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First published online November 19, 2004
Journal of Experimental Biology 207, 4393-4405 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01318

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

R. Pirow^*,, C. Bäumer and R. J. Paul

Institut für Zoophysiologie, Westfälische Wilhelms-Universität, Hindenburgplatz 55, 48143 Münster, Germany

^* Author for correspondence (e-mail: pirow{at}uni-muenster.de)

Accepted 29 September 2004

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 µmol l^–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.

Key words: Crustacea, Branchiopoda, Cladocera, Daphnia, zooplankton, oxygen transport, ventilation, circulatory system, diffusion, convection, haemoglobin, hypoxia, phosphorescence lifetime imaging

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