British crayfish could be wiped out by alien species with the plague

And these alien invaders can reproduce in great numbers at great speed without mating - if just one enters the waterways it will be capable of establishing a population.

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British crayfish have been up against larger American cousins for 20 years, leading to a rapid decline in numbers.

Now marine experts say the emergence of the new marbled crayfish presents an even grimmer future for the British freshwater crustacean, highly susceptible to the plague.

Trading of marbled crayfish is banned in EU countries because they carry the crayfish plague.

But it is believed that private collectors on the continent have illegally imported them to put them in aquariums.

The authorities became aware of a small number of collections in Britain after a woman from Southampton took some in to an aquatic pet shop.

Experts say the fish take their owners by surprise by the speed in which they reproduce - and that leads to dumping in rivers.

Although the woman was quizzed by officials from the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) she could not recall who supplied them to her.

Alasdair Scott, a senior fish health inspector at CEFAS, said: "We have become aware of an increase in numbers of illegal crayfish in Britain because we have had species handed to us from aquatic outlets.

"The woman in Southampton bought a female that was carrying eggs and then the next minute her small aquarium was overrun with juvenile crayfish.

"She handed them over to a pet shop and that is how we cane to realise somebody has brought them into the country.

"We spoke to her but she couldn't remember who she got them from.

"This could be a one-off incident but we suspect there are others here.

"The fear is that is these can reproduce then all of a sudden people don't know what to do with the juveniles and end up dumping them in the wild which is very bad news for our crayfish population.

"Because you don't need a pair of them they are able to breed on their own.

"Marbled crayfish is thought to be an American species and therefore a carrier of crayfish plague - a fungal infection to which our native crayfish are highly susceptible."

The crayfish are a brown-green colour and grow up to three inches long. They are larger and more aggressive than native varieties, giving them a competitive edge in the hunt for food.

Gerhard Scholtz, a German biologist from the Humboldt University of Berlin, has studied the marbled crayfish in the past.

He said: "This crayfish could become a menace to European freshwater ecosystems, as the release of even one into the wild would be enough to found a population that might out-compete native crayfish.

"As an American species, it is a potential transmitter of the crayfish plague that almost caused the extinction of the native European crayfish and which still threatens wild and farmed populations."

The red signal crayfish - Pacifastacus leniusculus in Latin - were imported to Britain in the 1970s to be commercially bred for food.

When the live crayfish farming market collapsed during the mid 1980s, commercial stocks were abandoned or neglected and ended up colonising our waterways.