White-clawed crayfish (Austropotamobius pallipes).
White-clawed crayfish prefer to live in streams and rivers where the water is clear, shallow and fast flowing, although they can also be found in ponds and lakes. They are also usually only found in areas where the water is 'hard' (ie, contains a lot of calcium carbonate), as they need calcium to grow the hard case covering their bodies. Therefore, they are most common in central and northern England where there are areas of limestone and the water is harder.
White-clawed crayfish can grow to about 10cm long and look a bit like lobsters. They have a brown body with five pairs of legs, though the front pair has turned into large claws. The eyes are like round beads stuck on its head, and above each eye is a sharp ridge.
Crayfish are nocturnal. During the day they hide in holes in the riverbank, or under tree roots or stones, and at night they come out to feed. They walk about on the river bed but they can also swim backwards quickly if they need to by flicking their strong tail.
Crayfish are omnivores, eating plants, other animals and dead things. They also eat each other. A crayfish catches its prey – such as a snail, insect larvae or a small fish – in its large claws and then uses these to tear it apart before putting it in its mouth. However, they are also preyed upon by fish, birds, rats and otters.
Crayfish mate in the autumn. When the female lays eggs they stick to the underside of her tail section and she carries them around until they hatch in late spring.
The hard case that protects their soft body doesn’t grow, but the crayfish inside does, which means that as it gets bigger it has to shed its hard case and grow a new one. This happens several times during its first year, but then just once a year when it is an adult. While growing its new hard case, crayfish are most at risk of being eaten by another animal. Crayfish can live for up to about 12 years.
The white-clawed crayfish is Britain’s only native species of crayfish. However, they are threatened by the spread of other species of crayfish, which were originally introduced in the 1970s and 1980s to supply restaurants. The signal crayfish in particular is extremely aggressive, and also carries a disease called crayfish plague which doesn’t really affect signal crayfish but can kill white-clawed crayfish.
White-clawed crayfish are present in the top part of the River Derwent, which flows south from Fylingdales Moor towards West Ayton. Some parts of the Derwent catchment still remain free from competition from the signal crayfish, and the National Park Authority works with local people and environmental organisations to help conserve the native crayfish population.
Habitat improvements include: putting in fences to stop cattle and sheep from damaging riverbanks; looking after trees along the riverbank; and building paths to help people enjoy the river without damaging the banks.
Anglers can also play an important role in keeping white-clawed crayfish healthy by disinfecting all fishing tackle and boots thoroughly if they have fished in rivers where signal crayfish may be present.