Shell morphology changes in the scallop Aequipecten tehuelchus during its life span: A geometric morphometric approach

Abstract

Shell morphology is a central feature of bivalve biology in fields such as taxonomy, evolution, and functional anatomy. When allometric shell growth occurs, traditional morphometric methods usually fail to provide robust, size-free shape variables. We used a more integrative approach, geometric morphometrics, to examine ontogenetic changes in the shell of the scallop Aequipecten tehuelchus. A single cohort that settled early in 2004 at a site in San José Gulf in northern Patagonia, Argentina, was sampled at irregular intervals over 5 yr. Different developmental stages had significant differences in shell shape. There was significant ontogenetic allometry, mainly reflected in the shape of the shell disc and the symmetry of the auricles. The most noticeable morphological changes in shell shape and size took place within the first 2 yr of life. Three different shell ecophenotypes were discriminated: spat, juveniles, and adults. Spat had a relatively large anterior auricle and a circular disc; auricles in juveniles were more symmetrical and the shell disc more elongated; and during the adult stage the auricles were small and asymmetrical and the disc elliptical. These 3 phenotypes may reflect changes in the scallop's life habits, as individuals develop from attached spat to actively swimming juveniles to more sedentary adults.