Water flea. Polarised light micrograph of a Daphnia sp. water flea (order Cladocera) with immature young (dark) in its brood pouch inside its body. 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. Typically the young hatch and remain in the brood pouch for around three days before being released into the water. Magnification: x100 when printed 10 centimetres wide.