water flea

any of various small crustaceans that move about in the water like fleas, as those of the genus Daphnia.

[1575-85]

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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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▪ crustacean

 any member of the crustacean order Anomopoda (class Branchiopoda), a large group containing about 450 species distributed worldwide. Most forms are found in freshwater habitats, but a few occur in marine environments. The best known genus is Daphnia, ubiquitous in ponds and streams in Europe and North America. The water flea is microscopic in size, typically measuring only about 0.2 to 3.0 millimetres (0.01 to 0.12 inch) long. It has a discrete head bearing antennae and a bivalve carapace that encloses all or most of the trunk and abdomen. An exception is the predatory giant Leptodora, which grows as long as 18 mm and whose carapace is reduced to a small brood sac. Most species swim by means of powerful strokes of the antennae; in some species the successive strokes produce a characteristic hopping and sinking motion. Apart from a few predatory forms, water fleas feed on microscopic particles of organic matter, which they filter from the water with specialized thoracic limbs. They in turn are eaten by fish. Certain water fleas provide, for example, the basic food of nearly all commercial fishes of the Great Lakes of North America.

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