No matter what their favourite diet, most fish tackle a snack by the same approach; they suck it up. Despite these unvarying tactics, almost every species seems to have come up with a remarkably different mouth shape. Given that all of these structures have evolved to achieve the same goal, is it possible to divine any unifying principles of mouth structure evolution that might explain their extraordinarily diverse morphology? Timothy Higham from the University of California, Davis, thinks so, and outlines in this issue of The Journal of Experimental Biology how two suction feeders have honed their suck to their own tastes(p. 2713).

Visualising fluid flow with digital particle image velocimetry, Higham and his colleagues, Peter Wainwright and mechanical engineer Steven Day, tempted largemouth bass and bluegill sunfish with their favourite snacks. Having filmed each animal's lunge and calculated the fluid flow as they gulped down a meal, the team was able to calculate the trajectory of each gulp to find out how each species had optimised their chances of catching their favourite meal.

According to Higham and his colleagues, the bluegill sunfish's mouth generated high fluid speeds, relative to their surroundings, with high accuracy; both of which are ideal for picking off their preferred diet of zooplankton and larvae. However, the largemouth bass gulped down huge mouthfuls of water while closing in fast on their prey, generating high flows relative to their bodies, which is ideal for predators intent on trapping evasive fish and larger crustaceans.

So what overarching principles have the team been able to glean from analysing the feeding fish? They suspect that species generating high flow speeds with high accuracy will have small mouths, while fish gulping down huge mouthfuls to engulf elusive prey will be able to throw their mouths wide open,but with relatively low force.

Higham, T. E., Day, S. W. and Wainwright, P. C.(
). Multidimensional analysis of suction feeding performance in fishes: fluid speed, acceleration, strike accuracy and the ingested volume of water.
J. Exp. Biol.