The body curvature displayed by fishes differs remarkably between species. Some nonmuscular features (e.g., number of vertebrae) are known to influence axial flexibility, but we have poor knowledge of the influence of the musculotendinous system (myosepta and muscles). Whereas this system has been described in stiff-bodied fishes, we have little data on flexible fishes. In this study, we present new data on the musculotendinous system of a highly flexible fish and compare them to existing data on rigid fishes. We use microdissections with polarized light microscopy to study the three-dimensional anatomy of myoseptal tendons, histology and immunohistology to study the insertion of muscle fiber types into tendons, and μ-CT scans to study skeletal anatomy. Results are compared with published data from stiff-bodied fishes. We identify four important morphological differences between stiff-bodied fishes and Carapus acus: (1) Carapus bears short tendons in the horizontal septum, whereas rigid fishes have elongated tendons. (2) Carapus bears short lateral tendons in its myosepta, whereas stiff-bodied fishes bear elongated tendons. Because of its short myoseptal tendons, Carapus retains high axial flexibility. In contrast, elongated tendons restrict axial flexibility in rigid fishes but are able to transmit anteriorly generated muscle forces through long tendons down to the tail. (3) Carapus bears distinct epineural and epipleural tendons in its myosepta, whereas these tendons are weak or absent in rigid fishes. As these tendons firmly connect vertebral axis and skin in Carapus, we consider them to constrain lateral displacement of the vertebral axis during extreme body flexures. (4) Ossifications of myoseptal tendons are only present in C. acus and other more flexible fishes but are absent in rigid fishes. The functional reasons for this remain unexplained.
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