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First published online January 12, 2004
Journal of Experimental Biology 207, 683-696 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00812
The dichotomous oxyregulatory behaviour of the planktonic crustacean Daphnia magna
Institut für Zoophysiologie, Westfälische Wilhelms-Universität, Hindenburgplatz 55, 48143 Münster, Germany
* Author for correspondence (e-mail: pirow{at}uni-muenster.de)
Accepted 24 November 2003
The dual function of appendage movement (food acquisition, ventilation) proved to be the key to explaining the peculiar oxyregulatory repertoire of the planktonic filter feeder Daphnia magna. Short-term hypoxic exposure experiments with normoxia-acclimated animals under varying food concentrations revealed a dichotomous response pattern with a compensatory tachycardia under food-free conditions and a ventilatory compensation prevailing under food-rich conditions. Food-free, normoxic conditions resulted in maximum appendage beating rates (fA) and half-maximum heart rates (fH), which restricted the scope for oxyregulation to the circulatory system. Food-rich conditions (105 algal cells ml-1), on the contrary, had a depressing effect on fA whereas fH increased to 83% of the maximum. In this physiological state, D. magna was able to respond to progressive hypoxia with a compensatory increase in ventilation. A conceptual and mathematical model was developed to analyse the efficiency of ventilatory and circulatory adjustments in improving oxygen transport to tissue. Model predictions showed that an increase in perfusion rate was most effective under both food-free and food-rich conditions in reducing the critical ambient oxygen tension (PO2crit) at which oxygen supply to the tissue started to become impeded. By contrast, a hypothetical increase in ventilation rate had almost no effect on PO2crit under food-free conditions, indicating that appendage movement is driven by nutritive rather than respiratory requirements. However, the model predicted a moderate reduction of PO2crit by hyperventilation under food-rich conditions. Since the regulatory scope for an adjustment in fH was found to be limited in D. magna under these conditions, the increase in ventilation rate is the means of choice for a fed animal to cope with short-term, moderate reductions in ambient oxygen availability. Under long-term and more severe hypoxic conditions, however, the increase in the concentration and oxygen affinity of haemoglobin represents the one and only measure for improving the transport of oxygen from environment to cells.
Key words: Crustacea, Branchiopoda, Cladocera, Daphnia, zooplankton, nutrition, oxygen transport, ventilatory and circulatory system, diffusion, convection, mathematical modelling
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