|MadSci Network: General Biology|
Hi Katherine, Daphnia absorb any chemicals in the water surrounding them straight into their bodies. This means that any chemical with a physiological effect affects the Daphnia extremely quickly and for this reason, Daphnia are actually used as an indicator to find out how toxic a liquid is by measuring its effect on the organism's heart rate. Some chemicals (such as caffeine) increase Daphnia heart rate. Others such as oil or ethanol (ethyl alcohol) show their toxicity by a general depressive effect on nerve transmissions which causes a slowing of the heart rate. This occurs through action on enzymes which control heart rate by either slowing or raising it, and these enzymes may be activated or suppressed by different chemicals. The precise details would be complex as oils often contain many chemicals with different specific effects, but I hope this explains the basic ideas. Yours, Dr David Hubble, UK
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