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The mechanism of mouth expansion in fish, consisting of jaws, suspensoria (j) and hyoids (h) has been modelled by a four-bar isosceles linkage. This model provides insight into limitations and demands of the expansion system used in feeding, as it can be optimized with regard to maximum mouth volume increase. The optimum length ratio of hyoids and jaws was found to be h/j = 0–7. This optimum is modified by mouth bottom depression, jaw protrusion and swimming.

To expand the mouth, at least two forces are required; one exerted by the sternohyoid and ventral body muscles, the other by the epaxial muscles through transmission in the quadrato-articular joints. (Data from EMG experiments confirm the synchronous activity of these muscle groups.) Force transmission and mouth volume increase are constraining quantities, which can be compromised. This leads to a model of the initial mouth shape which is actually found in many ‘generalized’ fishes, and to demands concerning volume and physiological cross-sectional area of the muscles involved.

Options for specific relative lengths of jaws and hyoids (h/j-ratios) are, for various fish species, compared with model predictions. The applicability of the model approach is shown by the obtained results.