Sessile Zoothamnium sp. ciliates colonise the invasive Oriental river prawn Macrobrachium nipponense in a brackish estuary in Ukraine.

Abstract

Sessilid ciliates are common epibionts of aquatic crustaceans, where mass colonisation of economically important prawns may lead to adverse effects on host health and ecosystem dynamics. The Oriental river prawn Macrobrachium nipponense, which is native to Asia, has recently expanded into European freshwater and brackish systems. As a host species, it may act as a vector for sessilid ciliates into new habitats, posing risks to native biota and aquaculture. We examined M. nipponense populations in brackish water bodies of southwestern Ukraine to assess the prevalence and localization of sessilid ciliates. Live and preserved specimens were microscopically examined. Morphological analysis focused on the distribution and abundance of Zoothamnium colonies on various body parts. Molecular material was also preserved for future species identification. Over 80% of prawns were found to carry large colonies of Zoothamnium sp., with high infestation intensity particularly on pereiopods and uropods. Ciliate aggregations often appeared near articulation points, which may obstruct mobility or respiration. Infestation patterns were consistent across sexes, though some correlations were noted between parasite load and reproductive condition in females. These results suggest that sessilid ciliates may exhibit parasitic characteristics under favourable conditions, undermining host performance. The expansion of M. nipponense may facilitate the introduction and establishment of such organisms across Europe. This study highlights the ecological implications of overlooked epibionts and stresses the importance of monitoring invasive host-epibiont complexes in aquatic environments.