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The crabs are large and dominant organisms, able to crush other animals
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 yaldwyni) 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) from 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.
The crabs and their tracks were abundant
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 (local) 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.
Read more at www.bbc.co.ukFishing 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.
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