It might look as if a largemouth bass is simply puckering up for a kiss when it slurps up a tasty morsel, but the routine motion is an extremely sophisticated and coordinated manoeuvre. ‘The mouth and throat of fish are built from over 15 mobile bones’, says Aaron Olsen, from Brown University, who explains that the animals rapidly unfold the intricate bone network to expand their mouths and suck in water; Ariel Camp likens the movement to unfurling an umbrella. The bones are so well connected that even a dead fish can pop its mouth open if you gently rotate the opercular bone covering the gills. Based on the bone structure, scientists were able to build simulations that successfully reproduced many aspects of the movement, but they failed to accurately reproduce the movement of the lower jaw. As Camp had previously collected high-speed X-ray movies that allowed her to follow the motion of the elaborate bone structure in minute detail, Olsen and Elizabeth Brainerd began the complex challenge of simulating the movements of the bones relative to each other in an attempt to reproduce the jaw swinging motion.
After identifying the key bones (the suspensorium and interoperculum) that link the operculum to the lower jaw, Olsen then represented each structure as a rigid rod, and linked all four together with hinge joints to form a quadrilateral structure (known as a four-bar linkage), before rotating the rod that represented the operculum to see how the movement propagated through the bones to swing the jaw open. However, the simple model failed to reproduce the inward swinging motion of the jaw bone that accompanied the downward rotation that Camp had seen as the fish opened its mouth.
In the next simulation, Olsen and Brainerd wondered whether the joints at both ends of the interoperculum might rotate in three dimensions, like a ball-and-socket, so they replaced the hinge joints at the ends of the interoperculum with more mobile ball-and-socket joints, but this also failed to reproduce the correct jaw movement. And when they replaced all of the hinges with ball-and-socket joints, the simulated motion of the flexible system was more realistic, but still did not recapture the true jaw motion. It was only when the team placed ball-and-socket joints at the operculum–suspensorium joint, the operculum–interoperculum joint and the interoperculum–lower jaw joint, while retaining a simple hinge at the lower jaw–suspensorium joint, that the simulation most closely mimicked the jaw bone's inward and downward rotations. However, Olsen admits that the team was surprised by how much the lower jaw swings inward as it opens, and they suspect that it may also twist a little, suggesting that there may be some give in the ball-and-socket that links it to the interoperculum.
So there is more flexibility in the joints that link the opercular to the jaw bone than had been thought previously, and the team is hoping that their new approach could help them to better understand how other suction feeders rapidly expand their mouths while vacuuming up food.