ABSTRACT: Protozoa, a ubiquitous component of microbial biofilms, are able to rapidly colonise new substrata and reach high abundances, yet their potential to influence invertebrate settlement on these biofilms remains largely unknown. We conducted still-water, no-choice settlement assays to determine the individual effects of 4 different species of marine biofilm-dwelling ciliates on settlement of the polychaete worm Galeolaria caespitosa. The ciliates included vagile and planktonic species common to natural assemblages. Each ciliate species significantly inhibited the settlement of G. caespitosa, with the exception of Litonotus sp. The extent of inhibition differed significantly among ciliate species. Settlement was significantly reduced in the presence of Amphisiella sp. (37.5%), Euplotes minuta (43.7%) and Uronema marinum (39.6%) over 72 h compared to the control. Settlement rates in the presence of filtrate from ciliate cultures were at no point significantly different from a control, ruling out a dissolved chemical cue; settlement was only inhibited in the physical presence of ciliates. Video analysis showed that E. minuta elicits more erratic movement and substrate avoidance in surface exploration by G. caespitosa larvae. In addition, the distribution of bacteria in the ciliate treatments was significantly more clustered than in controls, indicating a potential indirect mechanism by which ciliates impacted larval settlement. This is the first report of a behavioural response of invertebrate larvae following direct contact with ciliates, which, together with possible indirect interactions involving bacteria, may constrain invertebrate recruitment in nature.
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