The influence of biofilm-associated ciliates on the settlement of Galeolaria caespitosa

The settlement and metamorphosis of many marine invertebrates is strongly influenced by physical and chemical cues associated with microbial biofilms. Protozoa, a ubiquitous component of microbial biofilms, are able to rapidly... [ view full abstract ]

The settlement and metamorphosis of many marine invertebrates is strongly influenced by physical and chemical cues associated with microbial biofilms. Protozoa, a ubiquitous component of microbial biofilms, are able to rapidly colonise new substrata and over a short period of time reach high abundances within biofilms, yet their potential to influence invertebrate settlement remains largely unknown. We conducted still-water, no-choice settlement assays to determine the effects of marine biofilm-dwelling ciliates on settlement of the polychaete worm Galeolaria caespitosa. Settlement was compared among three biofilm conditions: a purely mono-species bacterial biofilm (Pseudoalteromonas luteoviolacea), the bacterial biofilm with a ciliate species present and the bacterial biofilm with filtrate from said ciliate culture. In response to the presence of common biofilm associated ciliates Euplotes, Uronema and Amphisiella, G. caespitosa showed a significant 36-45% reduction in settlement over 72 hours compared to that on the purely bacterial biofilm treatments. Settlement rates in the filtrate treatments were at no point significantly different from the controls. Settlement was only significantly inhibited in the physical presence of ciliates implicating a direct mechanism of influence. Through video analysis it was determined that the presence of Euplotes minuta elicits significant behavioural changes in the surface exploration of G. caespitosa larvae. In the absence of ciliates larvae follow a natural behavioural progression concluding with settlement and final metamorphosis, however, making direct contact with a ciliate caused this progression to reverse. This analysis successfully demonstrated a mechanism of influence that could explain the settlement inhibition observed in the settlement assays, and represents the first time that a behavioural response of invertebrate larvae caused by the presence of ciliates has been reported. The results of this study support the hypothesis that biofilm-dwelling ciliates are an important factor influencing the variability of invertebrate recruitment and ultimately the structure and dynamics of natural invertebrate assemblages.