The role of the lateral line and vision on body kinematics and hydrodynamic preference of rainbow trout in turbulent flow

The ability to detect water flow using the hair cells of the lateral line system is a unique feature found in anamniotic aquatic vertebrates. Fishes use their lateral line to locate prey, escape from predators and form cohesive schooling patterns. Despite the prevalence of complex flows in nature, almost nothing is known about the function of the lateral line and its relationship to other sensory modalities for freely swimming fishes in turbulent flows. Past studies indicate that under certain conditions the lateral line is not needed to swim steadily in uniform flow. This paper examines how the lateral line and vision affect body kinematics and hydrodynamic habitat selection of rainbow trout (Oncorhynchus mykiss) exposed to vortices generated behind a cylinder. Trout Karman gaiting (i.e. exploiting vortices to hold station in a vortex street) with a pharmacologically blocked lateral line display altered kinematics; body wavelength and wave speed increase compared to control animals. When visual cues are withheld by performing experiments in the dark, almost all Karman gait kinematics measured for fish with and without a functional lateral line are the same. The lateral line, rather than vision, plays a larger role in affecting body kinematics when trout hold station in a vortex street. Trout show a preference to Karman gait in the light but not in the dark, which may be attributed to physiological state rather than hydrodynamic or sensorimotor reasons. In the dark, trout both with and without a functional lateral line hold station near the downstream suction region of the cylinder wake (i.e. entraining) and avoid the vortex street. Vision therefore plays a larger role in the preference to associate with a turbulent vortex street. Trout in the light with a blocked lateral line show individual variation in their preference to Karman gait or entrain. In the dark, entraining trout with an intact lateral line will alternate between right and left sides of the cylinder throughout the experiment, showing an ability to explore their environment. By contrast, when the lateral line is blocked these fish display a strong fidelity to one side of the cylinder and are not inclined to explore other regions of the flow tank. Both entraining and Karman gaiting probably represent energetically favorable strategies for holding station relative to the earth frame of reference in fast flows. The ability to decipher how organisms collect and process sensory input from their environment has great potential in revealing the mechanistic basis of how locomotor behaviors are produced as well as how habitat selection is modulated.