A simple but fact filled video about the Signal Crayfish, including their diet, how and when they breed, where they are found and much more.
If you have seen a crayfish in the UK in the last 15 years, the chances are it likely that it would have been a signal crayfish. These were introduced from north America to various sites in this county in the 1970s as a species for farming, but of course, they escaped into the wild and were probably deliberately released by people who wanted to eat them. Unfortunately, Signal crayfish are larger than and spread a disease that is fatal to the native white claw crayfish and they have now almost gone extinct here.
Signal crayfish grow to measure up to 18cm in length and have 2 powerful claws. These are bright red on the underside in adult animals and have a noticeable bluey white patch on the upper-side of where their claws hinge. Alongside their claws crayfish have 4 pairs of smaller legs for walking and two long antennae that protrude from the front of their heads. Around their mandibles they have 3 pairs of leg like limbs called maxillipeds which they use to handle food and apparently stone, but also to keep water moving through their gills. They have abdomens which are made of 5 segments and a flattened tail at the end of their bodies. Males grow larger than females and usually have larger more prominent claws. Crayfish moult several times as they grow and when this happens they are able to completely regrow limbs that they may lose whilst fighting or to predators.
Signal crayfish predominantly live at the bottom of fresh watercourses, including in lakes, ponds, rivers and streams. They can swim, especially when threatened by folding their bodies up and propelling themselves backwards but they usually move by walking along the bottom, although they will sometimes climb the bank and have even been seen emerging from the water to get food from above the waterline. They are mainly nocturnal but during the summer time it isnt unusual to see the larger individuals out in the open.
They are opportunistic omnivores and will eat everything from vegetation and decomposing material to small fish, fish eggs, invertebrates and they will even eat each other if food supplies become scarce. Unfortunately this can have a knock on effect on the waterways in which they live as they can reach such a density that they strip the bank and bottom of the habitat almost completely bare.
Alongside their demand for food, Signal crayfish also cause harm to habitats by creating extensive interconnecting burrows up to 2 metres deep into the bankside, sometimes leaving it looking like a sponge and susceptible to erosion and collapse.
Signal crayfish are prolific breeders. In the Autumn large males hold territories and then attempt to seduce passing females by trying to pin them to the bottom. If successful, they pass over a pouch of sperm and let the female leave. She will then go to a sheltered spot and lay between 2 and 400 eggs which she fertilised with his sperm before attaching them to the underside of her body. She will carry these eggs until Spring when they hatch. The young crayfish, once free swimming leave their mother and become independent straight away. It takes signal crayfish 2 to 3 years until they are able to breed but as they can live for up to 20 years, they have the potential to produce lots of young in their lifetimes.
Signal crayfish are native to the united states and Canada but have been introduced across Europe and are now found in at least 25 European countries. They are also found across Russia, in some parts of northern and east Asia and in south Africa. People often ask if they can be controlled in any way and the answer is no, not without destroying the rest of the habitat for a period of time at least. In the UK, the best thing people can do to protect native crayfish is be very careful that they are not moving signal crayfish or the disease that they carry from one place.
#crayfish #wildlife #nature
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