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Found throughout England, these 15cm-long beasts are aggressive, breed faster than the native species, and damage banks with their burrowing.
Scientific name: Pacifastacus leniusculus
Family: Astacidae
Diet: An omnivore with a big appetite, it eats aquatic plants, algae and detritus, as well as various invertebrates including snails, insects and fish. They also prey on fish eggs and tadpoles, and have even been known to eat each other when food sources are low.
Size: 16-18cm
Weight: 60-110g
Lifespan: 20 years
American signal crayfish (also referred to as 'American signal' or 'signal crayfish') were introduced to Europe from North America in the 1960s and bred on farms for restaurants to serve as a delicacy.
After escaping the farms, signal crayfish spread quickly across England, Wales and part of Scotland. It's impossible to know how many of these invasive species there now are in the UK.
Drawn to mainly still or slow-flowing freshwater habitats, signal crayfish can burrow two meters deep into waterway banks, causing extensive damage and leaving canals vulnerable to collapse. They are able to cross land, and break through natural and artificial barriers.
As opportunistic omnivores, signal crayfish aren't fussy about what they eat. They consume anything from algae and aquatic plants to small invertebrates and fish, which is negatively impacting native ecosystems and food webs.
However, the species that has suffered the most is our native white-clawed crayfish, which is now protected and heading towards extinction. The American signal is larger and more aggressive, outcompeting its docile cousin for habitat and preying on their young. It also carries a fungal disease called 'crayfish plague', which is fatal for the white-clawed, and can be spread by wet footwear and equipment.
Schedule 9 of the Wildlife and Countryside Act 1981 means it is now an offence to release or allow the escape of American signal crayfish into the wild across England, Wales and Scotland. It is also an offence to keep any without a license except in areas of southern England - but an application for trapping must be made to the relevant environmental protection agency.
Lobster-like in appearance, American signals are brown-red in colour. Their claws have bright red undersides with a small white or turquoise mark on top, near the claw hinge.
American signal crayfish can be as much as 10cm larger than white-clawed crayfish. White clawed crayfish are a green-bronze colour, and have pale cream or soft pink claws that appear to have a granular texture. They are also nocturnal, unlike signals.
After mating season in autumn, females carry a clutch of 200-400 eggs on the underside of their tail throughout the winter. Once hatched, the juveniles remain attached to their mothers until late spring, at which point they visibly resemble adults and can feed on most plants and animals themselves.
Last Edited: 10 April 2025
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