Miriam L. Zelditch and William L. Fink

Abstract. Piranhas, like many teleosts, change their diets on both ontogentic and phylogentic time scales. Prior studies have suggested that pervasive morphological changes in body form on a phylogenetic time scale may be related to changes in diet, but previous reports have found little shape cahnge in piranhas on an ontogenetic time scale. We re-examine the post-transformational allometry of body form in one piranha, Pygocentrus nattereri (Kner), using the method of thin-plate splines decomposed by their partial warps. We find substantial evidence of allometry, primarily elongation of the mid-body relative to the more anterior and posterior regions, elongation of the postorbital and nape regions relative to the more anterior head and posterior body, and deepening of the head relative to the body. In addition to these pervasive changes throughout the body, there are some that are more localized, especially elongation of the postorbital region relative to eye diameter and snout, and an even more localized elongation of the snout relative to eye diameter. Initial dietary transitions are associated with changes in head and jaw proportions, but rates of shape change decelerate through growth, so tha the final transition to a diet increasingly dominated by small whole fish appears associated with change largely in overall body size.