HADFIELD, M. G.*; KOEHL, M. A. R.: Dissolved cues to invertebrate larval settlement: do they work in moving water?

Planktonic larvae of the tropical nudibranch Phestilla sibogae have been utilized for numerous studies of the patterns and mechanisms of settlement and metamorphosis in response to specific dissolved cues. A small, polar metabolite from the adult prey coral Porites compressa elicits specific, rapid metamorphosis. In this study we probe the effectiveness of such cues in the turbulent flow of coral reefs. Field and flume studies have shown that dissolved cue from a P. compressa reef in turbulent, wave-driven flow forms complex patterns of fine filaments above the corals. We tethered individual larvae of P. sibogae in a miniflume and exposed them to water flow that mimicked what a freely swimming larva would encounter. We then videotaped the responses of each larva as it was exposed to temporal patterns of cue filaments like those it would encounter at different distances above a reef. These observations revealed that larvae quickly retract the velum (but not the foot) and cease swimming when they encounter filaments of water containing coral metabolites, and resume swimming when they pass out of them. Because larvae sink when they quit swimming, the effect of entering successively wider filaments of coral metabolite as they near the reef is to bring about rapid contact and settlement. As in every other aspect of larval development studied in P. sibogae, from hatching time to age at competence, variance among behavioral responses to coral cue in flowing water is great.