Locomotion pattern and trunk musculoskeletal architecture among Urodela

Abstract

We comparatively examined the trunk musculature and prezygapophyseal angle of mid-trunk vertebra in eight urodele species with different locomotive modes (aquatic Siren intermedia, Amphiuma tridactylum, Necturus maculosus and Andrias japonicus; semi-aquatic Cynops pyrrhogaster, Cynops ensicauda; and terrestrial Hynobius nigrescens, Hynobius lichenatus and Ambystoma tigrinum). We found that the more terrestrial species were characterized by larger dorsal and abdominal muscle weight ratios compared with those of the more aquatic species, whereas muscle ratios of the lateral hypaxial musculature were larger in the more aquatic species. The lateral hypaxial muscles were thicker in the more aquatic species, whereas the M. rectus abdominis was more differentiated in the more terrestrial species. Our results suggest that larger lateral hypaxial muscles function for lateral bending during underwater locomotion in aquatic species. Larger dorsalis and abdominal muscles facilitate resistance against sagittal extension of the trunk, stabilization and support of the ventral contour line against gravity in terrestrial species. The more aquatic species possessed a more horizontal prezygapophyseal angle for more flexible lateral locomotion. In contrast, the more terrestrial species have an increasingly vertical prezygapophyseal angle to provide stronger column support against gravity. Thus, we conclude trunk structure in urodeles differs clearly according to their locomotive modes.

Keywords: Urodela; locomotion; muscle; trunk; vertebra.

Fig 1

—A. Dorsal view of perivertebral musculature after removal of M. dorsalis trunci. —B. Lateral view of trunk musculature. —C. Cross-sectional view.

Fig 2

—Frontal view of mid-trunk vertebra. —A. μ-CT scanned image of vertebra of Siren intermedia. —B. Frontal view of mid-trunk vertebra of Siren intermedia for measuring prezygapophyseal angle. —a. Prezygapophyseal angle. Scale bar = 1 cm.

Fig 3

—Lateral view of the trunk muscles of —A. Siren intermedia, —B. Cynops ensicauda, —C. Hynobius lichenatus. Scale bar = 5 mm.

Fig 4

—Cross-sections through the mid-trunk of —A. Siren intermedia, —B. Cynops ensicauda, —C. Hynobius lichenatus. Scale bar = 5 mm.

Fig 5

—Muscle weight ratios. Different superscript letters indicate significant differences (ANOVA and Tukey's test, P > 0.05). Black bar: aquatic species, grey bar: semi-aquatic species, white bar: terrestrial species.

Fig 6

—Prezygapophyseal angle of mid-trunk vertebra. Different superscript letters indicate significant differences (ANOVA and Tukey's test, P > 0.05). Black bar: aquatic species, grey bar: semi-aquatic species, white bar: terrestrial species.