Water flea is a generic term for a number of small aquatic crustacea characterise by their jumping or jerky mode of swimming. Most are between 0.1 mm and 3 mm in length. Most commonly, they will be species of Daphnia but a number of other genera are also commonly included, including Cyclops, and Diaptomus. Water fleas are commonly sold as aquarium food since they are easy to culture and reproduce rapidly. They are capable of existing in salt or fresh water, though different species inhabit each region. There are over 100 species of water flea.