Meeting Abstract
Pacus are a group of herbivorous fishes known to extensively process a variety of plant materials with their oral jaws by means of their robust incisiform teeth. This method of mechanical breakdown, while common in mammalian species, is less frequently observed in fishes, leaving the biomechanics of this processing behavior largely unknown. Using X-ray Reconstruction of Moving Morphology (XROMM), this study found similarities between the processing behavior of one species of pacu, Piaractus brachypomus, and a chondrichthyian species, Potamotrygon motoro,. Much like the freshwater stingray, P. brachypomus, alternates between periods of short compressive chews, where the amplitude of mandible rotation is small but food is still actively engaged between the teeth, and periods of extreme rotation of the lower jaw which result in the shearing of food between the mobile dentary and the stationary premaxillary teeth. In the pacu species, the exaggerated overbite motions of the lower jaw are facilitated in part by the morphology and motions of the hyomandibula. In most ray-finned fishes, the hyomandibula-neurocranial joint is a straight and flat hinge joint, effectively permitting only abduction and adduction of the suspensorium. However, the articular surface of the hyomandibula in P. brachypomus, is rounded at its joint with the neurocranium, contrary to the typical actinopterygian condition, and contrary to the largely carnivorous relatives of the pacu, piranhas. The rounded surface appears to permit rostro-caudal rotation of the suspensorium which in turn facilitates periods of overbite shearing during the pacu processing cycle.
