Four-bar linkage modelling in teleost pharyngeal jaws: computer simulations of bite kinetics

The pharyngeal arches of the red drum (Sciaenops ocellatus) possess large toothplates and a complex musculoskeletal design for biting and crushing hard prey. The morphology of the pharyngeal apparatus is described from dissections of six specimens, with a focus on the geometric conformation of contractile and rotational elements. Four major muscles operate the rotational 4th epibranchial (EB4) and 3rd pharyngobranchial (PB3) elements to create pharyngeal bite force, including the levator posterior (LP), levator externus 3/4 (LE), obliquus posterior (OP) and 3rd obliquus dorsalis (OD). A biomechanical model of upper pharyngeal jaw biting is developed using lever mechanics and four-bar linkage theory from mechanical engineering. A pharyngeal four-bar linkage is proposed that involves the posterior skull as the fixed link, the LP muscle as input link, the epibranchial bone as coupler link and the toothed pharyngobranchial as output link. We used a computer model to simulate contraction of the four major muscles, with the LP as the dominant muscle, the length of which determined the position of the linkage. When modelling lever mechanics, we found that the effective mechanical advantages of the pharyngeal elements were low, resulting in little resultant bite force. By contrast, the force advantage of the four-bar linkage was relatively high, transmitting approximately 50% of the total muscle force to the bite between the toothplates. Pharyngeal linkage modelling enables quantitative functional morphometry of a key component of the fish feeding system, and the model is now available for ontogenetic and comparative analyses of fishes with pharyngeal linkage mechanisms.