INTERACTIONS BETWEEN THE PHARYNGEAL JAW APPARATUS, FEEDING-BEHAVIOR, AND ONTOGENY IN THE CICHLID FISH, HAPLOCHROMIS-PICEATUS - A STUDY OF MORPHOLOGICAL CONSTRAINTS IN EVOLUTIONARY ECOLOGY

I found evidence of a morphological constraint on the food choice of young stages of a cichlid fish. The constraint is not a developmental one because it is not imposed by developmental changes. I found no evidence of developmental constraints. In gradual transitions developmental constraints may be rare, in contrast to their regular occurrence in sudden transitions. I suggest that flexible muscle activation and reserve capacity (structural capacity in excess of functional demand) provide organisms with mechanisms that buffer against the effects of form changes and as such facilitate ontogenetic and evolutionary changes. A mathematical model for piercing prey in the pharyngeal jaws is applied to a juvenile cichlid fish. The maximum possible biting forces and the best biting directions were calculated. Piercing forces were measured that were generated when natural prey were pierced. The results were compared with the feeding behaviour in the laboratory and in the field and with results from an adult. The model predicts great flexibility of the pharyngeal jaw apparatus. Furthermore, the model predicts that a shift in biting point, prey size, or predator size leads to changes in muscle recruitment. The m.transversus ventralis is predicted to play an important role in pharyngeal biting. The model explains why large fish can easily pierce Chaoborus larvae and pupae, whereas a fish of 2.5 cm is just able to pierce larvae (symmorphosis) and is unable to pierce pupae. The inability of small fish to pierce pupae is due to a morphological constraint. This constraint is imposed by the size of the cross-sectional areas of the muscles that generate the biting force. (C) 1993 Wiley-Liss, Inc.