We evaluated the development of metabolic capacities, immunocompetence, and growth patterns according to sex, age, and gametogenesis stages among scallops Placopecten magellanicus from the Magdalen Islands (Québec, Canada). Our 2-y study allowed us to evaluate in detail both seasonality and interannual variability of their physiological state. Enzymes— namely, phosphoglucose isomerase, phosphomannose isomerase, pyruvate kinase, aspartate aminotransferase, citrate synthase, chymotrypsin, and lipase were assessed in muscles and digestive glands of scallops. Immunocompetence was evaluated by measuring hemocyte activity in hemolymph and the efficiency of phagocytic cells. The various reproductive stages were identified with histological examination, and growth was estimated using digestive gland and gonadosomatic indices. Lipase and phagocytosis appeared to be good biomarkers of the physiological state of scallops in relation to their age and reproductive stage: Lipase activity is higher in young scallops (2 y old) as well as during the development and the ripe stages, and is associated with low phagocytic activity. Results suggest that the lipid digestion capacity may increase to improve lipid conversion during the development and ripe stages. After this period, metabolic reserves were presumably used to synthesize hemocytes at a higher rate and to rebuild gonadal tissue. The regulation of the reproductive cycle and immunocompetence was observed to depend directly on the environmental parameters without concomitant changes in the activity of the measured enzymes of the energy metabolism.