Developmental transition of touch response from slow muscle-mediated coilings to fast muscle-mediated burst swimming in zebrafish

► Zebrafish flii mutants show normal behavior at 24 h but slow swimming at 48 h. ► The flii mutants show actin disorganization in fast muscle but not in slow muscle. ► Specific inactivation of fast muscle affects motility at 48 h but not at 24 h. ► In contrast, slow muscle perturbation eliminates motility at 24 h but not at 48 h. ► The increasing contribution of fast muscle to motility plays a role in motor development.