How to measure the thermal death of Daphnia? A comparison of different heat tests and effects of heat injury

Abstract 1. 1. The thermal death point of the water flea Daphnia magna (age 50 ), induced by an acute 24 h heat exposure was 34.8°C. It was 37.8°C following a thermal shock for 15 min, and it was 39.4°C when a continuous temperature increase (0.2°C/min) was used. 2. 2. Heat death temperature of daphnids was related to the acute heating rate. 3. 3. The logarithm of median lethal time (Lt 50 ) of daphnids, kept at a constant high temperature, had a linear relationship to temperature (°C) within the range of 28.0–38.5°C. 4. 4. The mortality after heat exposure increased with recovery time at 20°C for up to 3 days. 5. 5. The animals which survived the heat exposure produced eggs and offspring. Furthermore, no time lag in development between the control and heat exposure group was observed. 6. 6. The comparison of the results made by different heat tests categorized to Methods 1 and 2 by Precht (1973), for use in the determination of lethal limits of ectotherms, has been discussed.