Hyoid morphology and movements relative to abducting forces during feeding in Astatotilapia elegans (Teleostei: Cichlidae)

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Abstract
In fishes, the abducting hyoid bars push the suspensoria outwards. This force transmission is generally assumed to be important during fast suction feeding (strenuous activity). In Astatotilapia elegans the hyoid symphysis can best be modelled as an oblique hinge. The relevance of this hinge morphology on the force transmission has been studied by means of a three-dimensional (3D) model simulating the displacements of the hyoid-suspensorial system. It appears that the transmission force factor increases throughout feeding in the case of the hinge model. Reduction of the hyoid symphysis to a point articulation (as was done formerly in attempts to quantify the transmission by means of planar models) suggests an unfavourable decline of the transmission force to zero during maximal mouth expansion. The angle between the hinge axis of the symphysis and the longitudinal axis of the hyoid bar is 45°. Such a configuration allows for a maximal increase in the volume of the buccal cavity for suction. This functional aspect, together with the apparent maximization of the force transmission during feeding, suggests that constructional and neuromotoric factors have been improved during the evolutionary development of the hyoid-suspensorial system.