Darkfield illuminated polarised light micrograph of a Daphnia sp. water flea with eggs (round) and an ostracod (orange). Water fleas are small crustaceans, commonly found in freshwater. They are filter feeders that ingest algae, protozoa or organic matter, and are a constituent of plankton. Ostracods are aquatic crustaceans. Magnification: x93 when printed 10 centimetres wide.