Wa·ter flea Zool. Any one of numerous species of small aquatic Entomostraca belonging to the genera Cyclops, Daphnia, etc; -- so called because they swim with sudden leaps, or starts.
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  water flea
       n 1: minute free-swimming freshwater copepod having a large
            median eye and pear-shaped body and long antennae used
            in swimming; important in some food chains and as
            intermediate hosts of parasitic worms that affect man
            e.g. Guinea worms [syn: cyclops]
       2: minute freshwater crustacean having a round body enclosed in
          a transparent shell; moves about like a flea by means of
          hairy branched antennae [syn: daphnia]