THE EFFECTS OF SPATIAL AND TEMPORAL HETEROGENEITY AS STRUCTURING FORCES FOR PARASITE COMMUNITIES IN HELISOMA-ANCEPS AND PHYSA-GYRINA

A total of 1231 Physa gyrina and 1532 Helisoma anceps were collected o ver a 12-mo period from Charlie's Pond, in Stokes County, North Caroli na. Similarity in the infra- and component parasite communities in Hel isoma anceps and Physa gyrina provided an opportunity to compare the e ffects of various life history traits on infection by larval trematode s in a number of different microhabitats. In order to assess the effec ts of microhabitat partitioning on the infection status of the two sna il species, collections were made twice per month. Site location, wate r depth, snail depth at capture, distance from shore, type of substrat um, infection status, and host size were recorded for each snail. Mult ivariate statistics were used to assess which variables determined a s nail's infection status. Data analysis indicated that certain of the m icrohabitat variables are better predictors of a snail's infection sta tus than others. Comparing uninfected snails to those infected with a specific parasite species, there was much disparity with regard to whi ch microhabitat characteristic varied significantly. Manipulation of i nfracommunity structure, by excluding certain parasites, allowed exami nation of the influence of temporal heterogeneity as compared to inter specific trematode interactions in structuring the infra- and componen t communities. Snails were maintained in cages in the field. Enclosure s were positioned a few centimeters above the substratum to prevent in fection of the snails via egg ingestion and effectively prevented the more prevalent parasite species (Halipegus occidualis in H. anceps and Halipegus eccentricus in P gyrina) from infecting the snails. Statist ical analyses indicate that preventing snails from ingesting eggs of s ome parasites had no affect on the recruitment of other parasites. Par asite prevalences in H. anceps and P gyrina were not different in cage d snails compared with those collected from the substrata of the pond.