Highlights • Negative gravitaxis in Ciona larvae is triggered by dimming illumination • Inhibitory input from the visual system suppresses gravitaxis until dimming • Output from the gravitaxis organ results in asymmetric motor inhibition • Reorientation behavior is evoked in downward- but not upward-facing larvae Summary Larvae of the tunicate Ciona intestinalis possess a central nervous system of 177 neurons. This simplicity has facilitated the generation of a complete synaptic connectome.
