On the functional significance of the loss of the interhyal during ontogeny in Clarias gariepinus Burchell, 1822 (Teleostei : siluroidei).

In larval Clarias gariepinus the cartilaginous interhyal is continuous with the dorsal and ventral part of the hyoid arch. During later ontogeny the interhyal becomes reduced and is completely lost in the adult specimens. The connection between the suspensorium and the hyoid is replaced by a stout ligament, the ligamentum hyomandibulo-ceratohyale. It is hypothesised that this will reduce the mobility of the hyoid bar. Some specific lifestyle adaptations of C. gariepinus overrule the necessity of highly mobile hyoid bars. Large hyoid bar depressions would destabilise the dorso-ventrally flattened skull when resting on the bottom. As the Clariidae are able to perform aerial respiration, no large hyoid depression is needed for an extensive aquatic respiration. As C. gariepinus is not a suction feeding species, no sudden volume increase of the orobranchial cavity is required through a large hyoid depression. Morphological evidence indicating a restricted possibility of the depression of these bars consists of a small skin fold between the hyoid and the lower jaw, allowing a restrained ventral excursion of the rostral tip of the hyoid. Due to the strongly dorso-ventrally flattened skull the suspensorium is relatively small, in its dorso-ventral direction, which restricts the lateral displacement of the caudal tip of the hyoids during depression. A short sternohyoideus muscle connects the hyoid bar to the pectoral girdle, thus enabling a restricted movement of the bar during contraction. The branchiostegal membrane is rather firm and can be little folded and unfolded.