RESUMEN
Anemonefish, Amphiprion spp., are socially controlled, protandrous sex changers with a monogamous mating system. Under certain conditions, sexually immature anemonefish with ambisexual gonads differentiate directly into males or females. Formation and maintenance of social rank in a group are considered key requirements for the induction of sex change or differentiation. Generally, each animal living in a social group experiences a different level of social stress in accordance with its social rank, and we hypothesize that the stress situation of individual anemonefish influences its sex determination. Groups of three sexually immature anemonefish were placed into each of five experimental tanks and kept for 10 days to allow for social rank formation and behavioral observation. The fish were then euthanized, and blood and brain samples were collected from each fish. The social rank of each individual was distinguishable from day 1 of the experiment. Aggressive behaviors were most frequent and blood Cortisol values were higher in dominant individuals. The transcription of mRNA for stress-related genes, i.e., those encoding for glucocorticoid and arginine vasotocin receptors, was higher in the brains of dominant individuals than in other social ranks. Furthermore, we detected higher transcription levels of gonad and brain aromatase genes, which encode the enzyme that converts androgens into estrogens, in the brains of dominant individuals. These results suggest that social rank reflects the blood Cortisol value, which in turn leads to sex differentiation by manipulating transcription of genes, including aromatase genes, in the brain.