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 °C) were studied in the microcrustacean Daphnia magna over three successive generations. A series of different morphological and physiological variables of 20 or 30 °C acclimated D. magna were investigated at 20 or 30 °C ambient temperature under the additional burden of oxygen limitation (hypoxia). Short-term exposure of 20 °C acclimated animals to 30 °C ambient temperature led to a mismatch of oxygen transport and energy demand under hypoxic conditions indicated by an upward shift of all critical PO2 values (Pc,O, Pc,N, Pc,A, Pc,H) by a factor of 1.3-1.5. This decrease of hypoxia tolerance was cancelled by a few days of acclimation to 30 °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.