Water Flea, tiny crustacean that swims with jerky movements. The "fleas," found in almost any drop of pond water, use powerful strokes of their antennae for propulsion. These branchiopods belong to the suborder Cladocera, a cosmopolitan, freshwater group of about 400 species. The water flea feeds on phytoplankton and detritus, filtered through bristles on its appendages. Sexes are separate; the female broods eggs in a dorsal chamber. Population sizes are highly seasonal and adverse times are passed in dormant stages. Numerous aspects of cladoceran biology are studied in Canada: there is high commercial potential in their cultivation for fish food.