Cold water can slow the heart rate of daphnia, according to "Practical Biology." As the water grows colder, the daphnia will begin to conserve energy in keeping warm by shutting off circulation to its extremities, making sure that blood is pumping to its vital organs before reaching less-vital appendages.
Adrenaline increases the daphnia's heart rate. The hormone circulates through the daphnia's blood and activates cell surface receptors in what is commonly called a pacemaker, or the sinoatrial node, which increases the rate of blood the heart ejects per minute. Noradrenaline, another hormone, and caffeine, a chemical, have the same effect.
Ethanol slows heart rate of a daphnia. Ethanol depresses the nervous system, making it move much more slowly, thus causing the muscles of the heart, which respond to stimuli from the nervous system, to move slowly, as well. Acetylcholine has a similar effect and slows the heart rate, as well.