Physiological and environmental conditions affect carbohydrate, specifically glucose levels in the crustacean hemolymph. Trehalose, naturally occurring nonreducing disaccharide (a1–1a of two glucose), shows a protective role in the organisms experiencing stressful conditions such as cold, salinity, and anhydrobiosis. As the predominant sugar in the blue crab hemolymph Callinectes sapidus, the trehalose synthesis gene (trehalose-6-phosphate synthase, TPS) is expressed in most internal tissues, including the hepatopancreas. The present study shows the function of TPS in carbohydrate metabolism in decapod crustaceans. The blue crabs show ontogeny variations in carbohydrate metabolism, especially hemolymph glucose and trehalose. Trehalose increases with the animals becoming large, whereas glucose is higher in adult animals than juveniles. Food intake elevates glucose and trehalose both in the hemolymph and hepatopancreas. The significance of hepatopancreas TPS is defined using a double-strand RNA of TPS injection study of the animals after feeding and emersion. The dsRNA-TPS injection results in 60%–70% of reduced TPS transcripts and its enzyme activity in the hepatopancreas. In the animals that received dsRNA-TPS, feeding has significantly reduced levels of changes in glucose and trehalose in the hemolymph and hepatopancreas, compared with the control animals (saline and double-stranded green fluorescent protein injection groups). In response to emersion, the animals injected with dsRNA-TPS show the glucose increases in the hemolymph the same as in controls but with trehalose and lactate significantly lower than the controls. The hepatopancreas of dsRNA-TPS–injected animals contains significantly reduced trehalose and glycogen, compared with control animals. Carbohydrate metabolism in crustaceans appears to be rather complicated with trehalose and glycogen; both can serve the glucose source for hyperglycemia and need further investigations.