| Abstract: | While simultaneous hermaphroditism occurs in most animal phyla, theories for its adaptive significance remain untested. Sex allocation theory predicts that combined sexes are favored in sedentary and sessile organisms because localized gamete dispersal and local mate competition (LMC) among gametes promote decelerating fitness “gain curves” that relate male investment to reproductive success. Under this LMC model, males fertilize all locally available eggs at low sperm output, additional output leads to proportionally fewer fertilizations, and combined sexes with female-biased sex allocation are favored. Decelerating male gain curves have been found in hermaphroditic flowering plants, but the present paper reports the first analysis in an animal. The colonial hermaphroditic bryozoan Celleporella hyalina forms unisexual male and female zooids that can be counted to estimate absolute and relative gender allocations. I placed “sperm donor” colonies—each with different numbers of male zooids, and each homozygous for diagnostic allozyme alleles—among target maternal colonies on field mating arrays, and estimated donor fertilization success by scoring allozyme markers in target-colony progeny. Fertilization success increased with numbers of donor male zooids, but linear and not decelerating curves fit the data best. Mean sex allocation was not female biased, consistent with nondecelerating male gain. Sperm donors, moreover, did not monopolize matings as expected under high LMC, but rather shared paternity with rival colonies. Hence localized water-borne gamete dispersal alone may not yield decelerating male gain and favor the maintenance of hermaphroditism; relaxed sperm competition in low density populations might also be required. In free-spawning marine organisms, males cannot control access to fertilizations, intense sperm competition may be commonplace, and high male sex allocation may be selected to enhance siring success under competition. |
