Carrying on our invasive species series, we turn this month to the aquatic environment, and the signal crayfish (Pacifastacus leniusculus). The species is listed on Schedule 9 of the Wildlife and Countryside Act 1981 (as amended), meaning it is an offence to release or allow this species to escape into the wild.
Signal crayfish were introduced to the UK from North America, to be farmed for human consumption. In 1976 there were escapes into the wild and the species then spread rapidly across the UK. They disperse downstream in water courses, there are also records of them travelling several hundred meters over land to reach new ponds. It is thought that their prolific spread has been aided by humans moving and releasing them, although it is now an offence to do so. Signal crayfish are found in streams, canals, rivers, lakes and ponds, and are also able to survive in brackish water. The UK is not the only country where this invasive species has been introduced, it is now found in 20 other European countries.
Identifying signal crayfish
Signal crayfish are lobster-like in appearance and grow up to 18cm. They are greenish/brown in colour with a bright red undersides, their claws also have a red underside, as well as a small turquoise/white spot on the upper surface at the claw hinge.
Our native white claw crayfish (pictured below) is easy to differentiate from the signal crayfish. It is much smaller, never reaching sizes over 12cm. The red underside on the body and claws found on signal crayfish is absent in white clawed crayfish.
Identifying signal crayfish
Signal crayfish are lobster-like in appearance and grow up to 18cm. They are greenish/brown in colour with a bright red undersides, their claws also have a red underside, as well as a small turquoise/white spot on the upper surface at the claw hinge.
Our native white claw crayfish is easy to differentiate from the signal crayfish. It is much smaller, never reaching sizes over 12cm. The red underside on the body and claws found on signal crayfish is absent in white clawed crayfish.
Intense competition
The introduction of the signal crayfish has had devastating consequences for our native white clawed crayfish (Austropotamobius pallipes), which has been driven almost to extinction. The white clawed crayfish was once a widespread and common species in UK rivers, but has suffered a decline of 50 – 80% across its European range in the last ten years. It is classified as ‘endangered’ on the IUCN red list of threatened species, since it is at risk of global extinction.
The signal crayfish can tolerate a wider range of conditions and are far more adaptable than the white claw crayfish. Not only do they outcompete our native species for refuges to shelter in and food, they grow faster, produce more young and carry a disease called crayfish plague (Aphanomyces astaci) which they have immunity to but to which our native crayfish are very susceptible.
As well as the effect on our native species of crayfish the signal crayfish causes other problems, such as eroding river and canal banks by causing destabilisation from where they dig out their burrows, and causing sediment pollution.
What can be done?
Now they are established across much of the UK, the best way to prevent their spread to the last remaining areas where white claw crayfish persist, is to maintain high standards of biosecurity. Crayfish plague is a fungus-like disease and the spores can survive for up to two weeks in damp conditions, it is therefore very easy to infect a new area through inadequate cleaning of equipment. By disinfecting and drying equipment, boats and footwear before entering water at a new location it will help stop the spread of this disease.