Is a convergently derived muscle-activity pattern driving novel raking behaviours in teleost fishes?

Behavioural differences across prey-capture and processing mechanisms may be governed by coupled or uncoupled feeding systems. Osteoglossomorph and salmonid fishes process prey in a convergently evolved tongue-bite apparatus (TBA), which is musculoskeletally coupled with the primary oral jaws. Altered muscle-activity patterns (MAPs) in these coupled jaw systems could be associated with the independent origin of a novel raking behaviour in these unrelated lineages. Substantial MAP changes in the evolution of novel behaviours have rarely been quantified so we examined MAP differences across strikes, chewing and rakes in a derived raking salmonid, the rainbow trout, Oncorhynchus mykiss. Electromyography, including activity onset timing, duration, mean amplitude and integrated area from five feeding muscles revealed significant differences between behaviour-specific MAPs. Specifically, early activity onset in the protractor hyoideus and adductor mandibularis muscles characterised raking, congruent with a recent biomechanical model of the component-mechanisms driving the raking preparatory and power-stroke phases. Oncorhynchus raking MAPs were then compared with a phylogenetically derived osteoglossomorph representative, the Australian arowana, Scleropages jardinii. In both taxa, early onset of protractor hyoideus and adductor mandibularis activity characterised the raking preparatory phase, indicating a convergently derived MAP, while more subtle inter-lineage divergence in raking MAPs resulted from onset-timing and duration differences in sternohyoideus and hypaxialis activity. Convergent TBA morphologies are thus powered by convergently derived MAPs, a phenomenon not previously demonstrated in feeding mechanisms. Between lineages, differences in TBA morphology and associated differences in the functional coupling of jaw systems appear to be important factors in shaping the diversification of raking behaviours.