Feeding Performance and Muscular Constraints in Fish

Analysis of the feeding kinematics of Astatotilapia (a small cichlid fish) suggests the presence of peripheral feedback modulation of the motor pattern, allowing the act of suction to be abbreviated. In this way, the effort spent in suction is minimized. The biological significance of the development of such a modulating feedback system is not immediately obvious from a ‘classical ecological’ point of view. It is postulated that the muscular metabolism itself might constrain the short, transient and strenuous motor output typical of suction feeding. Thus, reducing the suction effort makes sense when successive strenuous head-part movements are immediately required for additional suction, buccal transport or spitting. This hypothesis was tested by in vivo electrical stimulation of muscles important in feeding: the epaxials, which lift the skull and expand the buccal cavity. Reliable stimulation variables for the epaxial muscles were determined from preliminary stimulation experiments and from electromyographic recordings of these muscles in a specimen feeding on crickets. Stimulation trains of variable duration (<150ms) were applied in series of five trains. The intervals between trains were variable as well (<1s). The mechanical output was measured by means of an accelerometer, a force transducer or a magnetoresistive displacement transducer. In the latter case, the time course of the mechanical output could be recorded and analysed. The hypothesis predicts a decrease in the muscular output with increasing effort (long trains) and fast repetition (short intervals). The experimental results show the expected decline in mechanical output from one stimulation train to the next when longer stimulation bursts are imposed in quick succession. Statistical analyses (multiple regression) showed that train length, train rate and train number contribute significantly to the observed variation in mechanical output, supporting the hypothesis. Explanations for the phenomena are discussed.