Systemic and metabolic responses in Daphnia magna to anoxia

Abstract

The anoxic tolerance of the water flea Daphnia magna proved to be well-developed. During the first 1 or 2 h of anoxia, perfusion rate was not essentially impeded: admittedly, there was a 30–40% reduction of the amplitude of heart contractions, but the heart beat at a rate either similar to normoxia (150 min −1 at T=15°C) or markedly higher (in more than half of the experiments). During that period, energy was provided by a high rate of anaerobic metabolism: l-lactate partly accumulated in the animals (0.36 μmol lactate g −1 FW min −1), partly it was excreted into the medium (0.08 μmol lactate g −1 FW min −1). The anaerobiosis was accompanied by a decrease of intracellular as well as extracellular pH (metabolic acidosis). A metabolic depression, starting after about 1 or 2 h of anoxia, helped to prolong survival time during long-term anoxia (up to 24 h): in many experiments, heart and perfusion rate sharply declined to a much lower level (well below the normoxic rate), which was maintained during the next hours of anoxia. The accumulation of lactate stopped, while lactate excretion into the medium continued at an unchanged rate. If the anoxic period exceeded 20 min, long-term recovery processes, lasting up to 4 h, were found during normoxia: the heart rate increased and reached a maximum frequently after 40–50 min of recovery. There was also a restoration of pre-anoxic intra- and extracellular pH values. Additional experiments on the effects of altered ambient P CO 2 /pH on heart rate and blood pH revealed pH to be a possible control factor of heart rate.