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Local Population Size as a Determinant of Mating System and Sexual Composition in Two Tropical Marine Fishes (Thalassoma Spp.)

Robert R. Warner and Steven G. Hoffman
Evolution
Vol. 34, No. 3 (May, 1980), pp. 508-518
DOI: 10.2307/2408220
Stable URL: http://www.jstor.org/stable/2408220
Page Count: 11
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Local Population Size as a Determinant of Mating System and Sexual Composition in Two Tropical Marine Fishes (Thalassoma Spp.)
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Abstract

Two sexual types, one a non-sex-changing (primary) male and the other a protogynous hermaphrodite, coexist in the wrasses Thalassoma bifasciatum of the Atlantic and T. lucasanum of the Pacific. The adults of both species are found in relatively isolated local populations on spatially separated reefs of various sizes, and we suggest that the sexual polymorphism is maintained by differential selection in these local populations. On smaller reefs, populations of T. bifasciatum are small enough that large territorial males can successfully exclude small males from breeding. Although both sexual types can become territorial males, those individuals that reproduce as females while small and change sex only when they are large enough to compete successfully as males would be most fit on smaller reefs. In larger local populations, the density of small males at the prime mating site is sufficiently high that the territories of larger males are located elsewhere, and the majority of spawnings occur with the nonterritorial males. If there is a sufficient reproductive cost to the act of changing sex, then primary males would be more fit than hermaphrodites on larger reefs. This suggests that the overall frequency of the two sexual types should be a reflection of the underlying distribution of local population sizes. In accord with this prediction, T. lucasanum, which has the same mating behaviors but denser and larger local populations, has a higher proportion of primary males and a lower rate of sex change than T bifasciatum.

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