water flea

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various small chiefly freshwater branchiopod crustaceans of the order Cladocera, having a bivalve carapace and typically swimming with jerking flealike motions, and including the daphnias.
• noun Any of various other small aquatic crustaceans, such as some copepods, that swim with a similar motion.

from The Century Dictionary.

• noun One of numerous small or minute crustaceans which skip about in the water like fleas, as Daphnia pulex; any branchiopod. See Daphniidæ, Cladocera, Cyclops.

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of various aquatic crustacea of the order Cladocera, characterised by their jumping or jerky mode of swimming.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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
• noun minute freshwater crustacean having a round body enclosed in a transparent shell; moves about like a flea by means of hairy branched antennae