Determinants of body size composition in limited shelter space: why are anemonefishes protandrous?

I examined why anemonefishes, Amphiprion ocellaris, Amphiprion frenatus, and Amphiprion perideraion, which inhabit single host anemones, are monogamous and protandrous. Because they live in small groups (≤6 individuals) with a size hierarchy, they might have the environmental potential for polygyny. If the dominant fish were male, he could monopolize mating opportunities, and subordinates should be female. Female fecundity increases with body size, so that group body size composition largely influences his reproductive success. First, I developed an optimization model to predict the body size composition based on the carrying capacity of the host (C) and a fixed body size ratio (γ) or difference (δ) between individuals adjacent in rank. The γ and δ were assumed to be necessary for subordinates to avoid fatal eviction from the group. The model using δ and C could predict the body size composition of the 3 species. Next, I incorporated nonlinear female fecundity functions into the model to evaluate whether polygyny or monogamy was the better mating system for the dominant fish. I examined relationships between gonad weight and body size of A. frenatus and A. perideraion to estimate the functions. Assuming large δs and the nonlinear functions, dominant fish could have higher reproductive success in monogamous mating systems. The model also indicated that where C is limited, a larger δ resulted in one large, one small, and several very small subordinate individuals. This combination of body size composition model and nonlinear female fecundity functions can explain the function of protandry and monogamy under limited shelter space.