Evolution of complexity in motor patterns and jaw musculature of tetraodontiform fishes

The prey-processing behavior and jaw-adducting musculature of tetraodontifo rm fishes provide a novel system for studying the evolution of muscles and their function. The history of this clade has involved a pattern of repeate d 'duplications' of jaw muscles by physical subdivision of pre-existing mus cles. As a result, the number of adductor mandibulae muscles in different t axa varies from as few as two to as many as eight. We used electromyography (EMG) to quantify motor-pattern variation of adductor mandibulae muscles i n four tetraodontiform species during feeding events on prey items that var ied in durability and elusiveness. Statistical analyses of variation in EMG variables revealed significant differences in motor patterns between dupli cated muscles derived from a common ancestral muscle in seven of nine cases examined. Overall individual EMG timing variables (e.g. relative onset or duration of bursts) were slightly less likely to diverge functionally than amplitude variables (e.g. relative intensity of bursts). Functional diverge nce was found in significant overall differences between muscles and twice as frequently in significant muscle-by-prey interaction terms. Such interac tions represent an underappreciated way in which motor patterns can evolve and diversify. Regional variation was documented in undivided muscles in tw o species, indicating that it is possible for functional subdivision to pre cede anatomical subdivision. This study shows that phylogenetic increases i n the number of tetraodontiform jaw adductor muscles have been associated w ith increases in the functional complexity of the jaws at the level of musc le activation patterns.