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Bi-directional sex change in the gobiid fish Trimma sp.: does size-advantage exist?

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The size-advantage model predicts that a large dominant female should become male in fishes where large males monopolize mating opportunities with multiple females. We investigated the sexuality of the gobiid fish Trimma sp. to determine if a size advantage exists for this goby. Histological examination of the gonads showed that individuals functioned only as one sex at a time, even though the gonads contained both ovarian and testicular tissues. Experiments in aquaria demonstrated that bi-directional sex change occurred in Trimma sp. In natural populations, males were larger than females and the sex ratio was strongly biased towards females. This indicates that Trimma sp. may be polygynous and function primarily as a protogynous hermaphrodite. However, sex-size distributions largely overlapped and contained small males. Furthermore, in many three-female groups, the largest fish remained female and the smaller fish became male. The population density of Trimma sp. was about 12 times that of T. okinawae ; high population density is likely to allow the possibility that small males attain some degree of reproductive success, and this reproductive success selects for the small male strategy. This is in agreement with the predictions of the size-advantage model and a new version of the size-advantage hypothesis.

Affiliations: 1: Education Center, Kagoshima University, 1-21-30 Korimoto, Kagoshima 890-0065, Japan; 2: Faculty of Fisheries, Kagoshima University, Kagoshima 890-0056, Japan; 3: Diving Service Umi-Annai, 7-7 Masagohonmachi, Kagoshima 890-0067, Japan; 4: Kagoshima Immaculate Heart College, 4-22-1 Toso, Kagoshima 890-8525, Japan; 5: Tateyama Station, Field Science Center, Tokyo University of Marine Science and Technology, 670 Banda, Tateyama 294-0308, Japan


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