Jaw dropping moments happen to most of us from time to time. Fish, on the other hand, throw their jaws wide every time they spy a tasty snack. Sam Van Wassenbergh explains that how fish coordinate their complex jaws while opening wide was something of a mystery. Some fish species have as many as 60 bones in their heads, joined by complex networks of muscles and ligaments, so opening their mouths is much more complex than simply hinging around a single joint. Van Wassenbergh adds that morphologists had made suggestions about the mechanisms the fish could use to open their gaping jaws, but no one had ever analysed them in the act to see whether the fish used any, or all, of these proposed mechanisms. Puzzled by the feeding habits of the air-breathing catfish Clarias gariepinus, Van Wassenbergh and his colleagues Anthony Herrel, Dominique Adriaens and Peter Aerts began putting the fish through their paces and found that although they used some of the mechanisms that had been predicted, they didn't use all(p. 4627).

To find out how the fish orchestrate a gape, the team decided to analyse the fish's jaws with high speed X-ray movies. First Van Wassenbergh and Herrel had to fit the fish's skulls with tiny lead markers so they could track the fish's jaw movements using X-rays while they lunged for lunch. Once the fish had recovered from the surgery, Van Wassenbergh was ready to start filming,but even tempting the fish with chunks of cod and prawns couldn't persuade them to perform; terrified by noise and vibrations from the X-ray machine,they retreated into a corner of the tank and hid. Fortunately, the timid animals eventually became used to the cumbersome kit, and Van Wassenbergh was able to resume filming the fish's jaws as they snapped open.

Having recorded almost 200 jaw-dropping sequences Van Wassenbergh says `my finger still hurts from manually digitising all those markers'. But with all the data in hand, the team were ready to discover just how the catfish snap their jaws apart. They decided to compare the jaw movements that were predicted by the suggested mechanisms with the movements that the team saw in the X-ray movies. First Van Wassenbergh painstakingly measured the trajectories of several of the head's components involved in opening the jaws. Then he calculated the position predicted for the jaw by each of the jaw opening mechanisms before comparing jaw-fact with jaw-theory to see which mechanisms the fish were using to open their mouths.

Based on the comparison, the team realised that the fish began to open the mouth by rotating the operculum, as had been predicted from the morphology of the fish's head. Next the hyoid bone began retracting to continue the jaw's movement. Finally, the protractor hyoidei muscles, that link the hyoid to the jaw, began contracting to open the mouth fully. Surprisingly, the angulo-ceratohyal ligament, linking the hyoid to the lower jaw, had also been thought to participate in one of the opening mechanisms, but Van Wassenbergh couldn't find evidence that it contributes at all.

Van Wassenbergh is pleased that his observations agree well with some of the mechanisms that had been proposed and is keen to know whether other catfish use the same mechanisms when opening wide.

Van Wassenbergh, S., Herrel, A., Adriaens, D. and Aerts, P.(
). A test of mouth-opening and hyoid-depression mechanisms during prey capture in a catfish using high-speed cineradiography.
J. Exp. Biol.