The White-clawed Crayfish - Austropotamobius pallipes
The only freshwater crayfish native to the UK, this species was widespread throughout Britain before the 1970’s, but a large number of populations have now been lost. It has been in severe decline due to several factors.
Olive-green to brown in colour, this species can be up to 12 centimetres in length, though usually they are less than 10 cm. Their common name refers to the undersides of their claws which are off-white to pinkish in colour. The gender differences include that females tend to have slightly wider abdomens than males, and the males have larger claws. On the front of its head it has two pairs on antennae, one pair being longer than the other. It has four pairs of walking legs and lots of tiny legs on the underside which are called ‘swimmerets’.
It tends to be nocturnal and emerges at night to feed on its broad diet such as water worms, insects, larvae, snails and small fish. It has also been known to be cannibalistic, mainly on individual crayfish that have soft cuticles following their moult. During the day they are very secretive that like to hide under large stones or amongst plant life.
Mating happens in autumn and then the eggs develop attached to the mother's abdomen. The males place a sperm mass (spermatophore) underneath the female from its first two appendages which are specialised for this. The female then spends the winter with the eggs still attached to her. When eggs hatch, the young remain attached to the mother becoming independent at the beginning of summer. The young can moult more than 7 times during their first year, though after reaching maturity they tend to moult annually.
Crayfish plague, a virulent fungal disease, is probably the most devastating factor affecting the White Clawed Crayfish. It is carried by the aggressive species, the American Signal Crayfish (Pacifastacus leniusculus) which was introduced to the UK during the 1970’s. The spores of this disease can be spread in the water, on wet equipment and on fish.
Competition with the introduced Signal crayfish species has also affected its numbers. Signal crayfish have naturalised in the UK and are breeding successfully. Another two introduced species have also become established in the UK, the narrow-clawed (or Turkish) crayfish (Astacus leptodactylus) and the noble crayfish (Astacus astacus). Other factors that may have affected the species are pollution and river works.
Thankfully its declining numbers has been noticed and it has been targeted as a priority for conservation under the UK Biodiversity Action Plan (UK BAP). The Species Action Plan aims to maintain the current distribution of the species by restricting the spread of non-native crayfish and crayfish plague, as well providing suitable habitats. In order to restrict the spread of the non-native crayfish it is an offence under Schedule 9 of the Wildlife and Countryside Act to release the three introduced species of crayfish into the wild. The Environment Agency has also carried out research into potential methods of controlling the signal crayfish, and is currently investigating the use of pheromones in order to lure the invasive species into traps.