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Protogynous hermaphroditism in a temperate reef fish, the blackeye goby, Coryphopterus nicholsii (Pisces: Gobiidae)

University of British Columbia

In this thesis, I examine the occurrence of protogynous hermaphroditism in the blackeye goby, Coryphopterus nicholsii (Pisces, Gobiidae) at Bamfield, Vancouver Island, Canada. The female biased sex ratios, size distributions of female and male C. nicholsii, and histological data all confirm that the species is protogynous. Histological data strongly suggest two patterns of sexual ontogeny, with males resulting from either pre- or post-maturational sex change. Sex change is not common during, and immediately after the breeding season. The social organization of C. nicholsii is based upon the year-round defense of a territory, primarily by large individuals of both sexes. A territory always includes one or more shelter rocks. These are used as refuges by both sexes, and as nest sites by males during the breeding season. The defence of a shelter rock is important in reproduction, in the reduction of predation, and the avoidance of adverse environmental conditions. Suitable refuge and nest site rocks are a limited resource and intraspecific competition determines which individuals gain access to them. Since the outcome of competition for suitable shelter rocks is strongly size-specific, large males monopolize spawnings. Large males receive a disproportionate share of the matings: males smaller than 7.0 cm were never seen nesting in the field. Yet, approximately 30% of males are smaller than 7.0 cm, indicating that many females change sex at smaller sizes. This early sex change confers a growth advantage: small, non-nesting males grow faster than similarly-sized mature females. As a result, small males may obtain nesting male status more rapidly than females that change sex later in life. The presence of early sex changers may also be related to an alternative mating tactic, sneak spawning. Sex change is correlated with a reduction in whole-body concentration of 17(3-estradiol, and an increase in 11-ketotestosterone concentration. Both steroids occur naturally in female and male gobies. Administration of 11- ketotestosterone, its precursor 11-ketoadrenosterone, and an aromatase inhibitor (Fadrazole) resulted in complete sex change in mature females. These results strongly suggest that both 17B-estradiol and 11- ketotestosterone are involved in mediating sex change in this goby. These results are discussed with regards to the geographical disparity of protogynous hermaphroditism in teleosts. Protogyny occurs much more frequently on tropical coral reefs than on temperate rocky reefs. I propose that a difference in the occurrence of certain mating systems in tropical and temperate waters partly explains this disparity.

Thesis/Dissertation

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2009-03-30

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http://hdl.handle.net/2429/6649

Zoology

University of British Columbia

UBCV

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