Two different types of pharyngeal jaws have been described in A. citrinellus: papilliform and molariform. For both A. labiatus and A. zaliosus only papilliform pharyngeal jaws have been described previously in the literature (Meyer 1989, 1990a,b). All A. citrinellus individuals sampled in L. Managua, Tisma Pond and L. Masaya had papilliform pharyngeal jaws (see also Meyer 1990a). All A. citrinellus individuals collected in Ometepe in L. Nicaragua had molariform pharyngeal jaws. Among samples of A. citrinellus that were collected from L. Xiloá, L. Apoyo (A. zaliosus) and Isletas, L. Nicaragua both trophic morphs were found in sympatry. This study provides the first evidence of molariform pharyngeal jaws on fish from Lake Apoyo.
The pharyngeal jaw structure in C. citrinellum is correlated with differences in external morphology (Fig. 1, Meyer 1988). Molariform morphs have blunter, shorter snouts, larger eyes, wider heads, and deeper shorter bodies than papilliform morphs.
However, Matt Arnegard and I had to disagree with Ribbink and Konings about the function of the soft, enlarged lips. We believe they may be too delicate for constant friction against rocks. Our discovery that the lips are densely packed with taste buds suggests an alternative explanation; these lips, much like the barbels of catfishes, may allow A. labrosus to sense prey items deep in dark cracks by tasting them. Or, perhaps the two proposed functions are not mutually exclusive. A well-designed experiment could settle the matter.
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