Larval vertical migration and hierarchical selectivity of settlement in a brooding marine sponge

Knowledge of larval behaviours of sessile marine invertebrates from release to recruitment and of the role these behaviours play in determining adult distributions is limited. In manipulative experiments using larvae from the Great Barrier Reef sponge Rhopaloeides odorabile, we quantified larval behaviours associated with vertical migration, phototaxis and swimming ability. We also measured settlement responses to cues associated with light, settlement surface micro-topography, coral rubble and biofilms. Following an afternoon release, the majority of larvae (72%) migrated vertically to the surface (light) for 6 to 18 h. After 24 h, 55% of active larvae had moved from the surface to the bottom and maintained this position for up to 54 h before settling. Larvae did not display gregarious settlement patterns, or a preference for settlement surface topographies, but did preferentially settle to light-exposed surfaces. Initial settlement to biofilms or coral rubble was higher than in controls with no cue. However, the transition from initial settlement and attachment to metamorphosis was much higher when treatments comprised a combination of biofilm and coral rubble compared to biofilm-only treatments (49 vs. 9%). Overall, this demonstrates that hierarchical cues contribute to selective settlement. Vertical migration to surface waters facilitates passive dispersal via wind-driven surface currents and contributes to wide-scale dispersal, while a subsequent demersal phase, where larvae actively explore the benthos for settlement sites, enables dispersal over fine, micro-geographic spatial scales.