The effect of variation in population density on male fertilization success in a colonial ascidian

Abstract

Past investigations of the effect of variation in population density on fertilization processes under field conditions have focused entirely on the consequences for female fertilization success (the proportion of eggs fertilized). In contrast, we explored the effects of population size and density on male fertilization success (proportion of embryos fathered or eggs fertilized by individual males) via an in situ density manipulation that utilized allozyme markers to assay paternity after fertilization had occurred. Male-phase ascidian colonies in high density experimental populations experienced competition for fertilizations, which reduced the ability of a focal male to fertilize eggs brooded by individual female colonies. However, the concomitant increase in the number of females with eggs available to be fertilized offset this decline. Consequently, the overall, population-wide fertilization success of focal males did not vary between density treatments. Although population density did not affect the quantitative fertilization success of male colonies, an increase in the genetic diversity of offspring may nevertheless confer a fitness advantage to both males and females that reproduce in higher density populations.