Motor Patterns of Herbivorous Feeding: Electromyographic Analysis of Biting in the Parrotfishes Cetoscarus bicolor and Scarus iseri

Abstract

The motor patterns of the herbivorous feeding bite were investigated in two species of parrotfish (Cetoscarus bicolor and Scarus iseri) with functionally distinct biting modes using electromyographic recordings. Behavioral data revealed that S. iseri utilized faster bites, took more bites per feeding bout, and bit at a higher frequency than did C. bicolor. EMGs recorded from the epaxialis (EP), levator operculi (LOP), 3 subdivisions of the adductor mandibulae (A1–A3), and the sternohyoideus (SH) muscles displayed a high degree of within-individual variance. Duration of muscle activity and onset time relative to LOP were shorter in S. iseri than in C. bicolor and S. iseri displayed a greater EMG amplitude in the LOP and SH muscles than did C. bicolor. We calculated the duty factors of the muscles as the relative timing of EMG variables divided by the total feeding cycle time. Patterns of duty factors of the feeding muscles were similar in both species, though muscles were active for a longer portion of the total bite cycle in S. iseri. In addition to its typical bite, S. iseri employed additional motor patterns when taking particularly hard bites. A multivariate comparison of EMGs from biting and suction feeding taxa revealed that the biting motor pattern was significantly different from suction feeding, although there was a high degree of overlap among all feeding strikes. The activity of the sternohyoideus muscle was significantly different between suction feeders and biters. Despite strong similarities of the general motor pattern in a wide range of teleost fishes, components of this pattern are shown to be evolutionarily plastic.