Differences between inter- and intraspecific architectonic adaptations to pharyngeal mollusc crushing in cichlid fishes

Because fish heads are densely packed with muscles, ligaments, skeletal elements and other structures, transformations in one structure may influence surrounding structures. Transformations occur during phylogeny, ontogeny and as environmentally induced alterations, i.e. phenotypic plasticity. We describe differences in intra- and interspecific transformations of the pharyngeal jaw apparatus of haplochromine cichlids. Using multivariate clustering techniques we trace possible correlations in transformations of anatomical characters of the pharyngeal jaws and surrounding structures. The intraspecific transformation analysis is based on two environmentally induced morphs of Astatoreochromis alluaudi : a molluscivorous morph with a hypertrophied pharyngeal jaw apparatus and an insectivorous one with a non-hypertrophied apparatus. For the interspecific analysis five other haplochromine species from Lake Victoria with diets ranging from insects to molluscs were investigated. Although range in diet are the same, the anatomical ranges differ between A. alluaudi and the species cline. Besides similarities in anatomical changes of the pharyngeal jaw apparatus in the intra- and interspecific cline, differences were also observed. Apparently there are among haplochromines multiple pathways to achieve similar performance. In A. alluaudi architectonic and intrinsic plasticity constraints limit the adaptability of the pharyngeal jaw apparatus. In the species cline, these constraints have been overcome by genetical adaptation.