Crab Attack: the ground-hunting rats of Antarctica

 

The isolated, unique ecosystem of the New Zealand islands lay undisturbed for millennia with no ground-based predators. Flightless birds evolved, laying eggs in safe nests like this sweetly curled kiwi. Then humans turned up, along with rats: ground-based omnivores which have had a massive impact on all sorts of species. I was delighted to see that the tortoises of the Galapagos Islands, brought to near extinction by, amongst other things, rat predation, have just begun breeding in the wild. 

 

Those two examples highlight that some isolated ecosystems evolve species which are defenceless against types of predation which are common elsewhere. A zone where nothing exists which can crush bones develops fauna which are fragile, spindly and easily eaten should something come along with strong claws, hammers or teeth.

Crabs are skeleton crushers, durophagous in the language of science. Those big jaws come along and chew bones to make their bread. The bones may be scavenged or come from molluscs and other species living on the seafloor. In 2012 a scientific paper announced new research into a recently discovered large colony of king crabs living in Palmer Deep off the West Antarctic Peninsula. It came from a major  underwater survey done in 2010 which yielded enormous amounts of new data about the area. The paper asked how these animals had got there, how long ago and what their presence meant for the future.  The authors suggested that the crabs might spread up to shallower water and rapidly change the ecosystem there by preying on fauna which had no evolved defences.  Much as birds have suffered from rats in Aotearoa in fact.

Palmer Deep is a depression in the large Antarctic continental shelf, plunging down over 1000m below sea level, but surrounded so that anything reaching it must climb to at least 450m below the surface. An important feature is that in this area deeper layers are warmer than the shallower water above. The Upper Circumpolar Deep Water swirls at these lower levels, bringing in warmth. All the water is getting warmer, up by as much as 0.7°C since the 1990’s. The ice shelves are breaking up too. So the colony of some 1.5 million crabs is living in deep, (relatively) warm, salty water on a layer of thick sediment.

These crabs don’t like cold water.  If it goes below 0°C  they keep away, so at the moment they seem to stay under 850-750m deep. So one question is whether they are newly arrived, over that shallow sill, or have they been there for millennia? The first thoughts were that they are invaders, rats arriving (as immature forms) on warming currents: there are relatively few crabs in the Antarctic (though millions on the sub-Antarctic islands).

A 2013 study by the British Antarctic Survey argues that the crabs may have been happily breeding down there for a long time, a hypothesis based on considerable diversification and speciation. BAS also emphasises the sampling problems. Relatively few crab species have been found across the whole of Antarctica (below 60°S )and there are not good records on the types of capture. (In other words where crab records have been collected via fishing, we don’t always know what kind of trawler was being used, so we can’t tell much about the number of crabs within a population the vessel might have captured.) BAS agrees, though, that the key barrier to the crabs spreading seems to be temperature. 

They produce this great map showing the spread of lithodidae (the relevant species of crab) in the region. We’re talking about the ones in the blue band, which is sharply cut off by the much colder waters of the Weddell Sea to the east of the Antarctic Peninsula. 

In a way, of course, whether they’ve been there since the last glacial maximum (14500 years ago in Antarctica) or have just arrived is not relevant to potential damage caused if they reach new areas where the existing ecosystem is not designed for defence against their capabilities.In that sense, they’re more like humans. We just keep spreading and chewing up the environment when we get somewhere new. But it does help us think about whether and how they could spread, when and what, if anything we might do about it.

Because they do have a big potential impact. The original team, led by the Florida Institute of Technology (FIT) showed that the sediment is clearly disturbed by them.  This picture (from the 2012 article) shows the crab foraging and the gouges and tracks the animals leave behind them. Through their impact on the sediment and their way of eating, they are the top predators in their environment. It seems like Palmer Deep hosts a stable population and (especially given further analysis published this year) they are happily breeding.

They are voracious eaters, from dead stuff that drifts downwards to molluscs, worms, other crabs and even fish. The 2012 study also shows the huge difference in biodiversity between depths where the crabs are found and in shallower, colder water where so far they haven’t ventured.

In their latest article, published in September 2015, the FIT team reviewed both their earlier work and the BAS study and looked again at the barriers to the animals expanding their range. 

Both BAS and FIT concur that the crab's range appears constrained only by temperature. Salinity, depth, predators, prey do not seem to be a problem. They also point to the uniqueness of the Antarctic ecosystem compared to the greater similarity between Arctic areas where such species flourish in shallower waters. So it is crucial to understand that sea temperature is rising across Antarctica, but nowhere faster than on the western peninsula.  FIT's latest paper says:

”Summertime sea-surface temperatures and temperatures of the Antarctic Shelf Bottom Waters off the Western Antarctic Peninsula (WAP)  have risen by nearly 1.5 °C over the past 50 years, approximately double the globally averaged rate. Rising temperatures in shallow waters off the WAP will likely remove the thermal barrier to lithodids (and other reptant decapods) within the next several decades, facilitating their expansion into shallow, nearshore habitats.”

(A decapod is a class of crustacean to which crabs, lobsters and shrimps belong. Reptant animals are those which move with a creeping action. Scuttling sea-dwellers with a shell. I had to look it up, which confirms that I am not a scientist, just an avid reader and analyst.)

Despite the undoubted damage that would be caused by such a new predator in an unprotected ecosystem, perhaps the biggest threat will come from the commercial viability of a crab fishery. A great success story of the second half of the twentieth century has been the resistance to exploitation on the continent. The crab biomass in Palmer’s Deep matches that found in the fisheries of Alaska and South Georgia. It will offer a temptation, especially if the crabs are seen as themselves a threat. Yet, once that dam is breached it will not easily be closed again, opening the way to other resource –usage and threatening the fragile consensus currently providing some protection to the continent as a whole.

 

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