River Monster

lobster-3No more than the size of your thumbnail, I found this young crayfish in the bank-side silts of the river Rib at the weekend. It is a young Signal Crayfish Pacifastacus leniusculus, a north american native species introduced to Europe via Sweden and Finland in the 1960s. This was to ‘replace’ the diminishing numbers of native crayfish that were suffering from crayfish plague. What was not known was that the Signal was also a carrier of the plague and over the last 50 years the Signal has decimated native crayfish populations to the point of extinction in large geographical areas.
Following a typical Astacidae life-cycle, the signal crayfish female typically lays 200-400 eggs after mating in the autumn. They are carried by the female under the tail until spring-time at which point the eggs hatch into juveniles. The young moult three times before leaving their mother and can live for up to 20 years.
Efforts are being made to manage adult populations through licensed extraction for the restaurant trade, but the young are falling through the net as it were and present a long term challenge to the chalk-streams of Hertfordshire and elsewhere in the UK and Europe.

UK Signal Crayfish distribution. Source: National Biodiversity Network

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