The question whether a stress event can have a long-lasting effect on susceptibility to parasites was studied using a freshwater bivalve clam and its crustacean parasite as a model system. Anodonta piscinalis clams were collected from 2 populations during August–September 2002. Clams were transported to the laboratory and marked. The stressed clams were subjected to low oxygen for 25 days, while the unstressed control clams were caged in their lakes of origin for the same period. Then the clams were transported to a third lake where they were exposed to natural infections by the ergasilid copepod, Paraergasilus rylovi, 11 months after the stress event. The stressed clams were more intensively parasitized. They also showed lower growth, lower reproduction and lower survival than the unstressed control clams. The results indicate that susceptibility of A. piscinalis to P. rylovi infection may be condition dependent, and that stress may have a long-lasting, increasing effect on host susceptibility to parasitism in natural populations.
