Noble crayfish  (Astacus astacus)

The noble crayfish is classified as Vulnerable (VU) on the IUCN Red List (1). It is listed under Appendix III of the Berne Convention on the Conservation of European Wildlife and Natural Habitats (3) and on Annex V of the EC Habitats Directive (4).

This crustacean varies in colour from green or blue to brown and sometimes black. The undersides of the claws are dark red (2). The head and internal organs of all crayfish are protected by the carapace and the six segments of the abdomen are individually encased with a flexible membrane between them to allow movement. Crayfish have a pair of large claws at the front end, followed by four pairs of walking legs and then four pairs of small swimming legs called swimmerets. These swimmerets are covered with fine hairs to which the female attaches her eggs. A central tail flap is surrounded by four other flaps that are used to move the crayfish rapidly through the water, as well as curling up to form a brood chamber. There are two eyes on the end of eyestalks, but the senses of touch and taste are far more important, and are perceived using a pair of large feelers (or antennae) and a pair of small, fine, centrally located feelers (or antennules).

Previously prolific throughout Europe, the noble crayfish now has a scattered distribution through northern Europe and western Russia (1) (2).

A freshwater invertebrate, the noble crayfish is found along the banks of well oxygenated streams, rivers, lakes and ponds (2).

Feeding at night on worms, aquatic insects, molluscs and plants, the noble crayfish spends the day resting in a burrow. It undergoes periodic moults, shedding the hard exoskeleton in order to grow slightly larger, and then forming a new shell. Sexual maturity is reached during the fourth year of life, and breeding takes place between October and November. The fertilised eggs are carried by the female, attached to her legs, over the winter until May, when they hatch and disperse. Both juvenile and adult noble crayfish are preyed upon by mink, eel, perch, pike, otter, and muskrat (2).

Considered to be the finest edible crayfish, the noble crayfish was once abundant in Europe, but was expensive to buy. The American ‘signal crayfish’ (Pacifastacus leniusculus) was introduced to European waterways as a cheaper alternative, but the presence of this species was to have disastrous effects on the noble crayfish. An outbreak of crayfish plague, carried by the signal crayfish, spread through populations of all of Europe’s Astacus species. Caused by the freshwater fungus Aphanomyces astaci, crayfish plague infects and kills the noble crayfish, leaving the resistant signal crayfish unharmed (3).

Dredging of waterways has further threatened Astacus crayfish species as it leaves the water cloudy and disturbs the habitat. Acid rain, also causing a decrease in water quality, is thought to be responsible for a decline in breeding success, as the egg cases of young crayfish are unable to form properly (2).

Efforts to reintroduce the noble crayfish have been hindered by the presence of the signal crayfish, which competes for food resources, although in Norway some recent reintroductions look promising (5). In most European countries limits have been set for the trapping of noble crayfish, including restricting trapping to a period of about three months, and setting a minimum crayfish length of 10 cm. In some areas, trapping is completely prohibited. Elimination, or at least reduction, of signal crayfish populations, and protection of crayfish habitat are required to allow the noble crayfish to recover (2).

View information on this species at the UNEP World Conservation Monitoring Centre.