Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although fleas are insects and thus only very distantly related). They live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.

The two most readily available species of daphnia are Daphnia pulex (small size & most common), and Daphnia magna (large). They are often associated with a related species from the order Cladocera known as Moina (In the Moinidae family instead of Daphniidae and much smaller than Daphnia pulex (approximately half the maximum length)). Daphnia eggs for sale are generally enclosed in ephippia (a thick shell, consisting of two chitinous plates, that encloses and protects the winter eggs of a cladoceran.)