Residual tail twisting in ascidian larvae is stabilized by asymmetric myofibrils that resist bilateral symmetry restoration

Left-right (L-R) symmetry is a fundamental feature of bilaterian body plans, yet how it is actively maintained and corrected during development remains poorly understood. In the ascidian Ciona, embryos transiently exhibit clockwise tail twisting that is progressively reduced but stabilizes at a residual angle of approximately 10°. Inhibition of retinoic acid signaling or neuromuscular activity eliminated this residual twist, restoring full bilateral symmetry. Myofibrils in both left and right muscles adopted the same left-handed helical orientation, and mechanical modeling showed that this architecture generates net torque sufficient to stabilize residual twisting against full symmetry restoration. These findings reveal how bilaterian embryos balance symmetry restoration with morphogenetic asymmetry.