Kinetics of Fertilization in the Sea Urchin Strongylocentrotus franciscanus: Interaction of Gamete Dilution, Age, and Contact Time

Abstract

Determining fertilization success of free spawning organisms in the field requires knowledge of how eggs and sperm interact under varying encounter fre- quencies and durations. In the laboratory, we investigated the relative influence of sperm concentration, egg con- centration, sperm-egg contact time, and sperm age on fer- tilization in the sea urchin Strongylocentrotus francis- canus. Our results indicated that sperm concentration, sperm-egg contact time, sperm age, and individual vari- ability were sequentially the most important factors in- fluencing fertilization success. Egg concentration was not significant over the range tested. A theoretical model of fertilization (Vogel-Czihak-Chang-Wolf model) was used to estimate the two rate constants of fertilization kinetics: the rate constant of sperm-egg encounter and rate constant of fertilization. This model explained 9 1% of the variation in fertilization success, provided estimates of the rate con- stants involved in fertilization, and indicated the propor- tion (3%) of sperm-egg contacts that result in fertilization. Estimates of sperm swimming velocity and egg diameter were used to independently calculate the rate of sperm- egg encounter and confirm the predictions of the model. This model also predicts the non-significant effect of egg concentration on fertilization success found empirically.