Science Buddies: "Ask an Expert"

Hi,
I am doing a project that involves daphnia magna and giving them different substances (atropine,melatonin and epinephrine) and observing the effect on their heart rate. I already have the concentrations determined but I am trying to figure out how much time I should leave the daphnia in the solution before measuring the heart rate. Thank you in advance!

Hi,
There's really no one answer for how long to expose the Daphnia. In general, if you use a high concentration of test substance you would use a shorter exposure and vice versa. But, of course, then you must decide what is a 'high' concentration, and how can you find out that? Probably, the best thing is to do a preliminary experiment in which you place a number of Daphnia in a solution of one of your test compounds and measure heart rate at time zero and then at 5 or 10 minute intervals. If the heart rate changes drastically or the Daphnia die, then you know the concentration was too high. If you see no affect where you had expected to see an effect, then maybe you need to increase the concentration.

If I remember correctly, caffeine raises their heart rate, so you could use that as a positive control.

Let me know if you have more questions.

Sybee