How does a ‘box’ swim?
by David Catchpoole
With their ‘boxy’ shape and rigid bony carapace that covers most of
their body, boxfish look somewhat awkward compared to most other fish. As Science
journal commented recently, ‘One look at the aptly named boxfish, and you
might expect it to swim as well as a barn would fly.’1
In reality, boxfish are able to swim extremely smoothly.2 This is even more remarkable considering where they live—reefs
washed by highly turbulent and unpredictable waters. But even when continually buffeted
by swirling currents, boxfish make only the slightest of deviations from their straight
swimming paths, as they correct for unseen eddies and turbulence.
So what makes these ungainly-looking fish so stable and manoeuvrable? How do they
so efficiently keep to their swimming trajectory in swirling, surging waters?
According to recent research, the boxy shape is a major reason for their ‘hydrodynamic
stability’.3 Using a model of
the boxfish, Lactophrys triqueter (also known commonly as the ‘smooth
trunkfish’), in a water tunnel, the researchers found that, as the model was
tilted, its boxy shape changed the water flow, setting up counter-rotating currents
(vortices). These effectively act as self-correcting forces so that the fish is
automatically stabilized. Basically, if currents slant the boxfish upwards, a vortex
on top helps straighten it out.1 These findings excited
the researchers, who recognized that this same phenomenon is a hallmark of delta-wing
aircraft such as the Concorde and the space shuttle.4
The researchers found that the effect of varying pitch (tilting up/down) or yaw
(side-to-side movement) was the same—i.e. the self-correcting vortices that
develop around the boxfish’s body are the secret of its ‘unflappability’.
Apparently, this self-correction characteristic not only saves boxfish a lot of
energy, but it is also faster for them than using their fins to correct their position.
Navy engineers are showing interest in this, too, with a view to building more efficient
undersea robots.1
Who could have ever known that such an apparently simple ‘boxy’ shape
would be ideal for a fish that spends its life buffeted by the turbulent waters
swirling about the coral, hollows and overhangs of a reef? Who else but the Master
Designer—Creator of the heavens and the Earth and the sea and all that is
in them (Exodus
20:11).
References and notes
- Boxy swimmers, Science 299(5608):817,
2003. Return to text.
- Tilley, S., Smoothly does it, The Journal of Experimental Biology
206(4):637, 2003. Return to text.
- Bartol, I.K., Gharib, M., Weihs, D., Webb, P.W., Hove, J.R. and
Gordon, M.S., Hydrodynamic stability of swimming in ostraciid fishes: role of the
carapace in the smooth trunkfish Lactophrys triqueter (Teleostei: Ostraciidae),
The Journal of Experimental Biology 206(4):725–724,
2003. Return to text.
- In fact, ‘Lift coefficients of boxfish models were similar
to lift coefficients of delta wings, devices that also generate lift through vortex
generation.’ Ref. 3. Return to text.
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