It has to be said, boxfish are neither sleek nor flexible. With a rigid bony carapace that covers most of their body and a boxy shape, they look pretty awkward compared to most fish. But looks aren't everything. A novel combination of biological and aeronautical research has shown that water flows over the boxfish much like air does over the space shuttle, making them stable and able to swim more smoothly than you'd expect by looking at their ungainly build.
These efficient swimming patterns were first observed in nature. Boxfish are reefdwellers, living in highly turbulent and unpredictable waters. Despite the constant buffeting of the currents, boxfish make only the slightest of deviations from their straight swimming paths. Rather than being cumbersome,these fish are obviously very manoeuvrable. So how do they manage it?
To answer this question Ian Bartol, working with Malcolm Gordon at UCLA and Morteza Gharib at Caltech, studied the hydrodynamic stability of the smooth trunkfish, choosing this boxfish for its simple shape(). One of the collaborators, Daniel Weihs, is an engineer who has an interest in locomotion. He recommended that they joined forces with engineers at Caltech, which gave the group the opportunity to use three different engineering methods to analyse boxfish stability. But this analysis required a model of the fish to be made, so Bartol had a frozen specimen CT scanned at a hospital, raising some eyebrows as he stood in queue!
This model was examined using digital particle image velocimetry (DPIV). In this experiment light reflecting particles are suspended in a water tunnel. As the water flow changes over the object, the movements of the particles are digitally tracked. This experiment showed vortices forming at certain points on the carapace. Two further experiments measured force and pressure changes on the carapace surface. The combined results excited the engineers, who spotted the similarity to flow patterns for delta-winged aircraft and meant that they could use this comparison for analysis. In each experiment Bartol tilted the model to look at the effect of pitch, the up/down movement, or yaw,side-to-side movement.
`I was very happy to discover that all three methods collectively pointed to the fact that the carapace is important,' explains Bartol. As the smooth trunkfish tilts, the shape of the carapace alters the water flow so that the fish is automatically stabilised. This self-correction holds many advantages for these fish. `It is important from a biological standpoint,' says Bartol.`They save a lot on energy.' It is also faster for the fish than using their fins to correct their position. The results hold more than biological interest. The project is partly funded by the Office of Naval Research who want to apply the results to autonomous underwater vehicles.
Bartol and his colleagues are now extending this work to other types of boxfish. But the carapace is only part of the story. `They swim by using complex motions of their five fins,' he explains, so they are focussing on the effect that fin movements have on the carapace forces in live fish. Bartol is also looking at an improved method of DPIV — defocusing DPIV —that will allow him to study flow in three dimensions rather than two. `It's a very new technique — it's not even being used much in the engineering world.' It certainly seems that such combined approaches can offer benefits to all involved.
