All experiments were performed on the Siamese fighting fish (Betta splendens, Regan). Adult male fish were obtained from local commercial suppliers. All animals were housed in the laboratory for at least 1 week before use in behavioural studies. They were kept in isolated compartments, fed daily with minced lobster and squid meat, and the water changed routinely. In the protocol described below, only a single aspect of the agonistic display repertoire — the opercular abduction — is studied. All protocols for animal treatment adhered to guidelines
established by the National Institute of Health and were approved by the Northeastern University Institutional Animal Care and Utilization Committee.