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.

The southern king crab Lithodes santolla experiences low temperatures and unpredictable food conditions, and is under variable fishing pressure. To compare the reproductive investment of females, crabs were captured from three sites (Beagle Channel 55°S, Tierra del Fuego Atlantic coast 52-54°S and San Jorge Gulf 45-47°S). Fecundity, water and lipid contents were measured in midgut gland, ovary and muscle. To assess the maternal investment in the offspring, egg masses were analysed on lipid, protein, water and inorganic matter contents, egg volume and embryo size. Although fecundity was similar among sites, San Jorge Gulf females showed higher water and lower lipid contents in somatic and reproductive organs. The egg mass energy reserves varied among sites, showing the highest lipid values in the Beagle Channel, and the highest protein and inorganic matter content in San Jorge Gulf. Though water content was similar, egg volume and embryo size were higher in San Jorge Gulf. Moreover, maternal size did not correlate with egg volume, embryo size or lipid, protein and inorganic matter content of the eggs. This study suggests that reproductive investment of L. santolla varies among regions exposed to different environmental conditions, such as the quality of food available, denoting a female compensatory mechanism.