Muscle activation and strain during suction feeding in the largemouth bass Micropterus salmoides

Activation and strain in the sternohyoideus (SH) were measured in vivo in five largemouth bass Micropterus salmoides. The SH is thought to actuate lower jaw depression, hyoid depression and suspensorial abduction during suction feeding in teleost fish. Sonomicrometry was used to measure fascicle shortening and lower jaw kinematics, while activity was measured by electromyography (EMG). SH fascicles shortened by an average of 11% during suction feeding. In three fish SH fascicles consistently shortened during fast lower jaw depression, but in two individuals they contracted isometrically or lengthened slightly during fast lower jaw depression. The SH continued shortening after peak gape, presumably actuating hyoid depression and lateral expansion of the buccal cavity. Onset of SH relengthening and onset of lower jaw elevation were simultaneous, as were the return of the SH to resting length and gape closure. Activation followed the onset of shortening by an average of 23 ms, although the muscle was active an average of 15 ms before the onset of rapid shortening. SH fascicles reached sustained shortening velocities averaging -2.5 fascicle lengths per second, and generally increased shortening velocity after peak gape. The shortening velocities measured in this study suggest that the SH actively shortens to generate power during suction feeding. This study is the first direct measurement of in vivo muscle function during suction feeding, the most common mechanism of prey capture among aquatic vertebrates.