Crater landscape:: two-dimensional oxygen gradients in the circulatory system of the microcrustacean Daphnia magna

Oxygen transport processes in millimetre-sized animals can be very complex, because oxygen molecules do not exclusively follow the pathway predetermined by the circulating fluid but may also simultaneously move from the respiratory surfaces to the tissues along different paths by diffusion. The present study made use of the oxygen-sensitive phosphorescence probe Oxyphor R2 to analyze the internal oxygen pathway in the transparent microcrustacean Daphnia magna. Oxyphor R2 was injected into the circulatory system and the distribution of oxygen partial pressure (P-O2) in the haemolymph was measured by phosphorescence lifetime imaging in the P-O2 range 0-6 kPa (0-30% air saturation). There were substantial differences in the shape of the two-dimensional P-O2 profiles depending on the concentration of haemoglobin (Hb) in the haemolymph. A steep global gradient, from posterior to anterior, occurred in animals with low concentrations of Hb (90-167 mumol 1(-1) haem). In contrast, animals with a five- to sixfold higher concentration of Hb showed flat internal P-O2 gradients which, however, were only present under reduced ambient oxygen tensions P-O2amb=3-1 kPa), when Hb was maximally involved in oxygen transport. Under these conditions. the presence of Hb at high concentrations stabilized the unloading P-O2 in the central body to 0.9-0.4 kPa. Independent of Hb concentration and body size, the loading P-O2 was always 0.5 kPa below the P-O2amb. From these P-O2 profiles, it was possible (i) to follow the track of oxygen within the animal, and (ii) to visualize the shift from a diffusion-dominated to a convection-dominated transport as a result of increased Hb concentration.