Mating tactics in external fertilizers when sperm is limited

Among externally fertilizing animals in aquatic habitats, the proportion of a female's egg clutch that is successfully fertilized often falls below 100%. In many such species, particularly in coral reef fishes, males spawn daily at high frequencies, often with little or no sperm competition. A major evolutionary problem for such males is how best to allocate sperm over successive spawns. Females face the problem of ensuring complete fertilization of their egg clutch. Here we model male and female mating tactics when daily sperm production is limited and with various assumptions concerning how differences in the number of sperm released during a mating influence the number of eggs fertilized. The models reveal conditions under which males can maximize daily reproductive success, either by releasing a fixed number of sperm during all successive spawns or by matching sperm numbers to the clutch size of their mates. These patterns of sperm allocation exert different pressures on females, which may respond evolutionarily by developing various mating tactics of their own.