Adaptive gamete allocation when fertilization is external and sperm competition is absent: Optimization models and evaluation using coral reef fish

I develop a series of optimization models of adaptive sperm allocation by external fertilizers when sperm competition is absent. The models differ with respect to either one or two of the following assumptions: the dependence of fertilization success on egg concentration; the optimization criteria (the maximization of per-spawn vs long-term average fitness); and the limiting factor on male reproduction (daily mate and sperm availability vs per-spawn mating costs). I propose that, a priori, these (and similar) models are often equally plausible. Consequently, strong inferences concerning the selective regime that led to the putative adaptation would be possible only upon the evaluation of conflicting predictions generated by the different models. I demonstrate this using published data on sperm allocation and fertilization success in pair-spawning coral reef fish.