We might excuse some historical introductions due to naivety. Our understanding of natural history was thin in the 19th century, with Charles Darwin’s evolutionary theories controversial and, as a result, the potential impact of species such as the grey squirrel or muntjac upon our native fauna and flora was overlooked.
By the 1970s however, we really ought to have known better, but the introduction of the signal crayfish (Pacifastacus leniusculus) from North America, as a readily available food source, has created problems too complex to solve.
They are unfussy eaters, feeding on invertebrates, fish eggs and small fish. In some smaller habitats they are so prolific as to have all but created a monoculture, surviving through cannibalisation. They burrow into soft banks, causing erosion and colouring the water, which in turn reduces light levels and weed growth.
They also carry a crayfish plague, a type of water mould, to which they are immune but our native, white-clawed crayfish (Austropotamobius pallipes) has no resistance. The subsequent decline of white-claws has been extreme, and they are now considered endangered.
A further problem is the similarity between both species, especially when young. Both have a small, lobster-like form, with the bright red and turquoise on the claws giving the signal crayfish its name. The shape of the rostrum (the nose-like tip of the head) is the most reliable diagnostic feature, although the signals will grow larger, measuring up to 30cm long. Signal crayfish are widespread and here to stay, making them truly invasive.
Top image: Karim Vahed, a professor of entomology and England Manager at invertebrate conservation charity Buglife, inspects a signal crayfish he found in the waterway 'Markeaton Brook' in Markeaton Park in Derby. (Credit: Oli Scarff / Getty Images).
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