Peristaltic locomotion with a twisted and bent spine in Dermophis mexicanus.

Various aspects of movement, excavation, ecology, and muscle structure of Gymnophiona species have been reported. However, the unique mechanisms and dynamics of peristaltic locomotion and excavation are unclear, particularly for soil-inhabiting Gymnophiona spp. Previous studies primarily focused on forward locomotion and thrusting behavior, with less attention paid to the underlying vertebral and muscular mechanisms. Herein, using X-ray computed tomography, video analysis, and tissue specimens, we determined that the Mexican caecilian (Dermophis mexicanus) could generate peristaltic locomotion via coordinated bending and extension of the vertebral column and associated musculature, including the M. dorsalis trunci, M. subvertebralis, and external muscular sheath, facilitating both forward and backward movement. Thrust forces akin to a multistep spring mechanism are generated by extending their twisted and bent spines across the head, neck, chest, and abdomen. Each region flexes its vertebrae and modulates muscle size to fulfill various functions. The spinous processes of the vertebrae are loosely connected to independent segmental annuli via tendons. Unlike snakes, caecilians can move forward and backward by bending and stretching their spinal columns within their bodies, allowing movement through soil when there is space for their heads and bodies, which confers remarkable survival advantages. Dorsoventral and right-left dynamics of the spinal and muscular structures and the locomotion of Mexican caecilians was confirmed.