Increased Parasite Abundance Associated with Reproductive Maturity of the Clam

Several studies on vertebrates have demonstrated that re- productive activities may increase the parasite load, but this has not been shown in invertebrate hosts. We studied abundance of a potentially harmful gill parasite, the ergasilid copepod Paraergasilus rylovi, from the freshwater bivalve host Anodonta piscinalis in relation to reproduc- tive maturity of the host in the field. Prevalence of this previously un- studied parasite varied from 90 to 100%, and the mean parasite abun- dance from 16.3 to 28.8 among 3 study populations. Abundance of P. rylovi increased with host size. In the maturating age groups (3-5 yr) the length-adjusted mean parasite abundance among mature, reproduc- ing female clams that brooded glochidia larvae was 2 times higher than in nonreproducing females, the observed pattern being consistent among the 3 study lakes. Alternative, mutually nonexclusive explanations may be found for the result. For example, changes in clam behavior or fil- tration activity accompaning maturation can increase host exposure to parasites, or reproduction may decrease energy available to host im- munologic defense. However, the present result indicates that matura- tion, and reproduction, is associated with increased parasite abundance in A. piscinalis, an invertebrate host.