When the first fish dragged themselves out of the swamps and onto dry land, they promptly faced a problem: their mouths opened in the wrong direction. While nibbling at morsels suspended in the water is fine when your mouth opens forward, it is of little use when the surface that supports your next snack is horizontal beneath you. The first invaders would have needed flexible mouths to enjoy the nutritious feast awaiting them on the bank. Evidently, the earliest land dwellers managed to overcome the challenge and the rest is history, but Krijn Michel, Peter Aerts and Sam Van Wassenbergh from the University of Antwerp, Belgium, and Alice Gibb, from Northern Flagstaff University, USA, were curious to know more about the adaptations that may have permitted this ground-breaking life transition. ‘Records for terrestrial feeding exist for… largescale foureyes’, they explain, adding that the eccentrically named fish – which have specially adapted eyes that permit them to see above and below the water's surface – have been known to haul themselves onto the bank to ambush oblivious victims. In order to investigate the fish's technique for feeding on land, Michel purchased nine fish from a local supplier ready to film them in action.
However, before he could begin to dissect the fish's feeding mechanism, he needed to know more about the animal's jaw structure. Fish have many more bones in their intricate jaws than most terrestrial animals, so Michel carefully CT scanned the head of one of the largescale foureyes to reveal the complex architecture – including a structure known as the suspensorium (consisting of six bones that connect the jaw to the fish's skull) and the upper and lower jaws. Then, he enticed the hungry fish with a chunk of tasty brown prawn to lurch out of the water onto a ramp as he filmed the manoeuvre. He was amazed to see the fish's jaws almost leapt out of their heads, as the lower jaw rotated down by 90 deg while the upper jaw pushed forward as they engulfed the tasty treat. ‘The rotation seemed to be completely dissociated from any restrictions, as if the jaw was on a dislocated joint of some sort’, says Michel. And when he compared the fish's feeding technique in the water and on land, he could see that the land-feeders had to move their jaws significantly more than the fish that nibbled in the water.
But how were the largescale foureyes pulling off this remarkable feat? To get inside the fish's head, Michel built a mathematical model of the jaw structure, including a spring linking the lower and upper jaw, and then simulated how the bones might move to reproduce the fish's impressive jaw action. The simulations showed that a ligament linking the upper and lower jaws has to be flexible and placed high on the two bones to provide sufficient leverage to push and rotate the upper jaw far enough forward to snap up prey on the ground. Michel and colleagues also add that the modifications that allow the largescale foureyes’ jaws to leap out of its head are in line with the jaw adaptations found in other members of the cyprinodontiform fish family that allow them to manipulate food more deftly.