Signal Crayfish - Pacifastacus leniusculus

This American Signal Crayfish was introduced into southern England in the 1970s for farming. It escaped from many of the farms, and was also deliberately released into the wild where its population rapidly expanded across rivers and lakes. It can be found throughout the county and is out-competing our only native species the White-clawed Crayfish. The Signal Crayfish is also a carrier of crayfish plague that is fatal to our native crayfish.

Habitat

Rivers, streams, lakes, reservoirs and water-filled quarries.

Description

Adults normally grow up to 15cm in length, but have been recorded at 30cm. They have a smooth body that is bluish-brown to reddish-brown in colour (in some habitats they may be almost black in colour). Their robust, massive, smooth claws are red underneath with a white patch near the claw hinge which is moveable and gives the Signal Crayfish its common name.

Biology

Like all crayfish the Signal Crayfish is a solitary animal feeding on pretty much anything, its diet ranges from plants, decaying roots and leaves to meat including other crayfish.

The Signal Crayfish can walk overland quite easily in search of a new home, this way it rapidly spreads and colonises new freshwater sites.

They can breed from as early as two years old, once the female has reached 40mm long. She breeds once a year and can produce over 200 eggs at a time. The male fertilises the eggs in October to November and the female then carries them folded within her tail until April or May, when they’ve hatched, and she releases them. They can live up to 12 years.

Impacts

The Signal Crayfish is a big threat to our native White-clawed Crayfish as it carries the crayfish plague (a fungal disease) that is fatal to our native crayfish. They also out-compete our native crayfish as they are bigger, less fussy eaters (including eating other crayfish), rapidly colonise new areas and are much more aggressive. Because of this native crayfish numbers have plummeted.

Signal Crayfish also cause problems for the rest of the aquatic ecosystem. They compete with other species such as fish for shelter, the burrowing of signals can cause erosion of riverbanks and grazing of aquatic plants can cause change the structure of the habitat. The loss of plants means that there are fewer places for insect larvae to hide or for fish to lay their eggs.

Control

It is an offence under the Wildlife and Countryside Act 1981 to release, or allow to escape, any non-native animal to the wild in Great Britain except under license. Under these provisions, and others, it is an offence to release any crayfish species to the wild. The introduction of crayfish to water bodies from which they are able to escape, such as ornamental ponds, or farm ponds could also render the person making such introductions liable to prosecution under the Act.