First description of epizoic ciliates (<i>Sessilida</i> Stein, 1933) on <i>Bathyporeia</i> Lindström, 1855 (Peracarida, Amphipoda) and infestation patterns in brackish and marine waters

During a re-examination of macrozoobenthic samples from a long-term monitoring programme in the Dutch Oosterschelde and Westerschelde, the abundant presence of peritrich ciliates on specimens of Bathyporeia Lindström, 1855, was discovered. Out of the more than 3500 <i>Bathyporeia</i> specimens investigated, 44% contained ciliates. Although <i>Bathyporeia sarsi </i> Watkin, 1938 was significantly more often infested than <i>Bathyporeia pilosa</i> Lindström, 1855, these differences in infestation rates were largely due to differences between water bodies with higher infestation rates in the polyhaline than in the mesohaline reach. Observation of additionally collected living specimens and freshly preserved material showed that at least two, and likely three, species of ciliates are present of which two might be undescribed so far. One of the observed species matches <i>Zoothamnium nanum</i> Kahl, 1933. A second species belongs to the genus <i>Epistylis</i> Ehrenberg, 1830, but does not seem to match a so far known species. This also accounts for a possible third species belonging to the genus <i>Zoothamnium</i> Bory de St. Vincent, 1826, deviating from <i>Z. nanum</i> amongst others in the habitus of the stalk. The front part of <i>Bathyporeia</i> spp. and the antennae in particular, significantly more often harboured ciliates than the remainder of the body, where additional ciliates were only found on the ventral side. This shows that the peritrich ciliates benefit from the water currents induced by the basibiont, providing food items, but might also indicate that <i>Bathyporeia</i> spp. benefits from the presence of the epibionts as they are most prevalent on the body parts that are easiest to clean. Analyses of densities and distributions of epibiont and basibiont species gave first indications of the ecological niche of the peritrich ciliate communities