Analysis of ontogenetic changes in head shape and diet in a catfish with moderately enlarged jaw adductors (Clariallabes melas)

Jaw adductor hypertrophy, or the presence of enlarged jaw-closing muscles, has arisen several times independently in African clariid catfish. Previous work has demonstrated that species characterized by enlarged jaw adductors may have unusual ways of foraging such as terrestrial foraging and prey capture, and often include a large proportion of hard and terrestrial prey in their diet. However, relatively little is known about species of the genus Clariallabes with an intermediate degree of jaw adductor hypertrophy. In the present study we present data on head shape and diet for a range of sizes of specimens of a poorly known species of Clariallabes, C. melas. Our data show that growth patterns in this species and the previously studied C. longicauda are similar in some ways (e.g. positive allometry of the growth of the jaw muscles) but different in others (negative allometry in hyoid width). However, C. melas has a smaller head for its body size in all dimensions. Due to the large number of empty stomachs we encountered, dietary data remain preliminary, but suggest a varied diet including both hard and soft prey. Our data show that a large amount of variation in head shape may exist even among related species and that species with jaw adductor hypertrophy generally show positive allometry in the growth of the jaw adductors and associated structures. Whether jaw muscle hypertrophy is an adaptive trait in clariid catfish awaits further comparative analyses testing for the evolutionary association between jaw adductor hypertrophy and the inclusion of hard prey into the diet.