Aphanomyces astaci is a water mold that causes a disease known as crayfish plague, primarily affecting freshwater crayfish species. This pathogen is particularly harmful to native European crayfish, leading to significant population declines. It spreads through contaminated water or infected crayfish, making it a serious threat to aquatic ecosystems. The disease manifests through symptoms such as lethargy, loss of appetite, and discoloration in affected crayfish. Infected individuals often die within a few weeks, contributing to the decline of local crayfish populations. Aphanomyces astaci has also been linked to the decline of native species due to competition with invasive species like the North American signal crayfish.