Spatial and temporal patterns in the occurrence of peritrich ciliates as epibionts on calanoid copepods in the Chesapeake Bay, USA.

We investigated temporal and spatial patterns of distribution in two peritrich ciliates (i.e. Zoothamnium intermedium and Epistylis sp.) living as epibionts on calanoid copepods (i.e. Acartia tonsa and Eurytemora affinis) in Chesapeake Bay. Net tow samples collected along the main axis of the Bay were analyzed to estimate the occurrence of epibionts on copepods and to explore relationships among infestation prevalence, host abundance, and environmental variables. Zoothamnium intermedium and Epistylis sp. colonized populations of A. tonsa during spring and summer months, while only Z. intermedium colonized E. affinis during spring. Occurrence of epibionts on copepods showed high interannual variation, marked seasonality, and geographic heterogeneity. Extensive statistical analyses rejected simple scenarios of interactions between epibiosis, environmental variables, and host density, suggesting a more complex dynamics for the system. Analyses of epibiont colonies and zooids per host area (i.e. the sum of width and length of the body including antennae and swimming legs calculated assuming a cylindrical shape) were also performed. Overall, epibiont infestation prevalence (i.e. colonies/host area) and load (i.e. zooids/host area) were higher on copepodites than on adults for both host species, suggesting a preferential attachment to juveniles, or a higher predation pressure on adult stages. Infestation density and loads of both epibiont species were higher on the cephalothorax and abdomen of A. tonsa and E. affinis in comparison to the antennae and swimming legs, suggesting that ciliates can more easily colonize less active parts of the host.