A hierarchy of settlement cues influences larval behaviour in a coral reef sponge

For sessile marine invertebrates, processes contributing to larval release, dispersal, and settlement in favourable habitats are central to patterns of distribution and community structure. We quantified larval release patterns, phototactic behaviour, and settlement in response to environmental cues for the coral reef sponge Luffariella variabilis. Individual sponges released up to 830 larvae d–1. Larvae displayed phototactic behaviours by swimming upwards after release for brief periods (40 min), after which time most larvae (>95%) exhibited negative phototaxis. Light played a role in determining numbers and rates of larval settlement. Light levels of 56 μmol s–1 m–2 reduced the rate of settlement and inhibited larval settlement by 60% compared to dark controls. However, at lower light levels (0.7 to 0.34 μmol s–1 m–2), both time to settlement and numbers of larvae settling were consistent with settlement in dark controls. Larval settlement increased in the presence of other larvae, with >95% of larvae settling when placed in treatments with 50 individuals, compared to 50% settlement for treatments containing only 1 individual. The gregarious settlement of L. variabilis larvae was associated with conspecific larval settlement cues. Settlement in ‘conditioned’ water from which 200 larvae had previously settled and subsequently been removed was 80%, compared to 20% in controls. Our study unequivocally demonstrates that a conspecific cue not related to adults or other biotic or abiotic factors induces settlement in larvae. Our observations, the preference of larvae to settle in response to low light levels and of settlement increased by gregariousness, correspond with the cryptic and clumped distribution of L. variabilis in the field.