While slurping one's food is frowned upon in some societies, fish do it all the time; they simply throw their jaws wide, decrease the pressure within, and suck. In 1982, Mees Muller and his colleagues derived a sophisticated mathematical model that allowed scientists to calculate both the pressure in the mouth and speed of fluid drawn in by simply measuring the mouth's dimensions and its rate of expansion. However, no one had ever directly measured the fluid flows generated by a gulping fish while simultaneously recording the mouth pressure until Timothy Higham, Steven Day and Peter Wainwright put bluegill sunfish and largemouth bass, two members of the centrarchid family, into a DPIV tank().
Digital particle image velocimetry (DPIV) is mainly used to calculate the hydrodynamic forces acting on an animal as it scythes through the water; a fish swims through a suspension of microscopic reflective beads illuminated by a thin plane of laser light, allowing scientists to track the eddies generated by the fish. By swimming the fish in a flow tank that matches the water's flow to the speed of the swimming fish, it's possible to hold the fish in one position within the laser plane and record the glittering flow patters. However, this isn't an option when you're measuring fluid flows near to a lunging fish's mouth. Fortunately Higham realised that he could entice the fish to lunge reproducibly in the laser plane by tempting them with a tasty morsel suspended in the laser light; they always open their mouths at the same point when approaching a meal. But even then, the majority of the fish weren't correctly positioned in the plane of light and Higham had to discard the data.
After months of patiently filming fish feeding while simultaneously recording the pressures in their mouths, Higham was able to calculate the fluid speed as each fish sucked. Comparing the sunfish with the bass, Higham could see that the fluid speeds were strongly correlated with the time it took the sunfish to open its mouth fully, but the relationship was weaker for the bass. The sunfish's powerful suck was generated simply by opening its mouth,but Higham explains that the weaker relationship between the time the bass took to open its mouth fully and fluid flow suggests that the animal modulates its slurp with other mouth structures.
Next, Higham compared his measurements with fluid speed predictions from Muller's model and found that the model consistently overestimated the fluid speed over a range of distances in front of the fish's mouth. Ever since the model's development, fish kinematicists have used mouth pressure measurements to predict fluid flow speeds, but Higham's simultaneous recordings suggest that simply knowing the pressure in a fish's mouth isn't enough to accurately estimate the flow from Muller's model. However, he suspects that it is now feasible to increase the model's complexity to better reflect the true movements in a gaping fish's mouth, given the massive increase in computing power since the early 1980s.
