Morphology, mechanics, and locomotion: the relation between the notochord and swimming motions in sturgeon

Synopsis

To examine the relation between morphology and performance, notochordal morphology was correlated with notochordal mechanics and with steady swimming motions in white sturgeon, Acipenser transmontanus. In a still-water tank, motions of four sturgeon varied with changes in swimming speed and axial position along the body. For a 1.34 m sturgeon, slow and fast swimming modes were distinguished, with speeds at the fast mode more than two times those at the slow mode without changes in tailbeat frequency. This increase in speed is correlated with an increase in the body’s maximal midline curvature (m⁻¹), suggesting a role for curvature-related mechanical properties of the notochord. Maximal midline curvature also varied with axial position, and surprisingly was uncorrelated with axial changes in the notochord’s cross-sectional shape — as measured by height, width, inner diameter, and lateral thickness of the sheaths. On the other hand, maximal midline curvature was negatively correlated with the axial changes in the notochord’s angular stiffness (N m rad⁻¹) and change in internal pressure (% change from baseline of 58.6 kPa), both of which were measured during in vitro bending tests. In vivo curvature and in vitro angular stiffness were then used to estimate the bending moments (N m) in the notochord during swimming. In the precaudal notochord, the axial pattern of maximal stiffness moments was congruent with the pattern of maximal notochordal curvature in the precaudal region, but in the caudal notochord maximal angular stiffness was located craniad to maximal curvature. One interpretation of this pattern is that the precaudal notochord resists bending moments generated by the muscles and that the caudal notochord resists bending moments generated by hydrodynamic forces acting on the tail.