Components of fertilization success in the bluehead wrasse, Thalassoma bifasciatum

In many species of marine organisms, males and females release gametes directly into the water column. Although free-spawning marine invertebrates appear to have highly variable fertilization success, in tropical reef fishes the average fertilization success is quite high, typically over 90%; nevertheless, substantial variation has been reported, and fertilization has a direct effect on fitness. We investigated the factors affecting fertilization success in natural spawnings of the bluehead wrasse, Thalassoma bifasciatum. During a two-year study at a site in St. Croix, we found extensive and predictable variation in fertilization success in pair spawns of this reef fish. Fertilization success averaged 95%, but was affected by the amount of sperm released, the water velocity at a site, the mating success of the male, and the size of the female. As sperm released in a spawn increases, and as water velocity at a site decreases, sperm concentrations should remain higher in the vicinity of eggs for a longer period of time, and both of these factors are correlated with increasing fertilization success. The recent history of individuals with partners or sites did not affect the fertilization success of their spawn. In an evolutionary context, the real and predictable variance in fertilization success in this species may influence the mating choices of males and females. However, there is currently no evidence that females use differences in fertilization success among males or sites in their reproductive decisions.