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A Kinematic Jaw Model for the Rapid Wide Opening and the Closing of the Mouth in Some Labrinae (Pisces, Perciformes)

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

This paper is the third one of a series of articles (cf. VAN HASSELT, 1978, 1979a) on the morphology and the kinematics of the jaw apparatus of the labrid fishes Labrus berggylta Ascanius, Crenilabrus melops (L.), and Ctenolabrus rupestris (L.). These fishes show very rapid jaw movements during the catching of highly mobile prey (e.g., shrimps, small crabs). By the application of a deductive methodology, a kinematic qualitative jaw model was designed and developed to obtain a mechanism with very rapid, wide opening and rapid closing of the mouth brought about by a very small and relatively slow input movement, as well as closing of the protruded mouth. Premaxillary protrusion will be described in a separate paper. Once construction was begun with some assumptions or axioms, the development of the model progressed on the basis of simple kinematics and incorporation of topographical and biological demands and specifications. The main shape of the model became a symmetrical hollow pyramid-like configuration of essential structures arranged in the walls of the pyramid. These structures comprise a transverse axis of jaw rotation for each jaw at the jaw suspension, an articulating symphyseal line for transverse abduction and adduction, and a point for coupling between the upper and lower jaws. This is the basic model. Small adduction caused by the adducting suspensorium proves to be the most effective input movement for the rapid large jaw-rotations. In the labrids, the suspensorium-jaw apparatus shows adduction (VAN HASSELT, 1978). The location of these structures for the best working of the model appear to diverge somewhat from the positions in the basic model. In particular, the relatively stationary neurocranium and the ab-/adducting suspensorium demand different positions for the transverse axes. The model shows the upper-jaw axis and suspension in an area of minimal ab-/adductive movements, i.e., the area where the distal tips of the maxilla, the palatine, the nasal, and the lacrimal meet and are interconnected by a tangle of short ligaments. A safety mechanism is postulated to avoid any accidental rapid jaw action. In the model, the jaw axes appear to move slightly paramedially. A system with slightly approaching or separating jaw-axes causing rapid large jaw-rotations can be integrated into the basic-model system. A mechanical coupling between these two rapid systems reduces the need for muscles and nervous controls: the labrids have a ligament between the quadrate and the maxilla, and a movably connected suspensorium and palatine (in Ctenolabrus there is a narrow fusion) bearing the lower and upper jaw axes, respectively. The demands for the closing of a protruded mouth, with a maximal cavity inside a non-rotating protruded upper jaw, are incorporated into the basic-model system.

Affiliations: 1: Department of Morphology, Zoological Laboratory, University of Leiden, The Netherlands


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