When flying or swimming, animals must adjust their own movement to compensate for displacements induced by the flow of the surrounding air or water1. These flow-induced displacements can most easily be detected as visual whole-field motion with respect to the animal’s frame of reference2. In spite of this, many aquatic animals consistently orient and swim against oncoming flows (a behavior known as rheotaxis) even in the absence of visual cues3,4. How animals achieve this task, and its underlying sensory basis, is still unknown. Here we show that in the absence of visual information, larval zebrafish (Danio rerio) perform rheotaxis by using flow velocity gradients as navigational cues. We present behavioral data that support a novel algorithm based on such local velocity gradients that fish use to efficiently avoid getting dragged by flowing water. Specifically, we show that fish use their mechanosensory lateral line to first sense the curl (or vorticity) of the local velocity vector field to detect the presence of flow and, second, measure its temporal change following swim bouts to deduce flow direction. These results reveal an elegant navigational strategy based on the sensing of flow velocity gradients and provide a comprehensive behavioral algorithm, also applicable for robotic design, that generalizes to a wide range of animal behaviors in moving fluids.

SignificanceWe explore the utility of expansion microscopy (ExM) in neuroscience and developmental biology using the zebrafish model. Regarding neuroscience studies, ExM enables the tracing of cellular processes in the zebrafish brain, as well as the imaging of synapses and their biomolecular content and organization. Regarding development, ExM enables the resolving of nuclear compartments, particularly nuclear invaginations and channels, and helps relate such cellular nanostructures to proteins of the cytoskeleton during embryogenesis.

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All animals need to differentiate between exafferent stimuli, which are caused by the environment, and reafferent stimuli, which are caused by their own movement. In the case of mechanosensation in aquatic animals, the exafferent inputs are water vibrations in the animal’s proximity, which need to be distinguished from the reafferent inputs arising from fluid drag due to locomotion. Both of these inputs are detected by the lateral line, a collection of mechanosensory organs distributed along the surface of the body.In this study, we characterize in detail how the hair cells, which are the receptor cells of the lateral line, discriminate between such reafferent and exafferent signals in zebrafish larvae. Using dye labeling of the lateral line nerve, we visualize two parallel descending inputs that can influence lateral line sensitivity. We combine functional imaging with ultra-structural EM circuit reconstruction to show that cholinergic signals originating from the hindbrain transmit efference copies that cancel out self-generated reafferent stimulation during locomotion, and that dopaminergic signals from the hypothalamus may have a role in threshold modulation both in response to locomotion and salient stimuli. We further gain direct mechanistic insight into the core components of this circuit by loss-of-function perturbations using targeted ablations and gene knockouts.We propose that this simple circuit is the core implementation of mechanosensory reafferent suppression in these young animals and that it might form the first instantiation of state-dependent modulation found at later stages in development.

In the print version of this Letter, owing to an error during the production process, some of the symbols in equation (1) were missing. The correct equation (1) is shown below. The equation displayed correctly in the HTML and PDF versions of the original Letter.

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