The cellular and behavioral blueprints of chordate rheotaxis.

The oceans are filled with life-forms exhibiting complex adaptations to help navigate fluid environments and take advantage of fluid motion to locomote and disperse. However, the neuronal and behavioral underpinnings of navigating in fluid environments outside vertebrates are poorly understood. We present behavioral and computational modelling-based evidence that the pre-vertebrate chordate Ciona intestinalis actively modulates its heading and angular velocities to counter oncoming flows and perform positive rheotaxis. We demonstrate that a distributed network of ciliated peripheral sensory neurons is responsible for sensing hydrodynamic information such as flow velocity and direction. Pharmacological removal of sensory cilia impedes rheotactic behavior and reduces stimulus evoked neuronal activity. Whole-brain calcium imaging experiments reveal that the central nervous system of Ciona can encode the onset and offset of flow, flow direction, and flow velocity, suggesting that miniature chordate brain is capable of surprisingly sophisticated computations.