NUMBER OF PREOPTIC GNRH-IMMUNOREACTIVE CELLS CORRELATES WITH SEXUAL PHASE IN A PROTANDROUSLY HERMAPHRODITIC FISH, THE DUSKY ANEMONEFISH (AMPHIPRION MELANOPUS)

The populations of gonadotropin-releasing hormone (GnRH)-producing cel ls within the preoptic area (POA) and terminal nerve (TN) of the brain have been suggested as the neuronal systems mediating social control of sex and gonadogenesis in sequentially hermaphroditic teleosts. In t he present study, the number and soma size of GnRH-immunoreactive (GnR H-ir) cells in the POA and TN were studied in male, female and juvenil e individuals of the dusky anemonefish (Amphiprion melanopus), a speci es which displays both male to female sex change and socially controll ed sexual maturation. The results showed that the number of POA (but n ot TN) GnRH-ir cells differ significantly between sexual phases, with males displaying higher cell numbers than both females and juveniles. Soma sizes of POA and TN GnRH-ir cells were larger in females than in males and juveniles. However, this relationship was fully explained by differences in body size. The results indicate that high POA GnRH cel l numbers are part of a masculinizing mechanism and support the hypoth esis that the POA GnRH cell population plays a central role in initiat ing or mediating the process of socially induced gonadal and/or behavi oural transformations in sequential hermaphrodites.