The Collaborative International Dictionary of English v.0.48:

Daphnia \Daph"ni*a\, n. [NL.] (Zool.)
   A genus of the genus Daphnia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

daphnia
    n 1: 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, water flea]