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

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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.

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