Role of freshwater inflow and salinity on population regulation in the hydrozoan inquiline symbiont Eutima sp

The hydrozoan Eutima sp. is an inquiline symbiont known to infest the gills of the eastern oyster Crassostrea virginica (Gmelin, 1791) in Florida waters. Although it is unclear whether Eutima sp. polyps impact oyster fitness, access to bivalve hosts plays a role in sustaining Eutima sp. populations within estuaries. In addition, reduced salinities associated with high levels of freshwater inflow (e.g., monsoons) have been implicated in influencing the distribution and abundance of hydrozoan medusae and polyps. We investigated the influence of variable freshwater inflow and salinity on medusa and polyp stages of Eutima sp. in Estero Bay, an estuary subjected to seasonal extremes in precipitation. Medusae were collected using plankton nets, and polyps were identified from oysters sampled along two transects within the bay. Polyp prevalence (%) at one site was zero when freshwater inflow 7 wks prior to sampling exceeded 0.5 m3 s−1, was reduced upstream along one of the transects at a site that experienced significantly reduced salinities, and was positively related to salinity. Centers of distribution for medusae were relocated seaward and away from oyster reefs during periods of high freshwater inflow. This relocation of medusae away from future bivalve hosts has the potential to create a spatial gap between planktonic (medusae) and benthic (polyps) habitats of Eutima sp. Our results suggest that freshwater inflow may not only limit the distribution and abundance of polyps inhabiting oysters by reducing salinities but may also impact the settlement of planulae onto reefs in the first place.