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 also copepod.
Water flea of the genus Daphnia (magnified about 30)—Eric V. Grave/Photo Researchers
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