Reproductive success in large populations: empirical measures and theoretical predictions of fertilization in the sea biscuit Clypeaster rosaceus

Abstract

For organisms that free-spawn gametes into the environment, sperm limitation can be a major determinant of reproductive success. Previous tests of sperm limitation have been restricted to very small experimental populations. Here we test and then use a fertilization model to explore sperm limitation in large populations. Predictions of the fertilization model are compared with measures of dye diffusion and in situ fertilization of the sea biscuit Clypeaster rosaceus (Linnaeus). The model could not be rejected in either test. Then this model was used to simulate large-scale spawning events in a natural population of C. rosaceus. The results of our simulations indicate that both population size and population density are important to fertilization over a very large range (2 to over 250000 individuals), but we also found an important interaction between population size and density. The importance of high density was great in small populations but negligible in large populations. This result may provide insight into why aggregation during spawning is not universally seen in nature. Overall, results indicate that sperm limitation can both constrain reproductive success and mediate social behaviors in a wide range in population sizes.