sm-220-king-crab-kheirman-ghent-university

Antarctic shelf waters have been too cold for king crabs for the last 14 million years or so, said researcher Craig Smith. (Katrien Heirman, Ghent University )

Giant king crabs are invading the waters off the Antarctic Peninsula, which are warming with climate change, say researchers.

Craig Smith, a professor of oceanography at the University of Hawaii, and colleagues, reported their findings on king crabs (Neolithodes yaldwyni) Wednesday in the Proceedings of the Royal Society B.

"For the last 14 million years or so, these kinds of crabs have been excluded from the Antarctic shelf waters because it's too cold for them," said Smith. "But on the Antarctic Peninsula, the water is warming very rapidly."

Smith and colleagues stumbled across the large population of king crabs while studying the sea floor off the West Antarctic Peninsula for the effects of climate change.

"As soon as we saw them, we realized how significant they might be so we did a careful survey," said Smith.

The researchers used a remotely-operated robotic vehicle, connected to a fibre optic cable, to observe the crabs in an underwater basin off the Antarctic Peninsula called the Palmer Deep.

The researchers found 42 crabs, including a female carrying eggs, lurking on the sea floor between 1400 metres and 850 metres deep at temperatures above 1.4C.

They estimate there is a population of more than one million crabs in the Palmer Deep.

Warming waters

In Antarctica, the surface waters are cooler than water lower down, and this has so far excluded the king crabs from rising above a certain depth.

The researchers measured the temperature of the water at different depths and looked at past records to work out the rate at which the water there is warming.

Based on this they calculated that the crabs probably colonized the Palmer Deep 30 to 40 years ago and will invade the rest of the shelf in the next two decades.

"It looks like within the next 20 years or so the water up on the shelf at 500 metres will be warm enough for them," said Smith.

Voracious predators

King crabs have 12-centimetre wide bodies and extend 40 centimetres from tip to tip of their legs.

"They can crush clams and other kinds of animals with hard shells so they are very voracious predators," said Smith.

"They also forage very actively in the sediments. They dig through sediments and capture animals for feeding."

For several millions of years, the native fauna of Antarctica have evolved in the absence of such predators, and are therefore very vulnerable to the crabs, said Smith.

He and colleagues surveyed for echinoderms, such as sea urchins, sea lilies and starfish, which are known prey of king crabs.

"We saw none of those in the 'crab zone', but when we went 100 metres above we saw lots of them," said Smith.

They also found the overall number of different species in the crab zone was much lower than above it.

"It looks like the crabs are causing local extinctions of a variety of species that have evolved in the Antarctic," said Smith.

Ecosystem engineers

The researchers also observed the crabs feeding and digging for animals in the sediment, and making traces in the sediment as they moved around.

Smith says the crabs are not only reducing biodiversity but also changing the geochemistry of the sediment through eating the animals in it.

"As these crabs invade the Antarctic they are changing the natural ecosystem there," he says. "We call them ecosystem engineers"

Smith says the next step is to find out more about where the crabs are coming from.

They are many thousands of kilometres away from the Ross Sea, south of New Zealand, where single individual king crabs have previously been found.

Because the Antarctic Pensinsula is warming faster than anywhere else in Antarctica, this is where king crab invasions are expected to occur fast, says Smith.