Daphnia pulex is the most common species of the water flea, an organism which can be found in almost every permanent, eutrophic (nutrient-rich) water body. A few species are marine, but generally Daphnia, including Daphnia pulex, are freshwater organisms.

Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Native ); australian (Native )

Daphnia pulex is the most common species of the group of organisms known as water fleas. Their common name was given because of their general appearance and jerky swimming motions which resembles that of the land flea. They are, in reality, a type of small crustacean and are generally 0.2-3.0 mm long. Their bodies are not distinctly segmented, but an important feature of their anatomy is the carapace, a folded shell-like structure which covers the animal and opens both ventrally and posteriorly. Studying the anatomy of this organism is made easier by the fact that most of its outer covering is clear, showing most of the internal organs at work, including the heart. The head of the organism contains both a darkly colored compound eye and numerous antennae used for feeling and swimming. Many Daphnia, including D. pulex and D. magna have a specialized light-sensing organ similiar to a tiny eye called an ocellus. Located posteriorly at the junction of the head are small, hard to see mouthparts. They mainly consist of the mandibles which are in constant motion and used by the organism to crush and grind its food. In a live specimen food particles can be seen passing through the intestine which terminates at the anus located on the postabdomen. The postabdomen is the most posterior part of the body and terminates itself in two hooklike cuticular claws used by the organism to clear debris out of the carapace. The fine teeth located on these claws are often used for species identification. The central portion of the body is the thorax and contains four to six pairs of flattened legs covered in setae. Daphnia males are generally smaller than females but have longer antennules and a modified postabdomen. Daphnia females posses a brood chamber located between the body wall and dorsal surface of the carapace used to carry their eggs.

Daphnia can be found in almost any permanent body of water, even in rain-filled tire ruts or several meters from the ground, growing in tree moss in a rainforest. They are mainly freshwater and the highest concentrations of Daphnia populations are found in the vegetation in most lakes and ponds. They are often the most abundant organism in a body of water. They live as plankton in the open water of lakes, or live either attached to vegetation or near the bottom of the body of water.

While very prolific in most freshwater bodies, Daphnia are too small and weak to live in a strong current, which they are unable to swim against. They live in a water column and are light enough to stay suspended by using their legs and antennae for movement. They live mainly in the upper portion of this water column near the algae-rich surface of the water, but they will often move up or down the column depending on seasons or predators in a process called diel vertical migration. They are often forced to expend a large amount of energy moving towards a lower depth during the day in order to avoid predators and coming towards the surface to eat at night. Their location is also controlled by seasonal variation in their phytoplankton food supply.

Aquatic Biomes: benthic ; lakes and ponds; rivers and streams; coastal

Daphnia are oftened used to clear fish tanks of algae "bloom" because of their diet of bacteria, fine detritus, and very small algae particles. They are filter feeders meaning they do not usually actively seek food; they merely create a constant movement of water using their thoraic legs through their carapace where they are able to filter out any food particles with the setae and direct these towards the mouth. If a mass of food becomes entangled in the mandibles it is cleared by the spines located on the first legs and then kicked out of the carapace by the postabdomen. Not all algae is eaten by Daphnia, such as blue-green algae which has too tough of an outer cell wall and filamentous green algae which can be detrimental to the organism's health. While most species of Daphnia, including D. pulex, are herbivorous or detritivorous (feeding on phytoplankton), a few are carnivorous and prey on other water fleas.

Daphnia pulex reproduces both sexually and asexually in a process called parthenogenesis, where male gametes are unnecessary. Parthenogenesis occurs mainly in the summer, so that during summer an entire population of Daphnia pulex will consist almost completely of females. This process begins in the female, which then molt the carapace to increase their size and develope anywhere from two to twenty eggs in their brood chamber. Even without fertilization from a male, these eggs will develope into immature females which are released after the next molting stage. The young that are produced in this way are more precocial or well-developed than in the process of producing altricial fertilized eggs. This stage of reproduction is most used for a rapid increase in Daphnia growth but requires more favorable conditions.

The sexual stage of Daphnia reproduction occurs mainly in the winter during less favorable conditions caused by overcrowding, accumulation of wastes, lower food availability, and lower temperatures. First, some of the eggs that were produced by parthenogenesis hatch into males instead of females. These males then copulate with the females to form fertilized eggs which are then kept in the female's brood chamber. After the female's next molt she releases these eggs which have the ability to overwinter. They can resist freezing and drying while encased in a purselike ephippium that protects the egg as it rests in the sediment at the bottom of the water body until spring. These eggs remain in this stage of arrested developement, lasting up to twenty years, until the conditions become more favorable for hatching.

Daphnia usually live about ten to thirty days and can live up to one hundred days if their environment is free of predators. An individual will generally have ten to twenty instars, or periods of growth, during their lifetime.

Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual ; asexual ; fertilization (Internal ); oviparous

Daphnia are extremely widespread and common throughout the world. However, they are often used as a food source for aquarium fish and although some of these are raised specifically for this purpose, many are harvested from lakes or ponds. While this practice is unlikely to erradicate all Daphnia species, it could damage some rare populations with a limited range.

In general, Daphnia are beneficial to an aquatic environment, but they will occasionally limit the population size of other organisms as they compete for food and oxygen. Although they are often used in fish tanks to clear the water of algal bloom, fish are not able to be kept in the same tank with a high number of Daphnia because of a limit on the oxygen availability.

Although Daphnia are not used by humans as a food source directly, they are involved in many of the foodchains necessary to sustain fish that we consume or use commercially such as sticklebacks, minnows and young Sockeye salmon. They also are a primary food supply for those animals that trout and many other popular fish depend on. Also, almost any freshwater ecosystem is dependent on Daphnia's ability to convert phytoplankton and decaying matter into a more usable form.

Daphnia are also very frequently used by scientists for experimentation. They are small, cheap, and very easy to keep alive in a laboratory environment. Their almost transparent shell makes their internal functions easier to study and they are very susceptible to changes in temperature, food supplies, or dissolved oxygen content in their environment. Aquarium owners often use Daphnia both as a food source for their fish and to clear the water of debris.