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

Measurements of fertilization rates in free-spawning marine invertebrates h ave shown that reproductive success is related to both behavioural and envi ronmental 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 d istance from sperm source but were frequently greater than 20% at distances >10 m downstream. Current speed did not have a significant effect on ferti lization rate over the range of velocities examined. Fertilization rates di rectly downstream from spawning individuals were higher in shallow water (< 1 m) than in deep 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 s perm source. Once lateral diffusion of sperm was accounted for, the model p redicted little overall difference in fertilization rates for shallow and d eep spawners. These results from Coscinasterias indicate that movements int o shallow water at the time of spawning, which have been reported in severa l asteroid species, may provide little reproductive advantage in environmen ts experiencing net flow conditions.