They are often associated with a related genus in the order Cladocera: Moina, which is in the Moinidae group instead of the Daphniidae, and is much smaller than D. pulex (roughly half the maximum length).

Daphnia spp., like many animals, are prone to alcohol intoxication, and make excellent subjects for studying the effects of the depressant on the nervous system due to the translucent exoskeleton and the visibly altered heart rate.

The understanding of species boundaries has been hindered by phenotypic plasticity, hybridization, intercontinental introductions, and poor taxonomic descriptions.

[10] Daphnia species are normally r-selected, meaning that they invest in early reproduction, so have short lifespans.

An individual Daphnia lifespan depends on factors such as temperature and the abundance of predators, but can be 13–14 months in some cold, oligotrophic, fish-free lakes.

are typically filter feeders, ingesting mainly unicellular algae and various sorts of organic detritus including protists and bacteria[2][12] Beating of the legs produces a constant current through the carapace, which brings such material into the digestive tract.

are known to show behavioral changes or modifications to their morphology in the presence of predator kairomones (chemical signals), including larger size at hatching, increased bulkiness, and the development of "neck-teeth".

For example, juveniles of D. pulex and D. magna have a larger size after hatching, along with developing neck-teeth at the back of the head, when in the presence of Chaoborus kairomones.

Chitin-related genes (deacetylases) are thought to play an important part in the expression/development of these morphological defenses in Daphnia.

[15] Most Daphnia species have a lifecycle based on "cyclical parthenogenesis", alternating between parthenogenetic (asexual) and sexual reproduction.

They are then released into the water, and pass through a further four to six instars over 5–10 days (longer in poor conditions) before reaching an age where they are able to reproduce.

In either case, the resting eggs are protected by a hardened coat (consisting of two chitinous plates) called the ephippium, and are cast off at the female's next molt.

The ephippia can withstand periods of extreme cold, drought, or poor food availability, and hatch – when conditions improve – into females (They are close to being classed as extremophiles) .

[17] They may be used in certain environments to test the effects of toxins on an ecosystem, which makes them an indicator genus, particularly useful because of their short lifespans and reproductive capabilities.

[18] Daphnia is also commonly used for experiments to test climate change aspects, as UVB that seriously damages zooplankton species (e.g. decrease feeding activity[19]).

are an important base of the food chain in freshwater lakes (and vernal pools), they become a nuisance when they are unable to be eaten by native macroscopic predators, and some concern exists that the original spineless and hookless water fleas and spp.