Page last updated at 04:51 GMT, Wednesday, 15 October 2008 05:51 UK

Aquatic alien 'thugs' set to meet

By Rebecca Morelle
Science reporter, BBC News

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Claw to claw - how do the Chinese mitten crab and non-native crayfish size up?

Two of the UK's worst aquatic invasive species are set to meet.

Scientists believe that the ranges of the plague-carrying non-native crayfish and the fierce Chinese mitten crab are beginning to overlap.

Since their introduction, both of these animals have had a dramatic impact on local ecology and are especially harmful to native species in the UK.

Scientists are unsure what will happen when the two alien invaders eventually cross paths.

Various sites around the UK have been highlighted as potential meeting places; they include the River Lee, in the South, and the River Ouse and the Aire in Yorkshire.

The non-native crayfish was first introduced to the UK in the late 1970s for aquaculture, but it soon began to spread.

White-clawed crayfish (S.Peay)
Where the non-natives move in, the white claws are lost
Stephanie Peay

Several different species now exist in the UK's waterways, but the signal crayfish (Pacifastacus leniusculus), which comes from North America, has so far caused the most damage - thanks, in part, to a disease it carries.

Ecologist Stephanie Peay told the BBC: "The plague does not seem to harm the non-natives, but if our native white-clawed crayfish [Austropotamobius pallipes] encounter it, they die within weeks."

Even if the non-native crayfish do not carry the plague, they still cause problems for the UK's white-clawed species.

Ms Peay describes the crustaceans as "aquatic thugs".

See distribution of signal crayfish

She explained: "Where the non-natives move in, the white claws are lost. Survey work has shown that it only takes between four and seven years from first arrival to achieve a complete local extinction.

"The only future for the white claws is in isolated water bodies that are completely free from non-native crayfish."

See distribution of Chinese mitten crabs

And it is not only the white-clawed crayfish that are at risk.

The alien species is also having a big impact on other elements of the aquatic ecosystem and its burrows are also causing huge damage to water banks.

Voracious predator

Until now, non-native crayfish have been seen as one of the worst invaders in the waterways - but could another alien crustacean knock them off top spot?

The Chinese mitten crab - so called because of its furry, mitten-like claws - was first recorded in the UK in 1935.

Chinese mitten crab
They eat anything that they can get their claws into
Paul Clark, NHM

However, it really began to take hold in the 1980s after large numbers were thought to have been unleashed into the Thames and other rivers around the UK from ships' ballast water.

Paul Clark from the Natural History Museum said: "Since the late 80s, their range has been expanding massively.

"It is difficult to estimate the exact population, but I suspect the numbers are in the millions."

The crabs are voracious predators.

Dr Clark said: "They eat anything that they can get their claws into: weeds, fish eggs, snails, molluscs - anything.

"They are just an opportunistic feeder.

Bloody red shrimp (Noaa)

"It is hugely disruptive to local ecosystems - nobody has been able to do an assessment on how much damage they actually cause, but it could be quite significant."

Chinese mitten crabs inhabit both fresh and saltwater.

The young are born in coastal regions or estuaries, they then migrate up the river to spend their adult lives in freshwater, before returning back to saltwater years later to breed.

Once in freshwater, they can move huge distances.

Dr Clark said that in China, some studies have shown they can migrate up to 1,500km (930 miles).

Alien versus predator

In the UK, year upon year, the mittens have been increasing their range.

Philine zu Ermgassen, an aquatic ecologist at the University of Cambridge, said: "Mitten crabs are spreading so quickly at the moment.

Signal crayfish
You could predict a clash between the two
Philine zu Ermgassen

"They are now moving further up rivers into areas where there are crayfish - it seems they are starting to encroach upon crayfish terrain."

Scientists do not know what will happen when the two meet.

Ms zu Ermgassen said: "You could predict a clash between the two.

"If this is the case, the mitten crabs do seem to be more dominant as a species - the crabs are very aggressive and very strong. They will also be directly competing for food."

More worrying, however, could be the effect on the local ecosystem of the presence of two aggressive alien crustaceans.

She added: "You could end up with an additive effect. They are both broadly omnivorous and both burrow under banks."

Stephanie Peay says that this could be bad news.

She told the BBC: "What tends to happen when successful invasive non-native species move in is that you tend to get a reduction in the diversity of native species and a reduction in abundance.

"That will be bad for the native environment overall."

Chinese mitten crab/crayfish compared

Scientific name:
Pacifastacus leniusculus
Scientific name:
Eriocheir sinensis
Originally comes from North America Comes from Asia
Habitat: Found in lakes and watercourses Found in coastal regions, estuaries, lakes, watercourses
Eats anything available, including other crayfish Voracious predator, eats anything it can lay its claws on
Life expectancy - up to seven years in the wild Can live up to nine years in the wild


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SEE ALSO
Crayfish plague spreads to river
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'Kill Crayfish on sight' appeal
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15 Jul 08 |  North West Wales

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