Tuesday, 5 August 2008

Freshwater crayfish - one good, one very bad.

A quick post about freshwater crayfish and how our very own native species is on the verge of extinction thanks to yet another exotic alien invader...

Our poor old White-clawed crayfish Austropotamobius pallipes was doing very well thankyou very much until about 20 years ago. Some muppet decided that it would be a great idea to start breeding the Signal crayfish Pacifastacus lenuisculus in fish farms in the UK, primarily for the restaurant trade.

Of course, if history has told us anything it's that introduced species always escape, and sure enough by the late 1980's this bloody menace was at large in British waterways. Now, 20 years on, the White-clawed crayfish is without doubt one of the most at risk species in the British Isles and some sources have estimated that within 30 years it will be extinct on mainland Britain.

The images below were taken on a recent crayfish handling course and the Signal crayfish concerned were destroyed in the correct manner and the utmost care was taken in regard to biosecurity.

White-clawed caryfish from the River Ribble - perhaps 5 years old?

Adult male White-clawed from a still water site - in the right conditions these animals will thrive at incredible densities.

Adult male White-clawed, showing the characteristic pale claws. Crayfish can be sexed by the presence of 'claspers' at the base of the thorax (just visible in this image).

bastard, sorry, Signal crayfish..showing the distinctive bright red underside to the claws. This colouration is apparent on all but the youngest animals.

Close-up of a smaller example - again the red colouration is evident. These animals live at even higher densities that the White-clawed, with up to 200 animals (including young of the year) per square metre.

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