Acclimation of the microcrustacean Daphnia magna to warm temperatures is dependent on haemoglobin expression

The effects, mechanisms and dynamics of acclimation to warm temperature (30 degrees C) were studied in the microcrustacean Daphnia magna over three successive generations. A series of different morphological and physiological variables of 20 or 30 degrees C acclimated D. magna were investigated at 20 or 30 degrees C ambient temperature under the additional burden of oxygen limitation (hypoxia). Short-term exposure of 20 degrees C acclimated animals to 30 degrees C ambient temperature led to a mismatch of oxygen transport and energy demand under hypoxic conditions indicated by an upward shift of all critical P-O2 values (P-c,P-O, P-c,P-N, P-c,P-A, P-c,P-H) by a factor of 1.3-1.5. This decrease of hypoxia tolerance was cancelled by a few days of acclimation to 30 degrees C, which was virtually brought about by a strong increase of haemoglobin (Hb) concentration. Other variables such as energy demand or convective processes (ventilation, perfusion) remained almost unchanged. These results confirm the oxygen-limited tolerance hypothesis for the process of warm acclimation and add to it changes of Hb expression as essential molecular mechanism of thermal acclimation in D. magna. (c) 2005 Elsevier Ltd. All rights reserved.