Morphological and physiological consequences of parasites encysted in the bulbus arteriosus of an estuarine fish, the sheepshead minnow, Cyprinodon variegatus.

The contribution of parasites to host population regulation is undefinable without knowledge of parasite effects on individual host performance and survival. Effects are best determined by examining performance of differentially parasitized hosts under the varying environmental conditions that characterize the host's habitat. The sheepshead minnow, Cyprinodon variegatus, an estuarine fish, is heavily parasitized in the bulbus arteriosus by the heterophyid trematode Ascocotyle pachycystis in semi-enclosed shallow bodies of water in northwestern Florida. Laboratory experiments were conducted to examine the morphological and physiological effects of parasitization of these fish. Differentially parasitized fish were compared for differences in ventricle weight and swimming performance. Parasitized fish ventricles were hypertrophied due to mechanical blockage of normal blood flow by metacercariae in the bulbus. Swimming performance generally was reduced by the presence of parasites, which had their most significant negative effect at cold temperatures and low dissolved oxygen levels.