Students commonly test the effects of chemical agents on the heart rate of the crustacean Daphnia magna, but the procedure has never been optimized. We determined the effects of three concentrations of ethanol, nicotine, and caffeine and of a control solution on heart rate in Daphnia. Ethanol at 5% and 10% (v/v) reduced mean heart rate to ∼50% and ∼20% of its initial value, respectively. Recovery was rapid after removing 5% ethanol, but recovery from 10% ethanol took 20–30 minutes. Nicotine at 100 µM reversibly increased mean heart rate by ∼20%. Higher concentrations produced varied and sometimes irreversible effects. Caffeine at 0.1%, 0.5%, and 2% (w/v) had no convincing effect on heart rate. Of the three agents tested, nicotine's peculiar effects make it the least useful in an educational setting. Caffeine could be used to emphasize the need for blind observers because it does not increase heart rate in Daphnia. If students find that it does, their bias is revealed. Ethanol produces unambiguous effects at 5% and 10%. Heart rates recover quickly after removing 5% ethanol, which allows students to explore reversibility as an alternative to having a separate control group.
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