UYENO, T.A.: How squids bite: the functional morphology of cephalopod beaks and mandibular muscles

The cephalopod buccal mass, located at the center of the base of the arms and tentacles, contains the upper and lower beaks. Often compared to those of a parrot, the beaks perform analogous functions of manipulation and chewing food, but differ in form: the upper and lower beaks do not articulate. Thus, the muscles that connect the upper and lower beaks must create the pivot around which the beaks rotate. How do these mandibular muscles provide support for this pivot and also generate the opening and closing forces? Kear (1994. JMBA U.K. 74: 801-822) described the basic anatomy and muscle fiber orientations: the muscles consist of a large superior muscle that originates on the upper beak crest and inserts on the lower beak wings, the paired lateral muscles that originate on the upper lateral walls and insert on the lower lateral walls, and the thin inferior muscle that originates on the lower beak crest and inserts inside the upper beak hood. It was not in the scope of Kear’s paper to make a detailed description of the muscle and connective fiber trajectories and so it is unclear what portions of the musculature are responsible for creating a pivot and for opening and closing motions. Serial semi-thin glycol methacrylate sections of Lolliguncula brevis (Loliginidae) were used to reconstruct the relationship of the beak and the mandibular muscles and characterize the orientations of the muscle and connective tissue fibers. Preliminary data suggest that muscles are opposed by forces provided by elastic portions of the beaks, antagonistic muscles transmitting force through rigid beak portions and are also supported by the rigidity of adjacent, contracted muscles. Different portions of the superior and lateral mandibular muscles provide the majority of the opening and closing forces.