This study reports the basic nutritional requirement of subtropical Modiolus capax broodstock conditioned under nine diet—temperature treatments. The value of carbohydrate-rich diets was not only assessed to ensure maturation outside of the main breeding season but also to understand the interaction between diet—temperature over conditioning periods larger than the 30 days usually recommended for tropical bivalves. The experimental design included a natural diet prepared with a blend of the microalgae Tisochrysis lutea and Chaetoceros calcitrans (Micro100) and two carbohydrate-enriched diets composed of Micro100 wheat flour (MicroW) at 7% total dry weight of body tissues and Micro100 cornstarch (MicroC) at 7% total dry weight of body tissues. Maximum gonad development occurred in mussels fed MicroC at higher temperatures (26°C) where the highest frequency of partial spawning accompanied the highest lipid content in gonad and lower carbohydrate content in somatic tissues. This result reflects an early maturation compared with brood stock fed the natural Micro100 diet at 24°C. Lower temperatures (22°C) delayed maturation in all treatments and favored accumulation of carbohydrate reserves in somatic tissues with the MicroC diet. Natural diets supplemented with carbohydrate-rich sources by cereal flours are reliable alternatives to fuel and sustain gametogenesis in M. capax broodstock at high temperatures and to increase carbohydrate reserves in somatic tissues at low temperatures.