An initial description of alternative male reproductive phenotypes in the bluebanded goby, Lythrypnus dalli (Teleostei, Gobiidae)

Synopsis

Contrary to the generally accepted life history theory regarding sequential hermaphroditism (‘size-advantage 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.