Ontogeny of settlement behaviours in response to Grammatophora marina diatom biofilms in the marine polychaete, Platynereis dumerilii

Abstract Settlement, the transition of a swimming planktonic larva to a benthic juvenile, is a key process during the development of many marine invertebrates. Microbial biofilms act as positive cues for larval settlement across species, yet the behavioural processes preceding settlement are poorly understood. Here, we investigated age-dependent changes in settlement behaviour in marine polychaete Platynereis dumerilii larvae in response to Grammatophora marina diatom biofilms. Settlement behaviours (crawling, crawling speed and track straightness) were quantified from recordings of larvae ranging from the mid-trochophore to late-nectochaete in the presence or absence of diatom biofilms, using spot-tracking software. As larvae developed, the proportion of individuals crawling over the biofilm increased. Older larvae colonized biofilms more rapidly and showed greater discrimination between G. marina biofilms and controls. Older larvae straighten their movements compared with individuals swimming in the presence of biofilms, or behaviour witnessed in the absence of biofilms. The prevalence of these behaviours may reflect changes in the prioritization of sensory inputs from physical and chemical cues as larvae develop. Our findings suggest that settlement behaviours are developmentally programmed in P. dumerilii. Understanding settlement behaviours in P. dumerilii expands on this species’ behavioural repertoire and sheds light on the evolutionary relationship between marine larvae and microalgal biofilms.