Fertilization success was measured in the bluehead wrasse, Thalassoma bifasciatum, a tropical reef fish with external fertilization of pelagic eggs. This species exhibits intraspecific variation in its spawning behavior; females either spawn with single males (pair spawning) or spawn with a group of at least three and often > 20 males (group spawning). Fertilization success averaged almost-equal-to 75% and did not differ between pair and group spawning, despite an estimated 80-fold increase in sperm release in group spawns. There was also no evidence that pair-spawning males suffered sperm depletion over the course of the spawning period. Thalassoma bifasciatum occurs in a variety of habitats and is exposed to varying levels of water turbulence. Fertilization success varied among days, and decreased with rougher water conditions. Within a reef, the calmer spawning sites behind the reef relative to the current had higher fertilization success than those along the side of the reef. These data suggest that while the type of spawning occurring at the site does not affect selection for fertilization success, females may gain fertilizations by selecting particular locations or periods of calmer water conditions to spawn. This is an alternative hypothesis to explain temporal and spatial patterns of mating. We provide detailed methods on how to accurately obtain data on fertilization success. Our technique can be used to study natural spawning in a wide variety of reef fishes and other marine organisms with pelagic eggs, external fertilization, and predictable spawning.
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