Male Reproductive Success in Sessile Invertebrates: Competition for Fertilizations

Abstract

Recent in situ fertilization studies of free spawning and brooding marine organisms have focused almost exclusively on the yield of fertilized ova (female reproductive success). As a consequence, we known little about the factors that determine male reproductive success. If marine organisms compete for fertilizations (as do many terrestrial organisms), then a male's reproductive success should be reduced by the presence of other males. We tested this hypothesis via in situ experiments employing allozyme markers for both a colonial ascidian (Botryllus schlosseri) and a bryozoan (Celleporella hyalina). Under moderate density conditions, the presence of closer male—functioning colonies reduced the fertilization success of more distant males in both species. In C. hyalina, male fertilization success also increased with allocation to sperm production. In addition, selfing rates in this species were negatively correlated with the abundance of outcross sperm. These results suggest that male reproductive success in sessile marine invertebrates must be assessed as a function of the gamete output and spatial distribution of other males in a population, and that the performance of isolated males may yield overestimates of male fertilization success in natural populations.