Evol Ecol Res 12: 189-202 (2010)     Full PDF if your library subscribes.

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

Mary K. Hart1, Andrew W. Kratter2, Anne-Marie Svoboda3, Cara L. Lawrence1, R. Craig Sargent1 and Philip H. Crowley1

1Department of Biology, University of Kentucky, Lexington, Kentucky, USA, 2Florida Museum of Natural History, University of Florida, Gainesville, Florida, USA and  3Animal Ecology, Faculty of Earth and Life Sciences, Vrije Universiteit, Amsterdam, Netherlands

Correspondence: M.K. Hart, Department of Biology, University of Kentucky, Lexington, KY 40506, USA.
e-mail: mkhart@uky.edu


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.

Keywords: Caribbean, coral reefs, gonadal investment, mating systems, Panama, seabass, Serranus tortugarum, spawning behaviour, sperm competition, trade-offs.

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