Water flea head. Darkfield illuminated polarised light micrograph of the head of a Sida crystallina water flea, showing one of its eyes (round, top). Water fleas (order Cladocera) 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. Magnification: x200 when printed 10 centimetres wide.