Water flea and cyanobacteria. Polarised light micrograph of a water flea (order Cladocera, centre) amongst filamentous cyanobacteria colonies (long, thin). 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. Cyanobacteria are bacteria that obtain their energy through photosynthesis. Magnification: x200 when printed 10 centimetres wide.