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 2time 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.