Daphnia, or Water Flea, a freshwater shellfish that swims with jerky motions. It is a food for larger fishes. The daphnia is less than one-fourth of an inch (6 mm) long. Its internal organs are visible through its transparent body, making it useful to scientists in laboratory experiments. Because the daphnia reproduces rapidly, it is also often used in the study of heredity.

Daphnia is a genus of the order Clodocera, belonging to the class Crustacea.