|MadSci Network: General Biology|
Daphnia are cold-blooded animals, and you are correct, they do not thermoregulate. This means that their body temperature is the same as the water they are floating in. The chemical reactions that occur in the cells of Daphnia are dependent on certain enzymes, or proteins, to help the reactions proceed. As you increased the temperature of the water, the metabolism of the Daphnia increased as well, because chemical reactions occur faster at higher temperatures. This means that the heart rate will speed up in order to provide oxygen to the cells as the metabolism increases. However at 40 degrees Celsius, the enzymes break down, and the chemical reactions can no longer occur, so metabolism stops and the Daphnia dies. In its natural environment, Daphnia does not ever experience temperatures of 40 degrees, so this is unlikely to occur. However, they do experience changes in temperature in their natural environment, and their metabolism does increase or decrease as the temperature changes.
I hope this answers your question,
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