The metabolic activities of different developmental stages of the polychaete Marenzelleria viridis (Verrill 1873), which has successfully populated Baltic coastal waters since the 1980s, were determined under different temperatures, salinities and oxygen partial pressures by simultaneous calorimetry and respirometry. Larvae of M. viridis maintained their metabolic activity and met this fully aerobically down to severe hypoxia. Adult individuals gradually reduced their metabolic activity with decreasing oxygen partial pressures. An additional hyposmotic stress of 0.5 parts per thousand, salinity, referring to low salinities in the biotope, led to an approach of the response of M. viridis to the oxyconformer model. The results for adults were compared to those of the common indigenous polychaete Nereis (Hediste) diversicolor (O.F. Muller), which responded with maintained metabolic activities to hypoxia. The ecological efficiency of the different adaptation strategies to hypoxia are discussed.