Comparative study of lower pharyngeal jaw structure in two phenotypes of Astatoreochromis alluaudi (teleostei: Cichlidae)

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Abstract
The potentially molluscivorous East-African cichlid Astatoreochromis alluaudi is known to exhibit phenotypic plasticity in its pharyngeal jaw apparatus. We examined wild-caught (snail-eating) fish and specimens experimentally reared on soft food for differences in bone structure in their lower pharyngeal jaw (LPJ). The LPJ is built up of two halves, each of which consists of four structural units: a bony dentigerous, sutural and cortical plate, surrounding a medullary cavity containing sparse bone. Histomorphometric data and associated statistical analysis on serial microradiographs through the posterior third region of the LPJ, where crushing forces are assumed to be the highest, reveal differing growth trajectories: (1) compensating for fish size (standard length) the LPJ grows to a significantly larger size and volume in snail-eating specimens, (2) all structural units distinguished contribute to the volume increase of the LPJ in the hard versus the soft phenotype, and (3) the bone volume fraction in each of the units keeps pace with the growth of the unit proper, indicating that porosity does not change on one growth trajectory or from one phenotype to another. In addition, morphological observations show in hard food specimens: (1) the development of a structurally different bony layer along the inner side of the cortical plate, and (2) a reinforcement of the medullary cavity in the form of oriented trabeculae. Both are interpreted as a consolidation of the medullary cavity to resist the compressive forces exerted when hard food particles (mollusc shells) are crushed.