Additional Predators (Nonhosts) and a New Amphibian Host of the Digenetic Trematode Cercaria of Proterometra macrostoma in Laboratory Experiments

The consumption of digenetic trematode cercariae by nonhosts can affect energy transfer in aquatic ecosystems and simultaneously reduce the numbers of these infectious agents. Recently, crayfish (i.e., Faxonius juvenilis and Procambarus spp.) were documented as predators/nonhosts of the digenetic trematode cercaria of Proterometra macrostoma in laboratory experiments. The objective of this study was to assess additional invertebrate and vertebrate predators of this cercaria species during experiments, which included blue dasher dragonfly nymphs (Pachydiplax longipennis), orange bluet damselfly nymphs (Enallagma signatum), mosquitofish (Gambusia sp.), and Cope's gray tree frog tadpoles (Hyla chrysoscelis). Individuals of each species were divided into 2 groups and exposed to 2 levels of 2–3-hr-old cercariae in 10.8-cm culture dishes filled with 200 ml of spring water. The high exposure (HE; 4–6 cercariae/predator) level was approximately 2× the number of cercariae compared to the low exposure (LE; 2–3 cercariae/predator) level. All 4 potential predators ingested cercariae, but in terms of the average percentage of cercariae consumed following 7 hr of exposure in the LE and HE trials, Gambusia sp. (LE = 83.3%; HE = 80%) appeared to be the most successful predator, followed by P. longipennis (LE = 67%; HE = 45%), E. signatum (LE = 41.7%; HE = 33.3%), and H. chrysoscelis (LE = 20.8%; HE = 37.5%). There was a significant difference in the mean number of cercariae consumed after 7 hr postexposure (PE) in the LE vs. HE groups for Gambusia sp. For both Gambusia sp. and P. longipennis, LE and HE groups ingested the majority of the cercariae to which they were exposed within the first hour. By comparison, a more gradual consumption was noted for E. signatum and H. chrysoscelis. Living distomes of P. macrostoma were recovered from H. chrysoscelis stomachs following ingestion on days 1 and 6 PE. This suggests that some amphibians may serve as definitive hosts for this parasite, which has only previously been reported from centrarchid fish. Collectively, these results suggest a broad food niche for P. macrostoma cercariae in aquatic food webs with regard to nonhosts.