Species-specific phototaxis of coral larvae causes variation in vertical positioning during dispersal

Controlling vertical positioning is a key factor limiting the distance coral larvae can travel, as oceanic currents are faster closer to surface. Currently, the vertical position of coral larvae is assumed to be determined by buoyant, lipid-rich gametes. However, here we show that some, but not all, coral species can control vertical positioning by phototaxis. We first examined the effect of light on the vertical positioning of larvae from five different coral species in the laboratory. We found that larvae from P. verrucosa, but not from other coral species, show phototaxis towards light and accumulate near the surface. This behavior was consistent at any age and at any time during the day. In field experiments, using P. verrucosa larvae at three different depths (1, 7 and 15 m), the accumulation of larvae in the top half of transparent chambers was observed at all depths. However, such behavior failed to occur in dark chambers. We conclude that larvae from P. verrucosa, but not all coral species, accumulate close to the seawater surface as a result of actively swimming towards sunlight. This finding provides a new hypothesis that phototactic behavior is a key factor in regulating vertical positioning for the dispersal of coral larvae.