Boxfishes are not exactly the sleek streamlined shape most people would have in mind when designing a fish; their angular bodies appear to be in danger of rotating when buffeted by turbulent waters. But watch them darting around in their tropical-reef homes and you'll notice that boxfishes are remarkably fast and stable swimmers. To discover how these odd-shaped creatures manage to swim so smoothly, Ian Bartol and colleagues put several boxfish species through their paces(p. 327).

The ungainly appearance of these peculiar fishes is largely due to the rigid bony carapaces that cover their bodies. Carapaces produce slightly different body shapes in different boxfish species. To find out whether differently-shaped boxfishes use the same self-correcting mechanisms for stability while swimming, the team compared water flow along the carapaces of three boxfish species - the spotted boxfish, scrawled cowfish and buffalo trunkfish - using digital particle image velocimetry, pressure distribution measurements and force balance measurements. They found that the differently-shaped bodies of these three species directed water flow in very similar ways. Bartol explains that the ventral keels of these fishes are vortex generators, `producing flows around their bodies similar to those around the leading edges of delta-winged aircraft like the space shuttle.' The carapace surface pressures resulting from these flows contribute to forces on the carapace that allow the fish to make self-correcting movements while swimming, enabling them to swim in straight lines. Boxfishes' awkwardly-shaped bodies may also steady the fish while they hunt for coral-dwelling prey in their turbulent reef habitats.

Bartol, I. K., Gharib, M., Webb, P. W., Weihs, D. and Gordon, M. S. (
). Body-induced vortical flows: a common mechanism for self-corrective trimming control in boxfishes.
J. Exp. Biol.