Warming-induced killer crab invasion threatens Antarctic biodiversity

[Editor’s update: One of our observant readers and experts on deep-sea crabs, Dr Thomas Shirley of Texas A&M, points out that the photo above taken from the Mail article is of the tanner crab (Chionoecetes sp.), NOT the the giant king crab Neolithodes yaldwyni that is moving onto the Antarctic shelf. This is correct – the tanner crab is one of the species targeted in the northeast Pacific Fishery mentioned below in the reference to Deadliest Catch. He also noted that the big Japanese spider crab pictured is in fact a brachyuran or “true” crab, not an anomuran crab like N. yaldwyni. The video below, however, is accurate in depicting the giant king crab Neolithodes yaldwyni. Thanks to Tom for catching this.]

Climate change, we’re told, produces winners and losers. The most conspicuous losers so far have been polar ecosystems and the people that depend on them since the organisms that define these places are at the end of the line, so to speak, and have nowhere left to go as their world thaws. Think of the iconic photo of a lone polar bear searching for an ice floe in the liquid Arctic Ocean.

But that is only half the problem. The other half involves interlopers from warmer waters kicking them when they’re down.

Scientists have been concerned for some time about warming waters allowing the fox into the chicken coop of Antarctica’s spectacular and diverse seabed ecosystems.  New evidence from remotely operated subs now confirms those worries.

Antartica is a unique place: bitterly cold, very poor in plant nutrients, and — importantly — largely isolated biogeographically from the rest of the world by its subfreezing water temperatures and the belt of currents that flow perpetually around the continent unobstructed by other land masses. Over millions of years this isolation has produced a unique marine fauna that is surprisingly diverse given the region’s seemingly harsh conditions. Sponges, sea lilies and other sessile invertebrates grow luxuriously, albeit exceedingly slowly, in the cold clear waters beneath the ice. One probable reason for this luxuriance is that, like the isolated islands of Hawaii and New Zealand and and Mauritius, its animals have evolved over millions of years in blissful ignorance of predators.

But all that may soon be changing.

Writing in the Proceedings of the Royal Society (open access for this article!), deep-sea scientist Craig Smith and colleagues have confirmed that the giant king crab Neolithodes yaldwyni, apparently barred from Antarctica by  cold temperatures for at least 14 million years, has now entered the Palmer Deep, an area 120 km onto the Antarctic shelf. And they are plentiful, and reproducing. And looking hungrily, figuratively speaking, toward the Antarctic shelf.

"Crabzilla", the Japanese spider crab. 10' wingspan!

N. yaldwyni is a member of the king crab family, which supports the lucrative fisheries off Alaska made famous by Deadliest Catch and those awful Red Lobster commercials with platefuls of big spiny dismembered legs. It is arguably not the true “king” of crabs, however, a title that might better go to the Japanese spider crab (left), which must be the world’s biggest crustacean, and which I have vivid memories of seeing in a little seaside museum on visits to my aunt’s place in southern California when I was a lad.

Smith and colleagues piloted their Remotely Operated Vehicle (ROV) along transects over the Antarctic shelf to count the crabs, along with evidence of their activities in the form of telltale marks made in the sediment while walking, and the abundance of other animals. This allowed them to look for correlations between abundance of the crab predators and their prey.

So what happened? As the authors summarize:

Diversity curves for seafloor invertebrates where crabs and their traces were abundant (dashed line) and where they were absent (solid line). Ouch.

“The lithodid [i.e., crab] occurred at depths of more than 850 m and temperatures of more than 1.4°C in Palmer Deep, and was not found in extensive surveys of the colder shelf at depths of 430–725 m. Where N. yaldwyni occurred, crab traces were abundant, megafaunal diversity reduced and echinoderms absent, suggesting that the crabs have major ecological impacts. Antarctic Peninsula shelf waters are warming at approximately 0.01°C per year; if N. yaldwyni is currently limited by cold temperatures, it could spread up onto the shelf (400–600 m depths) within 1–2 decades. The Palmer Deep N. yaldwyni population provides an important model for the potential invasive impacts of crushing predators on vulnerable Antarctic shelf ecosystems.”

In other words, the barbarians are at the gate. They’ve already apparently eaten many of the species out of the deeper regions where the crabs are abundant (see figure above). All that’s needed is for the Antarctic waters to warm up a little and the hungry crabs will come marching (OK, crawling) up the slope and into the defenseless sponge gardens of the shelf. I suppose there is some solace in knowing that the crabs could then in principle be harvested and eaten, as their kin are elsewhere. But that would require trawling, which would presumably spell the end once and for all for the fragile, suspended-animation Antarctic benthic community.

Here’s a video taken by the remotely operated sub of these creatures scuttling along (at the glacial pace possible for cold-blooded animals in water slightly above freezing) and bearing down on their prey. Check out the fiendish glowing eyes — they totally remind me of the robotic monsters in cheapo Japanese scifi movies when I was a kid:

What next? That depends on on how warm it gets.

Original source: Smith CR, et al. 2011. A large population of king crabs in Palmer Deep on the west Antarctic Peninsula shelf and potential invasive impacts. Proceedings of the Royal Society of London B: Biological Sciences. doi: 10.1098/rspb.2011.1496.

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