Aphanomyces astaci is a water mold that causes a disease known as crayfish plague, primarily affecting freshwater crayfish. This pathogen is particularly harmful to native European crayfish species, 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. In contrast, North American crayfish species often show resistance to Aphanomyces astaci, which has contributed to the decline of European species following the introduction of these resistant species into European waters.