Sex allocation in a group-living simultaneous hermaphrodite: Effects of density at two different spatial scales

Questions: Is the effect of competitor density on the level of sperm competition a strong predictor of natural male gonadal allocation in a simultaneous hermaphroditic fish (Serranus tortugarum)? Is any such relationship consistent at different spatial scales? Does any variation in male gonadal allocation involve trade-offs in male and female gonadal investment previously proposed by sex-allocation theory? Data obtained: Density, proximity of conspecifics during spawning, rate of male-role competitive intrusions, and gonadal allocation; collected from multiple locations, at two spatial scales, on Caribbean coral reefs of northwestern Panama. Research methods: Data were obtained at two scales: (1) fine-scale: among different social groups within the same isolated population (n = 8 and 12 groups, 2 populations); and (2) large-scale: among populations on different (isolated) reefs (n = 9 reefs). Sex allocation (testes mass/total gonad mass, ovary/soma mass, and testes/soma mass) was compared with fish density and spawning behaviour in each social group and population. Conclusions: Our comparative approach confirmed theoretical predictions about density effects on sex allocation in a simultaneous hermaphrodite in nature. The proximity of conspecifics during spawning, intrusive behaviours by sperm competitors, and male allocation (testes mass/gonad mass) all increased with increasing density. Increases in male allocation were accompanied by reductions in ovary/soma mass, while testes/soma mass remained unchanged. These patterns were consistent among isolated social groups within a single population, as well as among isolated populations.