Aggressive American signal crayfish (Pacifastacus leniusculus) are threatening Britain’s native white-clawed crayfish (Austropotamobius pallipes) populations because they have better resistance to parasites and are less fussy about what they eat. The UK species suffers from two parasites: plague, which is carried by the American invader, as well as porcelain disease, which makes native crayfish sluggish and suppresses their appetite before eventually killing them a few years later.
The advance of the signal crayfish over its native rival has implications for the biodiversity of Britain’s rivers as the diversity of prey is reduced and the invaders’ appetite for fish eggs is causing a decline in the fish population.
The area of Yorkshire in the center of Britain is one of the last strongholds of the white-clawed crayfish, which is being increasingly driven out of the country’s waterways by its hardier American cousin. The signal crayfish was originally introduced to the UK in the 1970s for fish farming.
Although the white-clawed crayfish is now listed as an endangered species in the UK, the reasons for its decline and the American species’ success have not been well understood. Now, researchers from the University of Leeds’ Faculty of Biological Sciences have been working out why the signal crayfish has been gaining the upper hand, and how this information might be used in conservation projects.
Their study compared how quickly the two different species deal with food. The American signal crayfish ate up to 83 percent more food per day than did their native cousins. The research also showed that white-clawed crayfish are much more choosy about what they eat, preferring particular types of prey, while the signals eat equal amounts of all prey.
The white-clawed crayfish are also affected by a common parasite, known as porcelain disease, which affects their ability to catch prey, leading affected crayfish to eat 30 percent less. The American signal crayfish, on the other hand, seem unaffected by the parasite.
A Healthy Appetite
Dr. Alison Dunn of the Faculty of Biological Sciences at the University of Leeds, who led the study, explains: “The signals eat much more compared with the native crayfish. But the situation is exacerbated by a parasite that essentially changes the behavior of the white-clawed crayfish; it can’t eat or handle as much food as the signal, because the parasite weakens its muscles.
“The huge appetites of the signal crayfish can also have a massive effect on the whole ecosystem,” adds Dr. Dunn. “In particular it affects biodiversity because there is a reduction in the numbers of prey. In some Yorkshire rivers, for example, the fish population has declined because signal crayfish are eating large numbers of fish eggs.”
Dr. Dunn believes studying the effects of parasites on host species can offer vital clues about species conservation. “Parasites are a fascinating and vital part of any ecosystem and you have to consider their effects when looking at biological invasions,” she says. “We hope our findings will help us make predictions about how this invader might spread and help with management strategies.”
People are being urged to play a part in protecting the white-clawed crayfish by understanding how its rival spreads. The signal crayfish is able to move over land of its own accord, but it may also be inadvertently moved around by anglers who fail to spot it in damp fishing gear or by those using the crayfish as bait.
Reference: Neal R. Haddaway, Ruth H. Wilcox, Rachael E. A. Heptonstall, Hannah M. Griffiths, Robert J. G. Mortimer, Martin Christmas, Alison M. Dunn. Predatory Functional Response and Prey Choice Identify Predation Differences between Native/Invasive and Parasitised/Unparasitised Crayfish. PLoS ONE, 2012; 7 (2): e32229 DOI: <10.1371/journal.pone.0032229>