Water-borne and Tissue Endocrine Profiles of an Alternative Male Reproductive Phenotype in the Sex Changing Fish, Lythrypnus dalli

In the bi-directionally hermaphroditic fish, Lythrypnus dalli, two distinct male phenotypes have been described. The more conspicuous parenting males are larger, establish breeding territories, and display courtship, mating, and parenting behaviors. The alternative males, called mini males, have been postulated to have a parasitic reproductive strategy, although the behavioral ecology of mini males is not well understood. The mini male morph has been characterized based on size and anatomical differences, including sperm-filled accessory gonadal structures (as opposed to mostly mucous in nesting males), consistent with parasitic male morphs in other gobiid species. Here, we determined the endocrine profiles of mini males to gain further insight into their phenotype. Systemic (water-borne) 17β-estradiol (E₂) concentrations were higher than testosterone (T), and 11-ketotestosterone (KT) concentrations were lowest. Mini males in L. dalli are similar to parasitic males of other species in having higher T∶KT ratios than breeding males. In mini males, brain and reproductive tissue levels of T, E₂, and KT were higher than in the muscle. Among all the steroids, E₂ levels were high in all three tissues in mini males. Data from relative hormone levels in different tissues will lead to a better understanding of the endocrine regulation of behavioral, physiological, and morphological correlates of male sexual polymorphism.