rheotaxis

Rheotaxis is a directional movement of organisms in response to fluid flow. It can be categorized as positive rheotaxis, movement toward the direction of flow, and negative rheotaxis, movement against the flow. Rheotaxis is observed across a range of aquatic organisms, from microorganisms to larval animals.

The behavior arises from mechanosensory detection of how flow deforms surfaces, pressurizes the surrounding fluid, and creates shear. In many aquatic animals, flow is sensed through specialized sensory organs such as the lateral line system in fish and some crustaceans, or through the beating patterns of cilia and flagella in single-celled organisms. The physical interaction of cells with shear can bias their orientation and influence propulsion direction, producing a steering response that aligns movement with or against the flow.

Organisms exhibiting rheotaxis include bacteria and protozoa in microenvironments, aquatic invertebrates, and fish larvae. Nematodes and other small aquatic animals can also align with flow. In bacteria, shear can bias swimming by influencing flagellar motor rotation; in ciliates, flow fields interact with ciliary beating to steer trajectories.

Ecological and experimental relevance: Rheotaxis helps organisms maintain position in currents, locate refuge or feeding microhabitats, and influence migration and spacing in flowing habitats. It is routinely studied in microfluidic devices and flow chambers to quantify sensitivity to flow speed and direction, understand sensory biology, and separate populations by rheotactic preference.