Does spawning depth affect fertilization rates? Experimental data from the sea star Coscinasterias muricata

Measurements of fertilization rates in free-spawning marine invertebrates have shown that reproductive success is related to both behavioural and environmental factors. Water depth has been suggested as being one such factor affecting fertilization success. In experimental spawnings of the sea star Coscinasterias muricata, fertilization rates decreased exponentially with distance from sperm source but were frequently greater than 20% at distances >10 m downstream. Current speed did not have a significant effect on fertilization rate over the range of velocities examined. Fertilization rates directly downstream from spawning individuals were higher in shallow water (5 m). Diffusion models using empirically derived site-dependent diffusion parameters supported these findings but suggested that this would only be true for eggs released directly downstream from a sperm source. Once lateral diffusion of sperm was accounted for, the model predicted little overall difference in fertilization rates for shallow and deep spawners. These results from Coscinasterias indicate that movements into shallow water at the time of spawning, which have been reported in several asteroid species, may provide little reproductive advantage in environments experiencing net flow conditions.