DENSITY-DEPENDENT SELECTION ON GAMETE TRAITS IN THREE CONGENERIC SEA URCHINS

Abstract

Because male distribution and abundance strongly influence female fertilization success, density-dependent selection might act on gamete traits to maximize reproductive success. Intra- and interspecific variation in female fertilization success was investigated in the sea urchins Strongylocentrotus purpuratus, S. franciscanus, and S. droebachiensis. Male and female urchins of each species were induced to spawn within 25-m2 plots. The percentage of eggs fertilized, along with six covariates (male density, male–female distance, number of males, surge velocity, advection, and water depth), was measured for each spawning event. S. purpuratus had the highest levels of fertilization (94%), followed by S. franciscanus (64%) and S. droebachiensis (23%). The species with the highest densities, most males, and closest neighbors had the highest levels of fertilization. Depth and surge were not significant factors, but advection was associated with fertilization negatively in S. purpuratus and positively in S. droebachiensis. This interaction is probably caused by the higher degree of clumping in S. purpuratus than in S. droebachiensis. An analysis of covariance indicates that the rank order of least-squares means of percentage fertilization for these three species is the opposite of that for the uncorrected species means. S. droebachiensis performs best and S. purpuratus performs worst, when conditions are held equal. These differences in performance can best be explained by gamete traits. Additional experiments determined (1) that the 25-m2 plot size is the appropriate scale for investigation of patterns of fertilization in S. franciscanus, the species with the largest contiguous populations; (2) that S. purpuratus, the species commonly found in the intertidal and subtidal zones, had similarly high levels of fertilization in the two locations; and (3) that fertilization success in the experimental plots was similar to that during natural spawning events. These results are consistent with density-dependent selection on gamete traits. Gametes of the species that naturally occurs at the lowest population density and has the lowest natural fertilization rates have attributes that cause them to perform the best of the three under conditions of sperm limitation, and those of the species that occurs at the highest densities and has the highest levels of natural fertilization perform most efficiently under conditions of sperm competition. Historical demographic conditions appear to have resulted in selection on gamete traits to maximize reproductive success under conditions of either sperm limitation or sperm competition.