Daphnia, popularly known as water fleas, are small crustaceans that live in fresh water such as ponds, lakes, and streams. They serve as an important source of food for fish and other aquatic organisms. Daphnia are excellent organisms to use in bioassays because they are sensitive to changes in water chemistry and are simple and inexpensive to raise in an aquarium. They mature in just a few days, so it does not take long to grow a culture of test organisms.
Because Daphnia are transparent, it is possible to conduct bioassays using endpoints other than death. For example, through a microscope you can measure their heart rate or observe whether they have been eating. (Both of these signs are used to measure stress). If you are worried about killing Daphnia in your experiments, you could choose to measure one of these other endpoints instead. It is worth keeping in mind, though, that even under the best conditions these organisms live only a month or two, and in nature most of them get eaten within their first few days or weeks of life.