INTRASPECIFIC VARIATION IN SEX ALLOCATION IN A SIMULTANEOUS HERMAPHRODITE: THE EFFECT OF INDIVIDUAL SIZE

Within a population of simultaneous hermaphrodites, individuals may vary in both their current reproductive investment (biomass invested in gonads) and in how they allocate that investment between male and female function. In the chalk bass, Serranus tortugarum, estimates of both reproductive allocation and reproductive success as a male and a female can be made for individuals of different sizes. As individuals increase in size, their investment in gamete production increases, and there is a shift in allocation to a stronger female bias. Spawning frequency as a female in pair spawnings and as a male in both pair spawning and streaking (an alternative mating tactic) does not vary with individual size. As a result, larger individuals should release more sperm or eggs per spawn. Size-assortative pair spawning in this species leads to larger individuals having higher potential returns in total male reproductive success than smaller individuals, which should lead to increases in absolute levels of sperm production in larger individuals when individuals compete for fertilizations through sperm competition. However, smaller individuals contribute a smaller proportion of the sperm released in spawns with multiple spawners and thus are under more intense sperm competition than larger individuals, which should select for increases in male allocation in smaller individuals, all else equal. A local-mate-competition (LMC) model predicts that these factors select for increasing absolute male and female investment with individual size but a relative shift to more female-biased allocation as individual size increases. These predictions are supported by gonadal data. The predictions of average male allocation from the quantitative LMC model were 21.6% and 25.7%, whereas the collections averaged 21.3%. This close agreement of both the mean male allocation and its relative shift with individual size between model and data support the hypothesis that size-specific shifts in sex allocation in this species represent an adaptive response to patterns of mating success and sperm competition.