Reintroducing the Endangered White-Clawed Crayfish

Ecosystem engineers

Built with a mechanical yet agile exoskeleton, the white-clawed crayfish is well equipped to traverse fast-flowing streams and navigate rocky waters. As it burrows in search of food and buries to hide away from predators, the crayfish affects sediment buildup on stream and river beds. As such, their movement and feeding behaviour positively influences nutrient cycling and the quality of the habitat, benefiting other aquatic species.

A key player in trophic webs

The crayfish is also an important link in the web of interactions as both a predator and prey species. Armed with large imposing white claws, crayfish are omnivores that devour a wide variety of other species and matter. This easy-to-please eater has a diet consisting of macro-invertebrates, small vertebrates and aquatic plants. Feeding on both living and decaying plants and animals, the crayfish is one of nature’s cleaners. Studies show this opportunistic feeder affects the composition of species in the community. For example, by cropping stands of aquatic vegetation with its claws, crayfish control the amounts of plant-living invertebrates. Also, by shredding leaf litter, it supports collector species who mop up the remains of particulate matter.

An abundance of white-clawed crayfish is also vital for predating species. Many different mammals, fish and birds feed on them. Otters, eels and herons are a few examples of animals who predate on crayfish.

Bioindicators or surrogates for water quality 

White-clawed crayfish settle in streams, rivers and ditches by silt and stone beds with clear, well-oxygenated water and free of pollutants. Their preference for more pristine aquatic conditions means they act as a biological indicator of environmental integrity in regions like the Apennines. 

A novel invertebrate flagship species

Flagship species are those who hold an appeal to the public for being charismatic and ecologically significant. These roles are usually reserved for attractive mammal species, such as the panda, with invertebrates receiving far less popularity. The crayfish, however, challenges this trend and is considered a flagship species because of its outstanding ecological impact, longevity and cultural importance.

The ‘crayfish plague’

In the mid-nineteenth century in Italy, non-native species of crayfish were introduced to watercourses with the intention of restocking local populations. Unbeknown at the time, the non-native species were vectors of what’s known as the ‘crayfish plague’ caused by a fungus called Aphanomyces astaci. Invasive non-native species like the signal crayfish (Pacifastacus leniusculus), although immune to the disease themselves, are hosts for the pathogen. In 1859, the first occurrence of crayfish plague was recorded, wiping out populations of native species, who have no natural resistance. Still today, the disease has a devastating effect on white-clawed crayfish in many regions. With a high mortality rate, often 100%, whole populations are decimated only weeks after the disease enters a waterway.

Targeted reintroductions: Avoiding the plague

In spite of the threats, there have been positive results from conservation initiatives in the UK, France and Italy to restore native populations. As the crayfish plague requires a host, without it the fungus can only survive for a limited period (a few hours to several weeks). If efforts to reinforce populations after an outbreak are targeted at waterways where no alien crayfish exist, the risk of infection is greatly reduced.

Reintroductions should be complemented by measures that address the spread of disease in other ways. As the crayfish plague can be carried via contaminated equipment such as fishing gear, there is a need to raise awareness among local river users of the issue and inform them of best practices, such as disinfecting equipment.