Measuring recruitment of minute larvae in a complex field environment: The corallivorous nudibranch Phestilla sibogae (Bergh)

The nudibranch Phestilla sibogae feeds only on corals of the genus Porites. The nudibranch's minute (∼ 200 μm) larvae are specifically induced to settle and metamorphose by a chemical cue released by the coral, causing the larvae to recruit to reefs composed predominantly of Porites compressa. In this study, we investigated temporal and spatial patterns of recruitment of P. sibogae into coral reefs in Kaneʻohe Bay, HI. We collected heads of P. compressa at 3-week intervals for 3 years, brought them to the laboratory and maintained them in aquaria fed with filtered seawater for 2 weeks, and then examined them for the presence of juvenile P. sibogae that had grown large enough to be seen. We found that P. sibogae recruits to the Porites reefs of Kaneʻohe Bay sporadically and unpredictably throughout the year. Although most coral samples contained no or very few P. sibogae, three periods of intense recruitment (90-450 juvenile P. sibogae kg− 1 of coral) were recorded, all in different seasons. Size-frequency analysis of recruits on the coral revealed high rates of post-settlement mortality in the field, most likely due to predation. Given the short pre-competent larval period of P. sibogae, the low rate of flushing of Kaneʻohe Bay and the patterns of recruitment observed, we conclude that this population of P. sibogae is essentially a self-recruiting one. Two of the sampled reefs were characterized by unidirectional flow, allowing us to test a model of transport of larvae of P. sibogae responding to dissolved coral cue in turbulent, wavy flow. The model predicts that more larvae will be transported into upstream portions of a reef than into downstream portions, a prediction confirmed by analysis of the field-recruitment data. Furthermore, field releases of larval mimic particles also showed that most mimics landed in the upstream areas of reefs and down among the bases of coral branches, rather than at their tips.