Giant crabs make Antarctic leap

Richard Black BBC News 6 Sep 11;

King crabs have been found on the edge of Antarctica, probably as a result of warming in the region, scientists say.

Writing in the journal Proceedings B, scientists report a large, reproductive population of crabs in the Palmer Deep, a basin cut in the continental shelf.

They suggest the crabs were washed in during an upsurge of warmer water.

The crabs are voracious crushers of sea floor animals and will probably change the ecosystem profoundly if and when they spread further, researchers warn.

Related species have been found around islands off the Antarctic Peninsula and on the outer edge of the continental shelf.

But here the crabs (Neolithodes yaldwyn) are living and reproducing in abundance right on the edge of the continent itself.
Search for life

The researchers sent the Genesis, a submersible remotely operated vehicle (ROV) operated by the University of Ghent in Belgium, into the Palmer Deep in March last year.

The idea was to look at what life was down there, rather than specifically to look for crabs; and the team was somewhat surprised by how many they found.

Judging by the density of the crabs and their tracks, the scientists estimate there may be 1.5 million crabs in the basin.

A female crab retrieved from the area was found to be carrying mature eggs and larvae.

"Our best guess is there was an event, or maybe more than one, where warmer water flushed up across the shelf and carried some of the larvae into the basin," said project leader Craig Smith from the University of Hawaii.

It is believed that this species cannot tolerate water colder than 1.4C.

The seas here get warmer as you descend; and the crabs were only found below 850m.

The researchers calculate that they have probably been there only for 30-40 years; before that, the water would have been too cold even at the bottom of the Palmer Deep.

They cannot as yet survive on the continental shelf, which is at a depth of about 500m; but that could change.

"If you look at the rate at which the seas are warming, (the continental shelf) should be above 1.4C within a couple of decades, so the crabs are likely then to come into shallower waters," Professor Smith told BBC News.

The upper limit of the crab-dwelling zone - 850m - also marks the line between abundant seabed life above and depleted life below.

"Above the crab zone, the abundance and diversity of plants and animals was high, with echinoderms including brittlestars, sea lilies and sea cucumbers," said Professor Smith.

"We found none of them in the crab zone itself, and when we went 50-100m above we found very few - so we think the crabs are venturing up into shallow waters to feed.

"We would expect extinctions in some of these organisms."

These findings reinforce the belief of other scientists that king crabs will change the ecology of the Antarctic perimeter once they arrive - and that they would arrive at some point, washed from warmer waters along the South American coast, has long been expected.

With a legspan of up to a metre, the animals are generally top predators in the seafloor ecosystem.

The king (or stone) crabs are a group of about 120 species - and one member, the red king crab (Paralithodes camtschaticus) is already having an ecological impact in Norwegian waters following its slow spread from Russia.

However, in Northern latitudes they are also now important commercially, with Norwegian fishermen alone allocated a quota of thousands each year.

Fishing crabs for profit in this part of the Antarctic would not be permitted. But fishing could in time be used as a means to control them, said Professor Smith, if their ecological impacts become too severe.

Global warming brings crab threat to Antarctica
AFP Yahoo News 7 Sep 11;

The sea floor around the West Antarctica peninsula could become invaded by a voracious king crab, which is on the march thanks to global warming, biologists reported on Wednesday.

The worrisome intruder is a bright-red deep-sea predator that previously had been spotted only in the Ross Sea, on the other side of West Antarctica.

Taxonomists identified the crustacean just five years ago, bestowing it with the lengthy monicker of Neolithodes yaldwyni Ahyong and Dawson and placing it among the 121 species of king crab.

It is known as an "ecosystem engineer" because it digs into the sea floor to feast on worms and other tiny animals, an activity that in large numbers can have repercussions across the marine food web.

A team led by Laura Grange of the University of Hawaii at Manoa lowered a remote-controlled scoutcraft as part of a long-term probe into biodiversity in the waters off the Antarctic peninsula.

They looked at Palmer Deep, a mud-floored basin in the Weddell Sea located 120 kilometres (75 miles) from the edge of the continental shelf.

The robot's camera, trailed over two kilometres (1.2 miles), spotted 42 crabs, all of them at depths lower than 850 metres (2,760 feet), where the water was a relatively balmy 1.4 degrees Celsius (34.5 degrees Fahrenheit).

By extrapolation, the crab population in Palmer's deep -- an area measuring 14 kms (nine miles) long by eight kms (five miles) wide -- could be more than 1.5 million, says Grange.

That density is the same as commercial crab fisheries in Alaska and the British South Atlantic island of South Georgia.

The images gave a glimpse of the kind of damage caused by the foraging crustaceans.

The crabs, their shells measuring roughly 10 centimetres (four inches) across, had dug gashes up to 20 cms (one foot) into the soft ocean floor and thrown up lumps of sediment. The robot also retrieved a pregnant female crab, as proof that the species was reproducing.

None of the crabs was found at shallower depths, where the seas are colder.

The implication is that as global warming heats the frigid coastal-shelf waters, which lie at depths of 400 and 600 metres (1,300 and 1,950 feet), the way will be open for the crustacean to continue its creeping advance.

The evidence from sea-floor sediment is that no so-called lithodid, or crushing, crabs have inhabited the cold shallow waters of the West Antarctic peninsula for 14 million years.

Previous research has already named the peninsula as one of the most vulnerable regions in the world for global warming. The waters of its continental shelf are warming at the rate of 0.1 C (0.14 F) per decade.

"If N. yaldwyni is currently limited by cold temperatures, it could spread up onto the shelf within one to two decades," warns the study, published in the British scientific journal Proceedings of the Royal Society B.