Larval behaviour and settlement cues of a brooding coral reef sponge

Abstract

Patterns of larval release, dispersal and settlement in sponges are poorly understood despite their significance in explaining adult ecology. Time of release, swimming speeds, phototaxis and vertical migration were quantified for larvae of the dictyoceratid sponge Coscinoderma matthewsi. The influence of cues associated with biofilms and coral rubble on larval settlement and metamorphosis was also measured. C. matthewsi is a brooding sponge and releases tufted parenchymellae larvae during the day. Upon release, larvae (>90%) have no phototactic response, maintaining their position at the water surface for 80 min ± 0 (mean ± SE) regardless of a light cue (natural daylight) before exhibiting negative phototaxis. At 28 h post-release, the majority of larvae (94.7% ± 6.1) exposed to light from the surface migrated to the bottom and assumed a demersal phase. Without light, larvae occupied the surface for up to 28 h post-release (89.3% ± 1.8) before migrating to the bottom. Larvae did not settle gregariously and began to settle and metamorphose after 28 h post-release without a cue. Settlement and metamorphosis were faster in the presence of a biofilm (settlement = 15.0% ± 8.7 and metamorphosis = 12.5% ± 9.5 at 28 h post-release), while the addition of coral rubble accelerated metamorphosis further (settlement = 10.0% ± 4.1 and metamorphosis = 27.5% ± 10.3 at 28 h post-release) compared to controls (sterile surfaces) (settlement = 0% and metamorphosis = 0% at 28 h post-release). However, both biofilms and coral rubble decrease total metamorphosis (control = 92.5% ± 4.8, biofilms = 67.5% ± 7.5 and coral rubble = 55.0% ± 13.2) due to mortality after 76 h post-release.