Mantle cavity water oxygen partial pressure (Po2) in marine molluscs aligns with lifestyle

Abstract

Marine invertebrates with open circulatory system establish low and constant oxygen partial pressure (Po2) around their tissues. We hypothesized that as a first step towards maintenance of low haemolymph and tissue oxygenation, the Po2in molluscan mantle cavity water should be lowered against normoxic (21 kPa) seawater Po2, but balanced high enough to meet the energetic requirements in a given species. We recorded Po2in 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 Po2. Average mantle cavity water Po2correlates positively with standard metabolic rate (SMR): highest in scallops and lowest in mud clams. Scallops show typical Po2frequency distribution, with peaks between 3 and 10 kPa, whereas mud clams and limpets maintain mantle water Po2mostly <5 kPa. Only A. islandica and P. vulgata display distinguishable temporal patterns in Po2time series. Adjustment of mantle cavity Po2to 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 Po2in mantle cavity water correspond to molluscan ecotypes.