Functional Anatomy of Feeding in the Bluegill Sunfish, Lepomis Macrochirus: In Vivo Measurement of Bone Strain

ABSTRACT The suction feeding mechanism of the bluegill sunfish, Lepomis macrochirus, was studied in unrestrained fishes by the simultaneous recording of cranial muscle electromyograms, opercular cavity pressures, and opercular bone strain patterns. The electromyographic profile was similar to that of other advanced teleosts and consisted of preparatory, expansive, and compressive phases. The pectoral girdle remained nearly stationary during the strike. Opercular cavity pressures showed peak negative values of 145 cm H2O followed by a positive afterpressure of up to 50 cm H2O as water flowed out of the opercular chamber. Characteristic ‘coughing’ patterns showed an initial positive phase, followed by a negative phase, and then a final positive pressure pulse. Bone strain on the operculum was measured with rosette and single element strain gauges. Peak principal compression during the strike was 1800 με and the peak strain rate was − 615 × 103με/s, more than ten times that previously recorded in vertebrate bone during normal activity. Opercular bone strain results from the rapid reduction of pressure in the opercular cavity which causes the operculum to deform medially and does not result from muscle activity. The observed strain pattern is consistent with a stress regime of bending and twisting moments applied to the operculum during feeding. Two prominent orthogonal bony struts on the medial opercular surface are hypothesized to resist these bending and twisting moments.