Increasing temperature should increase heart rate.

Daphnia Heart Rate

Daphnia are ectotherms whose internal temperature varies as that of the environment does. They are invertebrates and members of the crustacea. They are common pond organisms and they filter feed on plankton.

The heart of a Daphnia can be seen through the transparent carapace, the name given to the main part of its exoskeleton.

Since the heart of Daphnia is easily visible they are often used in studies on the effects of changing environmental conditions. The heart rate can be counted if the flea is mounted on a cavity and observed at low power, say between 40x and 100x.