Alternative social and reproductive niches linked to intra-sexual color variation in a facultatively protogynous North American annual killifish, Millerichthys robustus.

Intra-sexual variation in coloration is often linked to comprehensive differences in reproductive behavior and life histories. Both males and females can express intrapopulation variation in coloration, and facultative hermaphrodism adds further complexity to the social and reproductive system. In replicated experimental tanks, we studied social interactions and reproductive behavior in males, females and sex-changing (transitional) intermediate individuals of the Mexican annual killifish, Millerichthys robustus, a facultative protogynous hermaphrodite with distinct intraspecific variation in male coloration (red, orange and yellow morphs) and female ornamentation (variable number of melanin-based ocelli). Territoriality was expressed in males and females but not in intermediates. Among males, the yellow morph was competitively inferior, but males of any morph were capable of establishing territories. Red and orange males possessed territories near the substrate. The territories of yellow males were located near the water surface. Males of all morphs also engaged in sneaking behavior and attempted to mate with females outside their own territory. Females with more ocelli were more aggressive and more likely to become dominant but were not courted more often by males. Intermediates rarely engaged in any reproductive behavior. Overall, we demonstrate differences in social and reproductive behavior corresponding to male and female coloration and associate them with variation in social and reproductive niches.