Daphnia
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<b>Daphnia</b> is a genus of small, planktonic crustaceans, 1–5 millimetres (0.04–0.20 in) in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resembles the movements of fleas. Daphnia live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.
 
Sometimes Daphnia may be used in certain environments to test the effects of toxins on an ecosystem, which makes them an indicator genus, particularly useful because of its short lifespan and reproductive capabilities. Because they are nearly transparent, their internal organs are easy to study in live specimens (an example might be to study the effect of temperature on the heart rate of these ectothermic organisms). Another reason they are often used is because of their thin membrane. This allows for drugs to be added to the daphnia. They are often fed to tadpoles or small species of amphibians such as the African dwarf frog (Hymenochirus boettgeri). Daphnia spp. are also a popular live food in tropical and marine fish keeping. They are also used to monitor affects of certain drugs, such as adrenaline or capsaicin, on the heart.</p>