Blind Mexican cave fish are fine at negotiating their cave homes. Having done away with their eyes they use other senses to guide them in the dark. Shane Windsor from the University of Auckland, New Zealand, explains that all fish sense their environment using velocity sensors on their skins and pressure gradient sensors along both sides (known as the lateral line). However, for blind cave fish these pressure and velocity sensors are their main senses for detecting their surroundings. Curious to know how the fish's surrounding hydrodynamic fields change as they encounter obstacles, Windsor and his PhD supervisors, Gordon Mallinson and John Montgomery, put blind Mexican cave fish in a digital particle image velocimetry (DPIV) rig built by Stuart Cameron to visualise the fluid flowing around them (p. 3819/3832).
Releasing individual fish (ranging in size from 40 to 60 mm) into the DPIV system, Cameron and Windsor shone a plane of laser light into the tank and filmed the water swirling as the fish swam through the laser plane. Filming the fish when they swam perpendicular to the wall was easy; ‘They follow surfaces so if you have a square tank they keep going round and round the outside,’ explains Windsor. However, recording the fluid flows as the fish approached a wall head on was more difficult. The duo had to direct the fish out into open water by placing an obstacle in their path, forcing them to head directly toward the opposite wall.
Analysing the velocity of the water flowing around the fish's nose and along its sides, Windsor was able to calculate the pressure field that the fish detects with its lateral line. Comparing the pressure field surrounding the fish in open water with the pressure field as it approached a wall head on (p. 3819), Windsor says, ‘When it's away from the wall there is a stagnation point – that's where the flow is coming straight in to the nose. From the point of view of the fish the flow stops and there is very high pressure.’ However, as the fish approached the wall head on, at a distance of about 8–12 mm the team saw the stagnation point widen and spread across the fish's nose as the pressure rocketed, warning the fish that it needed to change course to avoid a collision.
When the fish swam parallel to the wall (p. 3832), at distances less than 4–6mm, the team saw the stagnation point slip around to the side of the fish's head closest to the wall and spread wide as the pressure rose. The pressure at the side of the fish also dropped as the fish neared the wall.
Next the team was curious to find out how these pressure and velocity features varied as the fish swam at different speeds and distances from the wall. Windsor explains that the fish swim faster when introduced into a new setting and the team wanted to find out if increasing the fish's speed increased their sensitivity to looming objects. However, ‘You can't say “Swim this fast and this far from the wall,” to the fish. You have to take what you get,’ says Windsor, so he teamed up with Stuart Norris to run computational fluid dynamics (CFD) simulations where he varied the computational fish's swimming speed and distance to the wall to find out how the pressure and velocity fields were affected.
The team found that the fluid flow patterns hardly changed, even at the highest speeds. ‘Everything just scales with the velocity and the form doesn't change,’ says Windsor, and adds ‘if a fish is sensitive to a certain relative change, say a doubling, it will pick it up at pretty much the same distance irrespective of how fast it is swimming’. So, speeding up may not help the fish detect more distant objects because the hydrodynamic changes that they respond to occur at the same distance from obstacles regardless of their speed. Also, moving fast gives them less time to respond to structures, so why do they speed up in unfamiliar water?
Windsor suspects that by swimming fast the fish increase the fluid flow around their bodies, making the hydrodynamic signal stronger and easier to interpret in noisy environments. He also suspects that the fish probably keep track of the location of the stagnation point and other flow features on the surface of their bodies. ‘They can use that to interpret how things change in time and space,’ says Windsor, ‘to help them avoid obstacles in the dark’.