Effects of gamete dilution, age and contact time on fertilization success in the tropical sea urchin, Echinometra mathaei

A series of laboratory experiments were conducted to determine the effect of sperm dilution, egg concentration, sperm-egg contact time, and gamete aging on fertilization success in the tropical sea urchin, Echinometra mathaei. The results demonstrated that sperm dilution, sperm age, and sperm-egg contact time were sequentially the most important factors influencing fertilization success, while egg concentration was not significant over the range tested. Sperms retained their potency for more than two hours only in relatively dense sperm suspensions (≥10-4 dilution of ‘dry’ sperm) whereas they exhibited lower viability with increasing dilutions and age. In egg-sperm contact time experiments more than 80% fertilization was achieved at lower sperm dilutions (10-3-10-2) within 10 sec of mixing, while at higher sperm dilutions, longer times of contact were needed to achieve the similar fertilizations. Consequently, eggs remained in good quality for up to 3 h and there was no abnormality or adverse effects in fertilization were observed in a series of sperm dilution tested. These laboratory experiments suggest that sperm dilution and its limited longevity can play an important role in limiting the fertilization of sea urchin eggs in the field during natural spawnings. It follows, therefore, that sea urchin (E. mathaei) are under considerable selective pressures to spawn synchronously in order to generate high sperm concentrations and higher sperm-egg encounters in the water column to maximize the probability of successful fertilization.