Balitorid loaches are a family of fishes that exhibit morphological adaptations to living in fast flowing water, including an enlarged sacral rib that creates a ‘hip’-like skeletal connection between the pelvis and the axial skeleton. The presence of this sacral rib, the robustness of which varies across the family, is hypothesized to facilitate terrestrial locomotion seen in the family. Terrestrial locomotion in balitorids is unlike that of any known fish: the locomotion resembles that of terrestrial tetrapods. Emergence and convergence of terrestrial locomotion from water to land has been studied in fossils; however, studying balitorid walking provides a present-day natural laboratory to examine the convergent evolution of walking movements. We tested the hypothesis that balitorid species with more robust connections between the pelvic and axial skeleton (M3 morphotype) are more effective at walking than species with reduced connectivity (M1 morphotype). We predicted that robust connections would facilitate travel per step and increase mass support during movement. We collected high-speed video of walking in seven balitorid species to analyze kinematic variables. The connection between internal anatomy and locomotion on land are revealed herein with digitized video analysis, μCT scans, and in the context of the phylogenetic history of this family of fishes. Our species sampling covered the extremes of previously identified sacral rib morphotypes, M1 and M3. Although we hypothesized the robustness of the sacral rib to have a strong influence on walking performance, there was not a large reduction in walking ability in the species with the least modified rib (M1). Instead, walking kinematics varied between the two balitorid subfamilies with a generally more ‘walk-like’ behavior in the Balitorinae and more ‘swim-like’ behavior in the Homalopteroidinae. The type of terrestrial locomotion displayed in balitorids is unique among living fishes and aids in our understanding of the extent to which a sacral connection facilitates terrestrial walking.

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