AbstractA newly developed apparatus for the continuous measurement of respiration and activity was used to measure the metabolic rates of the fish Pomatoschistus microps and the shrimp Palaemon adspersus under conditions of diminishing O₂ concentration.Neither the fish nor the shrimp proved to be typical O₂ conformers or O₂ regulators. The shrimp retained its standard rate down to an O₂ saturation of 36 % and the goby down to 54 % (at 20 °C), whereafter both behaved as conformers.The O₂ extraction capacity of P. adspersus is more pronounced than that of P. microps.The regulation capacity of both species increases at low temperatures (5 °C).Salinities within the ecological range appear to have no effect on the regulation capacity.