Crayfish plague is an infectious disease that has decimated native crayfish populations, particularly in Europe. Caused by an invasive water mold, the illness spreads rapidly through freshwater, leading to high mortality rates in susceptible species. This has permanently altered aquatic ecosystems and represents an ongoing threat to crayfish biodiversity, highlighting the severe consequences of introducing non-native species.
The Pathogen and Its Carriers
The organism causing crayfish plague is a water mold, Aphanomyces astaci. While often called a fungus, it spreads through microscopic spores in aquatic environments. The pathogen is native to North America, where local crayfish have co-evolved and developed significant resistance.
Resistant North American crayfish, like the signal crayfish (Pacifastacus leniusculus), act as carriers. They harbor low-level infections without severe symptoms and continuously release spores. In contrast, crayfish native to Europe, Asia, and Australia have no natural immunity.
When these carriers are introduced into new ecosystems, local populations are exposed to a disease they cannot fight. A. astaci was introduced to Europe in the 19th century, linked to crayfish imported for aquaculture and the aquarium trade.
Identifying an Outbreak
An outbreak is identified by changes in crayfish behavior. Nocturnal crayfish may wander in open areas during the daytime. This is often accompanied by a loss of coordination, causing them to appear to walk on stilts with extended legs.
As the infection progresses, crayfish may show tail paralysis and lose their escape reflex. Fine, whitish, or brownish growths of the pathogen’s hyphae might also be visible on soft tissues around the leg joints, eyes, and under the tail.
Mass mortality is a hallmark of the disease. The mortality rate in susceptible populations often approaches 100% within weeks, and numerous dead crayfish on the riverbed confirms the plague’s presence.
How the Plague Spreads
The pathogen Aphanomyces astaci spreads through microscopic, free-swimming spores called zoospores. Infected carrier crayfish release vast quantities of these spores from their cuticle into the water, where they actively seek a new host to infect.
While spores facilitate local spread, the movement of the plague over longer distances is driven by other factors. The primary vector for introducing the plague to new areas is the translocation of live crayfish, whether intentional or accidental.
Human activities also contribute to the spread of spores without moving crayfish. Spores can hitchhike on damp equipment, and if these items are not properly cleaned and dried before use in a different waterway, they can trigger a new outbreak. Contaminated items can include:
- Fishing gear
- Boots and waders
- Boats
- Scientific survey equipment
Ecological Devastation
The loss of native crayfish negatively impacts the entire freshwater ecosystem. As a keystone species, crayfish serve as prey for fish, birds, and mammals. They also control smaller invertebrate populations and consume decaying organic matter.
Their burrowing and foraging also make them ecosystem engineers, creating shelter for other organisms and helping maintain water clarity. The removal of crayfish from a river or lake disrupts these processes.
Without crayfish, water quality can decline, and predators like eels and otters may struggle. In Europe, native species like the white-clawed and noble crayfish have been driven to local extinction. They are often replaced by the plague-carrying North American signal crayfish, altering the ecosystem.
Preventing Further Contamination
Preventing the spread of crayfish plague depends on the actions of people using freshwater environments. The “Check, Clean, Dry” protocol is the most effective strategy and should be applied to all equipment after use in any body of water.
First, “Check” all gear for visible mud, plants, or animals. These should be removed and left at the site before leaving, as spores can be embedded in soil and organic debris.
Next, “Clean” all equipment with hot water, preferably above 40°C (104°F), as the spores are sensitive to high temperatures. If hot water is not available, an environmentally safe disinfectant can be used.
Finally, “Dry” all gear completely for at least 48 hours. This step is important because the spores cannot survive desiccation. A longer drying time is recommended in cool or damp conditions.