Water flea. Macro photograph of the underside of a water flea (Daphnia sp.), showing the two lobes of its body (lower right), and its feathery antennae (centre and upper right). Water fleas are small crustaceans, commonly found in fresh water. They are filter feeders that ingest algae, protozoa or organic matter, and are a constituent of plankton. Daphnia reproduce through parthenogenesis (female asexual reproduction).