Hydrodynamic imaging by blind cave fish
During my PhD at the University of Auckland I researched the fluid flows and behaviour involved with hydrodynamic imaging. Blind Mexican cave fish (Astyanax fasciatus) live in caves in Mexico and do not have functioning eyes. In the complete darkness of the caves the fish are able to avoid colliding with obstacles and each other. Using their lateral line sensory system the fish are able to sense objects by the way the objects effect the flow around the fish as they swim through the water.
By studying the swimming kinematics and behaviour of cave fish in the lab I found that hydrodynamic imaging has a short range and that fish only reacted to obstacles when they are about 10% of their body length away. Interestingly, there was no significant correlation between how fast the fish swam and the distance at which they could detect objects. This was contrary to what we expected as blind cave fish are known to swim faster when put into a new environment and it had always been assumed that by swimming faster they were enhancing their hydrodynamic imaging ability.
We also found that blind cave fish frequently touch surfaces with their pectoral fins. We thought initially that the fish might be using this as another way of sensing surfaces. However, later experiments showed that fish rely on hydrodynamic information to sense surfaces and ignore their sense of touch if it gives conflicting information.
Flow visualisation and modelling
We measured the flow fields around freely swimming blind cave fish as they swam in open water and as they approached obstacles using particle image velocimetry (PIV). We then combined these measurements with computational fluid dynamics (CFD) modelling to estimate the stimulus to the lateral line.
Our results showed a high-pressure region around the nose of the fish, low pressure regions corresponding to accelerated flow around the widest part of the body and a thick laminar boundary layer down the body. In agreement with what we saw behaviourally there was no increase in the distance at which a fish could detect a wall if it swam faster (assuming that the fish are sensitive to a certain relative change in the flows around them). The signal to the lateral line decreased the further the fish were away from the wall and appeared to be insufficient for the fish to detect the wall when they were approximately 25% or more of their body length away.
All together our results showed that hydrodynamic imaging provides blind cave fish with detailed information about their surroundings at short range.