Daphnien

Daphnien, known in English as Daphnia, are small, transparent freshwater crustaceans that form a major component of the zooplankton in lakes, ponds, and streams worldwide. Commonly called water fleas, they range in size from about 0.2 to 5 millimeters and can be observed with a hand lens.

They belong to the genus Daphnia within the family Daphniidae, order Cladocera, class Branchiopoda. Members possess a bivalved carapace that encloses a simple body plan with antennae and limb pairs used for feeding and swimming. Females carry eggs in a brood chamber; under favorable conditions, many species reproduce by cyclical parthenogenesis, producing offspring asexually. When environmental conditions deteriorate, they can reproduce sexually, producing resting eggs inside protective ephippia that can settle into sediments and hatch later when conditions improve.

Ecology: Daphnien are filter feeders that consume microalgae, bacteria, and detritus, helping to control algal populations and transferring energy to higher trophic levels. They are a key prey item for many fish and invertebrates. Because their growth, reproduction, and survival respond quickly to pollutants, Daphnia species are widely used as model organisms in ecotoxicology and environmental monitoring. The most commonly studied species in laboratories is Daphnia magna, though many other species exist, including Daphnia pulex and Daphnia longispina.