<jats:p>Marine invertebrates with open circulatory system establish low and constant oxygen partial pressure (Po<jats:sub>2</jats:sub>) around their tissues. We hypothesized that as a first step towards maintenance of low haemolymph and tissue oxygenation, the Po<jats:sub>2</jats:sub>in molluscan mantle cavity water should be lowered against normoxic (21 kPa) seawater Po<jats:sub>2</jats:sub>, but balanced high enough to meet the energetic requirements in a given species. We recorded Po<jats:sub>2</jats:sub>in mantle cavity water of five molluscan species with different lifestyles, two pectinids ( Aequipecten opercularis , Pecten maximus ), two mud clams ( Arctica islandica , Mya arenaria ), and a limpet ( Patella vulgata ). All species maintain mantle cavity water oxygenation below normoxic Po<jats:sub>2</jats:sub>. Average mantle cavity water Po<jats:sub>2</jats:sub>correlates positively with standard metabolic rate (SMR): highest in scallops and lowest in mud clams. Scallops show typical Po<jats:sub>2</jats:sub>frequency distribution, with peaks between 3 and 10 kPa, whereas mud clams and limpets maintain mantle water Po<jats:sub>2</jats:sub>mostly &lt;5 kPa. Only A. islandica and P. vulgata display distinguishable temporal patterns in Po<jats:sub>2</jats:sub>time series. Adjustment of mantle cavity Po<jats:sub>2</jats:sub>to lower than ambient levels through controlled pumping prevents high oxygen gradients between bivalve tissues and surrounding fluid, limiting oxygen flux across the body surface. The patterns of Po<jats:sub>2</jats:sub>in mantle cavity water correspond to molluscan ecotypes.</jats:p>