Testing the relationship between muscle deformation and vertebral curvature during feeding in rainbow trout (Oncorhynchus mykiss).

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Muscle shortening underpins most skeletal motion, and ultimately animal performance. Shortening is usually assumed to be homogenous within a muscle. However, in swimming fish, the whole body deforms like a bending beam: as the vertebral column flexes laterally, muscle shortening increases along a medio-lateral gradient. Recently it has been shown that fish also bend their bodies dorsally during feeding, resulting in dorsoventral shortening gradients in some regions. This suggests these dorsal motions also follow beam-like bending, where the curvature of the vertebral column predicts the linear, dorsoventral shortening gradient in the muscle. We tested this hypothesis by measuring vertebral curvature and dorsal body (epaxial) muscle shortening during feeding motions in rainbow trout (Oncorhynchus mykiss). We used XROMM to measure dorsal and lateral curvature across the anterior vertebral column and used fluoromicrometry to measure shortening between markers throughout the epaxial muscles. Vertebral curvature and muscle marker position were used to predict muscle strain following beam theory. Predicted strains were compared to the measured strains from fluoromicrometry. Trout flexed the anterior vertebrae dorsally and laterally during feeding strikes. When curvature and marker position in both planes were included, the model’s predicted strain matched the measured strain well across most vertebral regions. Our model confirms dorsal flexion during feeding in trout can be modelled with beam theory, even when accompanied by lateral flexion. We find evidence of linear dorsoventral and mediolateral shortening gradients in the epaxial muscles, although it remains to be determined if these reflect shortening of the fibres within these muscles.