The plankton-filtering Daphnia has an eye of only 22 ommatidia, while the carnivorous Evadne and Leptodora have 80 and 300, respectively.
A few Daphnia prey on tiny crustaceans and rotifers, but most are filter feeders, ingesting mainly unicellular algae and various sorts of organic detritus including protists and bacteria.
Daphnia are crustaceans (subphylum Crustacea), a group of arthropods that includes the familiar crabs, lobsters, shrimps, barnacles, copepods, krill, and crayfish.
The lifespan of a Daphnia does not exceed one year and is largely temperature dependent.
Daphnia or daphnids are commonly called water fleas because of their jerky or jumping swimming style (although fleas are insects and thus these two groups are only very distantly related as being arthropods).
The order to which daphnia belong, Cladocera, is comprised of free-swimming organisms, and most orientate themselves with dorsal side up.
Ecologically, daphnia are important components of freshwater food chains, consuming algae and other small organisms, and in turn being prey for small fish and other carnivorous aquatic animals.
Most species of crustaceans are marine, but there are many freshwater forms, such as crayfish and Daphnia, and some terrestrial forms.
Sometimes Daphnia may be used in certain environments to test the effects of toxins on an ecosystem.
The newly hatched Daphnia must molt several times before they are fully grown into an adult usually after about two weeks.
Under favorable environmental conditions, Daphnia reproduce parthenogenetically, usually in the spring until the end of the summer.
The following daphnids are listed as vulnerable by IUCN: Daphnia nivalis, Daphnia coronata, Daphnia occidentalis, and Daphnia jollyi.