Many water-breathing animals have a strategy that consists of maintaining low blood PO₂ values in a large range of water oxygenation level (4-40 kPa). This study examines the postprandial changes in O₂ consumption, arterial blood PO₂, and tissue protein synthesis in the shore crab Carcinus maenas in normoxic, O₂-depleted, and O₂-enriched waters to study the effects of this strategy on the O₂ consumption and peptide bond formation after feeding. In normoxic water (21 kPa), the arterial PO₂ was 1.1 kPa before feeding and 1.2 kPa 24 h later. In water with a PO₂ of 3 kPa (arterial PO₂ 0.6 kPa), postprandial stimulation of protein synthesis and O₂ consumption were blocked. The blockade was partial at a water PO₂ of 4 kPa (arterial PO₂ 0.8 kPa). An increase in environmental PO₂ (60 kPa, arterial PO₂ 10 kPa) resulted in an increase in protein synthesis compared with normoxic rates. It is concluded that the arterial PO₂ spontaneously set in normoxic Carcinus limits the rates of protein synthesis. The rationale for such a strategy is discussed.