COMPARATIVE-STUDY OF LOWER PHARYNGEAL JAW STRUCTURE IN 2 PHENOTYPES OF ASTATOREOCHROMIS-ALLUAUDI (TELEOSTEI, CICHLIDAE)

The potentially molluscivorous East-African cichlid Astatoreochromis a lluaudi is known to exhibit phenotypic plasticity in its pharyngeal ja w 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 analys is on serial microradiographs through the posterior third region of th e LPJ, where crushing forces are assumed to be the highest, reveal dif fering growth trajectories: (1) compensating for fish size (standard l ength) the LPJ grows to a significantly larger size and volume in snai l-eating specimens, (2) all structural units distinguished contribute to the volume increase of the LPJ in the hard versus the soft phenotyp e, and (3) the bone volume fraction in each of the units keeps pace wi th the growth of the unit proper, indicating that porosity does not ch ange on one growth trajectory or from one phenotype to another. In add ition, morphological observations show in hard food specimens: (1) the development of a structurally different bony layer along the inner si de of the cortical plate, and (2) a reinforcement of the medullary cav ity in the form of oriented trabeculae. Both are interpreted as a cons olidation of the medullary cavity to resist the compressive forces exe rted when hard food particles (mollusc shells) are crushed. (C) 1994 W iley-Liss, Inc.