Cruising specialists and accelerators--are different types of fish locomotion driven by differently structured...

Locomotor specialists, such as accelerators and cruisers, have clearly differing body designs. For physical reasons these designs are mutually exclusive, i.e. cruisers necessarily have poor accelerating capabilities and vice versa. For the first time, we examine whether differences in the anatomy of the musculo-tendinous system of the trunk are present in addition to the differences in external body design. We investigated the myoseptal series of two closely related locomotor specialists, the cruiser Scomber scombrus and the accelerator Channa obscura, by microdissections combined with polarized light microscopy and histology. Our comparison includes 3D-morphology of myosepta, spatial arrangement and length of myoseptal tendons, their relation to red and white muscles, rostrocaudal changes in all these aspects and the musculo-tendinous system of the caudal fin. Regarding all these features, Channa has retained the plesiomorphic condition of its actinopterygian ancestor. In contrast, the derived morphology of Scomber is characterized by (i) lateral (LT) and myorhabdoid tendons (MT) that are lengthened to up to 20% of body length (compared to a maximum of 8.2% in Channa), (ii) posterior myoseptal cones that are subsequently linked by horizontal projections of merged LTs and MTs, (iii) an increased area of red muscle fibers that insert to LTs of myosepta, (iv) the reduction of epineural (ENTs) and epipleural tendons (EPTs) that connect backbone and skin, (v) specific caudal tendons that are identified to be serial homologues of LTs and MTs of more anterior myosepta, (vi) and a partial reduction of intrinsic caudal muscles. These results suggest the following functional adaptations in the cruiser Scomber. Red muscle forces may be transmitted through LTs and posterior cones to the prominent tendons of the caudal fin. The length of LTs and the intersegmental connections along the posterior cones may facilitate posterior force transmission and may be correlated with the long propulsive wavelength generally observed in cruising carangiform swimmers. Epineural and epipleural tendons are interpreted to minimize lateral backbone displacement during high body curvatures. This is consistent with the lack of these tendons in Scomber, because high body curvatures are not displayed in stiffer-bodied carangiform swimmers. It remains to be tested whether the specializations revealed in this initial study for Scomber represent general specializations of carangiform swimmers. Taking into account the geometry of myoseptal tendons and the horizontal septum we evaluate how local bending according to beam-theory can be generated by white or red muscle activity in Channa and Scomber. In both species, the musculo-tendinous anatomy of the caudal fin explains the functional asymmetry of the caudal fin that was experimentally revealed in previous studies.