SynopsisContrary to the generally accepted life history theory regarding sequential hermaphroditism ('sizeadvantage model'), we have recently identified 'mini males' in Lythrypnus dalli, the bluebanded goby. These are small (female-sized) fish that appear male based on their external genitalia. We investigated the reproductive anatomy and demography of these mini males. Based on their small size and the sexual plasticity of this species, we expected that mini males are not actually reproductively functioning males. However, when we examined their gonad anatomy, with particular attention to the male-typical accessory gonadal structure (AGS), we found that mini males have gonads that are comparable to those of nesting males in relative size and the percentage of tissue that is 'male' (testicular and AGS). Although this is contradictory to theories of sperm competition in alternative male phenotypes, reproductive strategies of these two types of males were clearly distinguishable based on their AGS content -as described in other examinations of differing ejaculate quality in alternative male phenotypes in the Gobiidae. Regular sampling of the L. dalli population showed that mini males make up a small fraction of the small size classes and that as the breeding season progressed, mini male frequency decreased and standard length increased significantly -thus allowing us to discuss their role within the social system. Based on these findings as well as comparisons with other species with multiple male phenotypes, we can begin to categorize L. dalli males as either bourgeois or parasitic, primarily based on their secondary sex characteristics and AGS contents.

Androgen signaling, via receptor binding, is critical for regulating the physiological and morphological foundations of male-typical reproductive behavior in vertebrates. Muscles essential for male courtship behavior and copulation are highly sensitive to androgens. Differences in the distribution and density of the androgen receptor (AR) are important for maintaining dimorphic musculature and thus may provide for anatomical identification of sexually selected traits. In Lythrypnus dalli, a bi-directional hermaphroditic teleost fish, both sexes produce agonistic approach displays, but reproductive behavior is sexually dimorphic. The male-specific courtship behavior is characterized by rapid jerky movements (involving dorsal fin erection) towards a female or around their nest. Activation of the supracarinalis muscle is involved in dorsal fin contributions to both agonistic and sociosexual behavior in other fishes, suggesting that differences in goby sexual behavior may be reflected in sexual dimorphism in AR signaling in this muscle. We examined sex differences in the local distribution of AR in supracarinalis muscle and spinal cord. Our results demonstrate that males do express more AR in the supracarinalis muscle relative to females, but there was no sex difference in the number of spinal motoneurons expressing AR. Interestingly, AR expression in the supracarinalis muscle was also related to rates of sociosexual behavior in males, providing evidence that sexual selection may influence muscle androgenic sensitivity to enhance display vigor. Sex differences in the distribution and number of cells expressing AR in the supracarinalis muscle may underlie the expression of dimorphic behaviors in L. dalli.

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