Abstract

The brain nanopeptides arginine vasotocin and isotocin are considered to be involved in the regulation of social and reproductive behavior in teleosts. We investigated the immunoreactivity of brain arginine vasotocin and isotocin neurons in four pairs of the protandrous false clown anemonefish Amphiprion ocellaris (Cuvier, 1830). After 450 days of pairing, the social rank of each individual was clearly distinguishable by body mass. The resident-intruder model test revealed that dominant individuals tended to display agonistic behavior more frequently than subordinate individuals, yet pairing failed to induce sex differentiation by social rank (i.e., gonadosomatic index and steroid profiles did not differ significantly and the gonads were ovotestes in both social rank individuals). However, dominant individuals had a larger size of arginine vasotocin neurons in the magnocellular layer and a greater number of isotocin neurons in the parvocellular layer of the brain preoptic area (POA) than did subordinate individuals. Arginine vasotocin and isotocin neurons of each layer of the POA showed different projection patterns: in the magnocellular layer, the fibers innervated the medial zone of the telencephalon and the mesencephalic tegmentum, but not in other layers. These results suggest that vasotocin and isotocin neurons in the brain of A. ocellaris regulate social behavior and have different roles.