Water flea of the genus Daphnia (magnified about 30)—Eric V. Grave/Photo Researchers
Any of about 450 species (order Anomopoda) of microscopic, mostly freshwater crustaceans distributed worldwide. Species in the genus Daphnia are ubiquitous in Europe and North America. Water fleas have a discrete head that bears antennae. The carapace (shell) encloses all or most of the body, except on the predatory giant Leptodora (up to 0.7 in. [18 mm] long), whose carapace is just a small brood sac. Most species swim by powerful strokes of the antennae, sometimes producing a hopping-and-sinking motion. All but a few predatory species use specialized thoracic limbs to filter organic matter from the water. See alsocopepod.