UYENO, Theodore A.; KIER, William M.; Univ. of North Carolina, Chapel Hill; Univ. of North Carolina, Chapel Hill: The function of cephalopod buccal mass musculature

The buccal mass of cephalopods includes a pair of jaws termed �beaks� that are used to masticate prey. The beaks perform complex movements, yet do not articulate with one another. Instead, they are embedded in a complexly arranged mass of muscle and connective tissue. A recent morphological analysis has identified and described four major muscle groups in the buccal mass: a pair of lateral mandibular muscles and the posterior, anterior and superior mandibular muscles. Analysis of the muscle and connective tissue fiber orientations in these muscles, in conjunction with observations of beak movements has generated predictions of the function of the muscles in beak movement and support. The lateral mandibular muscles are predicted to generate opening force and to provide a muscular fulcrum for the beak movements. The closing force is predicted to be generated by the other muscles. The current study is designed to test these predictions. Isolated buccal masses of Octopus bimaculoides exhibit a normal bite cycle. Electromyograms were recorded from the major beak muscles of isolated buccal masses while simultaneously recording beak displacements using an electronic movement monitor. The recordings allow identification of the muscles that are active during opening and closing movements and thus they provide a test of the predictions of the morphological analysis. The results suggest that the musculature itself creates a dynamic joint. This mechanism, termed here a �muscle articulation�, allows more diverse movements than a conventional joint and may be more widespread than currently recognized. This study was funded in part by the following grants to WMK: NSF IBN-972707, DARPA N66001-03-R-8043.