FUNCTIONAL-MORPHOLOGY OF PUFFERFISH INFLATION - MECHANISM OF THE BUCCAL PUMP

The mechanism of the buccal pump used to fill the stomach with water d uring inflation behavior was studied in the striped burrfish, Chilomyc terus schoepfi. The movement patterns of the pectoral girdle and the a nterior region of the hyoid apparatus were inferred from simultaneous impedance and buccal pressure recordings. Impedance was measured betwe en two electrodes; a stationary electrode implanted in the roof of the mouth and a second electrode implanted either in the anterior tip of the cleithrum or the musculature surrounding the anterior end of the h yoid apparatus. In addition, simultaneous buccal pressure and electrom yographic (EMG) recordings were made during inflation sequences to det ermine the activity patterns of eight cranial muscles. Impedance recor dings indicate that buccal expansion is caused by synchronous posterov entral rotation of the hyoid apparatus and the pectoral girdle, wherea s buccal compression is accomplished by hyoid and pectoral girdle ante rodorsal rotation. EMG data revealed that periods of reduced buccal pr essure occurring at the onset of each inflation cycle were associated with activity of several muscles that open the mouth (levator operculi , dilatator operculi), depress the hyoid apparatus (hyohyoideus abduct or, sternohyoideus), and retract the pectoral girdle (levator pectoral is). Periods of positive buccal pressure, during which water was force d into the stomach, were associated with muscular activity indicating mandibular adduction (adductor mandibulae), hyoid protraction (protrac tor hyoideus), and pectoral girdle protraction (protractor pectoralis) . The key components of the buccal pump that, among tetraodontiform fi shes, are derived for the Diodontidae plus the Tetraodontidae are the highly kinetic joint between the cleithrum and supracleithrum which pe rmits extensive pectoral girdle protraction and retraction and the mas sively developed hyohyoideus abductor muscle that flexes a joint betwe en the ceratohyal and the first branchiostegal ray, providing a novel mechanism of buccal door depression.