Rheotaxis revisited: a multi-behavioral and multisensory perspective on how fish orient to flow.

Here, we review fish rheotaxis (orientation to flow) with the goal of placing it within a larger behavioral and multisensory context. Rheotaxis is a flexible behavior that is used by fish in a variety of circumstances: to search for upstream sources of current-borne odors, to intercept invertebrate drift and, in general, to conserve energy while preventing downstream displacement. Sensory information available for rheotaxis includes water-motion cues to the lateral line and body-motion cues to visual, vestibular or tactile senses when fish are swept downstream. Although rheotaxis can be mediated by a single sense, each sense has its own limitations. For example, lateral line cues are limited by the spatial characteristics of flow, visual cues by water visibility, and vestibular and other body-motion cues by the ability of fish to withstand downstream displacement. The ability of multiple senses to compensate for any single-sense limitation enables rheotaxis to persist over a wide range of sensory and flow conditions. Here, we propose a mechanism of rheotaxis that can be activated in parallel by one or more senses; a major component of this mechanism is directional selectivity of central neurons to broad patterns of water and/or body motions. A review of central mechanisms for vertebrate orienting behaviors and optomotor reflexes reveals several motorsensory integration sites in the CNS that could be involved in rheotaxis. As such, rheotaxis provides an excellent opportunity for understanding the multisensory control of a simple vertebrate behavior and how a simple motor act is integrated with others to form complex behaviors.