The Neotropical cichlid fish Cichlasoma citrinellum exhibits a distinct trophic polymorphism in the structure of its pharyngeal jaws. Fish have either strong, sturdy molariform pharyngeal jaws or weak papilliform jaws. Intermediate forms are rare among adults. Ontogenetically, however, all fish start as papilliform morphs. Differences in morphology and allometry between morphs in external measurements (25 measurements) and measurements of the lower pharyngeal jaws (nine measurements) were analysed using a multivariate morphometric approach. The ontogenetic growth trajectories for the pharyngeal jaws of the respective morphs start to diverge early in ontogeny and therefore seem to preclude the existence of adult intermediate morphs. The isometric growth of the tooth diameter in molariform morphs (in relation to standard length), versus its relatively retarded (negatively allometric) growth in the papilliform morph, is the main cause for the development of distinct trophic morphs in pharyngeal-jaw morphology. At approximately 50 mm standard length, both pharyngeal morphs can be discerned clearly. In this study only larger fish of determined pharyngeal-jaw morphology were considered (n= 30 for molariform morphs, n= 31 for papilliform morphs). External morphology differs between morphs as well, but less perceptibly. Principal-components analyses revealed that only a few measures load highly on PC2 and therefore account for most of the differences between the two trophic morphs in the shape of the external morphology. Molariform morphs have blunter, shorter snouts, larger eyes and deeper, shorter bodies than papilliform morphs.

The two external morphologies resemble other known limnetic and benthic body shapes of polymorphic species of fishes, and are believed to enhance the ecological separation of the two pharyngeal-jaw morphs. Evolutionary implications of ontogenetic changes and adult differences in morphology and trophic polymorphism are manifold. The alternative adaptations in this polymorphic species will decrease intraspecific competition by differential niche utilization and habitat choice, thus allowing for larger population sizes. Alternative adaptations may eventually lead to genetic isolation and possibly speciation of the two trophic morphs.