Hydrodynamic imaging by blind Mexican cave fish

Blind Mexican cave fish (Astyanax fasciatus) lack a functioning visual system, and are known to use self-generated water motion to sense their surroundings; an ability termed hydrodynamic imaging. Nearby objects distort the flow field created by the motion of the fish. These flow distortions are sensed by the mechanosensory lateral line. Little is known about the fluid mechanics involved in hydrodynamic imaging, or how the behaviour of the fish might influence their ability to sense the world around them.Automated image analysis was used to study the effects of swimming kinematics on the ability of the fish to sense their surroundings when introduced into a novel environment. The fish reacted to avoid head-on collisions with a wall at a remarkably short mean distance of 4.0 ± 0.2 mm. The ability of the fish to react, was dependent on whether they were beating their tail as they approached the wall. When following surfaces, such as a wall, the fish changed their swimming kinematics significantly and used both tactile and hydrodynamic information. Measuring the tendency of the fish to follow a tightening curve showed the fish to be moderately thigmotactic.The flow fields around freely swimming fish were experimentally measured using Particle Image Velocimetry (PIV). A new algorithm was developed to calculate the pressure field around the fish based on the velocity field measured using PIV. The algorithm was validated against analytical and computational fluid dynamic (CFD) solutions.The flow fields around gliding fish and the stimuli to the lateral line of the fish were calculated using CFD models, validated against the experimental PIV data. The flow fields changed in characteristic ways as the fish approached a wall head-on or swam parallel to a wall. At 0.10 body lengths from a wall, the stimulus to the lateral line was estimated to be sufficient for the fish to be able to detect the wall, but this decreased rapidly with increasing distance from the wall. The CFD models suggested that the velocity of the fish does not affect the distance at which they detect an object.Hydrodynamic imaging is a short range sensory ability and blind cave fish require their sensitive lateral line and fast reactions in order to be able to use it to sense the world around them and avoid collisions. The information gained about the fluid mechanics of hydrodynamic imaging, and the flow measurement and modelling techniques developed here will be useful for further study of this remarkable ability.