This study surveyed gregarine parasites that infect the amphipod, Gammarus fasciatus, to investigate temporal dynamics in infracommunity structure. We sampled a population of hosts for 2 yr from the north branch of the Raritan River in New Jersey. These hosts were infected with 2 direct life cycle gregarine parasites, Rotundula gammari and Heliospora longissima. Infections were separated temporally, with the prevalence of R. gammari peaking within the amphipod population in the fall (prevalence = 78% year 1 and 97% year 2) and H. longissima peaking in early spring (prevalence = 41% year 1 and 52% year 2). Increases in host population density did not significantly correlate with the abundance of these 2 parasites. However, H. longissima abundance was positively correlated with host body weight while R. gammari showed no significant relationship. The mean body mass of amphipods infected with H. longissima was 20.7 ± 1. 2 mg, and with R. gammari 8.1 ± 0.2 mg, which suggests a sized-based infection pattern. Mixed species infections were infrequent with an overall prevalence of 4.6%. When both gregarine species co-infected the same host, the R. gammari but not the H. longissima infrapopulation size was significantly lower when compared to single-species infections, suggesting asymmetric interactions. We conclude that the observed temporal patterns of infection by the 2 parasites are driven by a seasonal change in host demographics and size-dependent infections. We argue that specificity for host developmental stages may have arisen as a mechanism to avoid overlap between these gregarine species.
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Vol. 103 • No. 1