Soluble settlement cue in slowly moving water within coral reefs induces larval adhesion to surfaces

Abstract

Larvae of many benthic marine animals are induced to settle and metamorphose by dissolved chemical cues released by organisms on the substratum. Can dissolved cues released into the turbulent, wave-dominated flow typical of many shallow coastal areas affect the adhesion of settling larvae to benthic surfaces? We addressed this question using larvae of the nudibranch, Phestilla sibogae, which settle and metamorphose in response to a water-borne, species-specific metabolite of their prey, Porites compressa, abundant corals forming reefs in shallow habitats in Hawaii. Field measurements of water velocities showed oscillatory wave-driven flow above reefs with peak instantaneous velocities of 0.10–0.40 m/s, much slower back-and-forth water movement through the spaces within reefs with peak velocities of 0.02–0.04 m/s, and net shoreward transport of water through reefs of ∼0.01 m/s. We used a water channel in the laboratory to measure the wall shear stresses required to dislodge larvae of P. sibogae from various surfaces. We found that cue from P. compressa is necessary for the larvae to attach to surfaces and, if cue is dissolved in water bathing the larvae, they can adhere to surfaces other than living P. compressa. After 2 h of exposure to cue and a surface, the adhesive strength of the larvae reached its peak value and did not change during the next 20 h. The mean nominal wall shear stress required to dislodge larvae of P. sibogae attached to P. compressa tissue (1.59±0.64 Pa, n=10 experiments) was not significantly different from that necessary to wash them off coralline algae (2.53±2.45 Pa, n=8) encrusting coral skeleton collected within reefs, but they stuck more tightly to glass (4.26±1.04 Pa, n=13). It is likely that most P. sibogae larvae initially settle onto surfaces within reefs because (1) settlement cue released by P. compressa and sinking larvae of P. sibogae accumulate in the slowly moving water within a reef; (2) larvae exposed to cue are able to stick to surfaces such as coralline algae that are common within the reef; and (3) P. sibogae develop their full attachment strength slowly, and the magnitude and frequency of peak shear stresses due to turbulent sweeps that might wash larvae off surfaces are much lower within than at the top of the reef.