Muscular basis of buccal pressure: inflation behavior in the striped burrfish Chilomycterus schoepfi

We examined the relationship between commonly measured features of cranial muscle activity and the magnitude of sub- and superambient pressure measured inside the buccal cavity of the striped burrfish Chilomycterus schoepfi during inflation behavior. Buccal pressure was recorded simultaneously with electromyographic (EMG) records of activity from three expansive-phase muscles (levator operculi, levator pectoralis and hyohyoideus abductor) and three compressive-phase muscles (adductor mandibulae, protractor hyoideus and protractor pectoralis) in eight individuals. We quantified EMG activity in approximately 30 inflation cycles per fish by measuring the burst duration, rectified integrated area, intensity of activity (area divided by duration) and onset time relative to the onset of subambient pressure at the beginning of the cycle. Multiple regressions were calculated separately for data from each fish to investigate the relationships between pressure and EMG variables. The percentage of variation in minimum buccal pressure or area under the subambient pressure curve explained by the multiple-regression models ranged among individuals from approximately 52 to 84 %. The regression models accounted for more variation in peak pressure and the integrated area of superambient pressure; r2 ranged from 76 % to 97 %. The strong relationship between EMG activity and superambient buccal pressure suggests that the latter is probably a direct function of the strength of compressive-muscle contraction. In contrast, the magnitude of subambient pressure is a complex function of the area of the oral opening and the rate of buccal expansion, factors that do not appear to be as directly indicated by the degree of muscle activity.