On the agonistic display of the Siamese fighting fish. I. The frontal display apparatus

The agonistic behavior of the Siamese fighting fish has long been a popular subject for ethologists. While this behavior is well documented, its physiological basis is still poorly understood. One of the most important components of this behavior is the frontal display, in which a fish faces its opponent directly with tonically extended gill covers. As a first step towards establishing this display as a model for behavioral physiology, the musculoskeletal structure of the frontal display apparatus is examined. The opercular bones and the opercular abductor, the opercular dilator muscle, appear to have undergone adaptive modifications that facilitate the display. The opercular bone rotates around the hyomandibular bone through a ball-and-socket joint. Due partly to a reduction of its articulation with the preopercular bone, the operculum can rotate as much as 90 degrees around this joint. The opercular dilator muscle consists of three parts: a deep belly--DO alpha, and two superficial bellies--DO beta and DO gamma. The three portions have different origins, but all three insert on the lateral surface of the opercular bone just dorsal to the spheroidal joint. In comparison, the opercular dilator muscle of the sunfish and the goldfish lacks the superficial bellies. Innervation of this muscle is derived from the maxillary division of the trigeminal nerve, all three portions are innervated by axons from the same fascicle, suggesting that they are embryologically related. The superficial bellies consist of uniform, large muscle fibers. The deep belly consists of an external group of large fibers around a central tendon, and an internal group of small fibers. Enzyme histochemistry shows that the external group of fibers consists mostly of fast-twitch fibers, whereas the internal group consists of slow, oxidative fibers. Direct stimulation demonstrates that all bellies can mediate opercular extension. The architectural and biochemical differences among the three portions suggest that they are functionally not equivalent. The fast-fatiguing muscles may mediate the initiation, while the large fibers of the superficial bellies and the small oxidative fibers of the deep belly may be involved in the maintenance of the display.