stewards of the planet,” he said.
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