<jats:title>Summary</jats:title> <jats:p>Respiration rate of meiofauna is difficult to measure, and the response to variations in the environmental oxygen concentrations has so far been mainly addressed through behavioral investigation. We investigated the effect of different oxygen concentrations on the physiology of the marine platyhelminth Macrostomum lignano. Respiration was measured using batches of 20 animals in a glass microtiter plate equipped with optical oxygen sensor spots. At higher oxygen saturations (&amp;gt;12kPa), animals showed a clear oxyconforming behavior. However, below this values, the flatworms kept respiration rates constant at 0.064±0.001 nmol O2 l-1 h-1 ind-1 down to 3 kPa PO2, and this rate was increased in 30% in animals that were reoxygenated after enduring a period of 1.5h in anoxia. Physiological changes related to tissue oxygenation were assessed using live imaging techniques with different fluorophores in animals maintained in normoxic (21 kPa), hyperoxic (40 kPa), near anoxic (≈0 kPa) conditions and subjected to anoxia-reoxygenation. Ageladine-A and BCECF both indicated that pHi under near anoxia increases by about 0.07 to 0.10 units. Mitochondrial membrane potential, Δψm, was higher in anoxic and hyperoxic compared to normoxic conditions (JC1). Staining with ROS sensitive dyes, DHE for detection of superoxide anion (O2•-) formation and C-H2DFFDA for other ROS species aside from O2•- (H2O2, HOO• and ONOO-), both showed increased ROS formation following anoxia reoxygenation treatment. Animals exposed to hyperoxic, normoxic and anoxic treatments displayed no significant differences in O2•- formation, whereas mitochondrial ROS formation as detected by C-H2DFFDA was higher after hyperoxic exposure and lowest under near anoxia compared to the normoxic control group. M. lignano seems to be a species tolerant to a wide range of oxygen concentrations (being able to maintain aerobic metabolism from extremely low PO2 and up to hyperoxic conditions) which is an essential prerequisite for successfully dealing with the drastic environmental oxygen variations that occur within intertidal sediments.</jats:p>