A test of mouth-opening and hyoid-depression mechanisms during prey capture in a catfish using high-speed cineradiography.

Detailed morphological analyses have identified a number of different mechanical pathways by which the morphologically complex cranial system of fishes can achieve mouth opening and hyoid depression. However, many of these proposed mechanisms remain untested. Furthermore, very little is known about the precise timing of activity of each of these mechanisms, and about the magnitude of each mechanism's total contribution to its proposed function. In the present study, all mouth opening and hyoid depression mechanisms described for Clarias gariepinus, an air-breathing catfish, are analysed. High-speed X-ray videos were recorded during prey capture of three catfish implanted with small, radio-opaque markers in the cranial elements potentially involved. A kinematic analysis was performed from which data were used as input in planar four-bar models. This analysis shows that the opercular mouth-opening mechanism initiates mouth opening, but is not able to cause the complete mouth openings as observed on the X-ray videos. The latter is accomplished through the protractor hyoidei muscles, which couple hyoid depression to lower jaw depression in a four-bar system and also reinforce lower jaw depression by shortening during the final stage of mouth opening. Although the angulo-ceratohyal ligament was previously hypothesised to play a part in mouth opening, our results show that it probably does not, but rather functions as a hyoid-elevator during mouth closure. Finally, hyoid depression is exclusively achieved by the four-bar mechanism involving neurocranial elevation and pectoral girdle retraction, generally without any reinforcement by shortening of the sternohyoideus muscle. In contrast to the results from a recent analysis on sunfish, the catfish's sternohyoideus gradually elongates during hyoid depression.