The aim of this study was to test the effect of food quantity on energy storage and defense capacities of oysters during a reproductive cycle. One-year-old Crassostrea gigas oysters were fed two different dietary rations (4% and 12% of oyster dry weight in algal dry weight per day) in controlled experimental conditions over an annual cycle. Oyster dry weights, carbohydrate and lipid contents, energetic adenylate charge, and hemocyte parameters of oysters were significantly affected by reproductive processes related to seasonal temperature variation and, to a lesser extent, by the dietary rations. Energy parameters decreased during gametogenesis as gonads developed then increased during the gonad resorption phase. The additional energy provided to oysters fed the 12% diet compared to oysters fed the 4% diet was allocated mainly to the development of more gonad tissue. Regardless of diet, hemocyte concentrations were also seasonally affected. Hemocyte concentrations were low during gametogenesis and significantly increased during the gonadal resorption and tissue restoration phase. Phagocytic activity and adhesive capacity of hemocytes were temporarily inhibited during gametogenesis and were at their lowest levels in June. Oysters fed the 12% diet had significantly higher hemocyte concentrations and lower phagocytosis activity and reactive oxygen species production compared to those fed the 4% diet.