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To examine physiological adaptations to the two combined stressors O 2 deprivation and extreme CO 2 concentrations, we compared respiratory responses of two nematode species occurring in natural CO 2 springs. The minimum O 2 concentration allowing maintenance of respiration in both species was 0.0176 μmol O 2 ml −1 (corresponds to 1.4% O 2 in air). After exposure to anoxia, individuals resumed respiration immediately when O 2 was added, but on a lower level compared to control and without showing a respiratory overshoot. A species-specific response was found in respiration rate during 20% CO 2 : the more tolerant species maintained respiration rates, whereas the sensitive species showed a decreased respiration rate as low as after anoxia. The results indicate that during 20% CO 2 the sensitive species undergo a survival state. We conclude, that the ability to maintain respiration even under low oxygen and high CO 2 concentrations may allow the better adapted species to occupy an ecological niche in the field, where others cannot exist. [ABSTRACT FROM AUTHOR]