Evolution of motor patterns in tetraodontiform fishes: does muscle duplication lead to functional diversification?

Several times within the teleost fish order Tetraodontiformes singular jaw adducting muscles have been effectively 'duplicated' by physical subdivision to produce new muscles. This morphological system provides an opportunity to investigate how the functional complexity of muscular systems changes with evolutionary increases in the number of component muscles. In this study we asked if muscle duplication has lead to functional diversification by comparing the motor patterns of muscles that result from subdivision events. The activity patterns of five different sets of duplicated muscles were quantified with electromyographic recordings (EMG) from four individuals in each of three species during processing of three prey types. Prey varied in durability and elusiveness (live fiddler crabs, pieces of squid tentacle and live paeneid shrimps). For each cycle of prey processing, measurements were made of the relative onset time of each adductor muscle, the duration of each burst of activity, and the relative intensity of each activity burst. Two types of functional divergence of muscles were observed in analyses of variance conducted on the EMG variables. In two of the 15 variables examined, the timing of activity of the descendant set of muscles differed. In another three of the 15 variables, there were significant interactions between muscle and prey type, indicating a prey effect which differed in the descendant muscles. Overall, evidence of motor divergence was found in three of five cases of muscle duplication. This indicates that muscle subdivision has led to increased functional complexity of the jaw-adductor muscle system in tetraodontiform fishes.