Water flea : 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.

Example:


Taxonomy
Water flea, free encyclopedia, different species, search, generic term, characterised, jerky, characteristic, body, shell, 3 mm, brackish waters, marine environments, grazers, detritus feeders, Unsourced, crustacean, Personal tools.

Defination from the web:
Water flea is a generic term for a number of small aquatic crustacea characterised by their jumping or jerky mode of swimming. ... There are probably many hundreds of different species covered by the term "water flea" ...