Daphnia are water fleas, a genus of branchy-billed crayfish. Body length about 3-4 mm. weight about 10 mg. Life expectancy about 3 months. They are widely distributed. They live in standing and shallow-flowing fresh water. 14 species are known in Russia. They are part of the zooplankton. Daphnia are food for fish. They are bred in fish farms as live food for young commercial fish.
     One of the largest (more than 50 valid species in the world fauna) and difficult for taxonomy genera of branchfishes. The type species is D. longispina O.F. Mueller, 1785. The most characteristic distinctive feature of the genus is antennae I of females fused with the head. Besides, rostrum of females is usually well developed and ventral margin of flaps is convex. In both sexes, the flaps, as a rule, bear spines and form an unpaired outgrowth, the caudal needle. Most species (except some Australian species, often referred to the genus Daphniopsis) have two eggs in the ephippium. All bristles of antennae II are common, with long setulae. Most systematists recognize the division of this genus into two subgenera, Daphnia (Daphnia) O.F. Mueller, 1785, and Daphnia (Ctenodaphnia) Dybowski et Grochowski, 1895. The subgenus Daphnia (Daphnia) lacks the notch of the head shield, the egg chambers of Ephippium are usually almost perpendicular to the dorsal margin of the flaps. Representatives of subgenus Daphnia (Ctenodaphnia) have notch of scutellum, ovipositor chambers of Ephippium are usually almost parallel to dorsal margin of leaflets. All species previously assigned to the genus Daphniopsis are included in the subgenus Daphnia (Ctenodaphnia), and many of them have primitive (absence of head shield notch) or evasive characters (one egg in the ephippium).
     Covers of females consist of a head shield and bipartite carapace. Usually they have a clearly visible pattern of rhombuses and polygons - reticulation. One hypodermic cell forms each such cell of the coverslip. There are spinules on the edge of the flaps and a tail needle covered with spinules on the posterior end. Many species have a row of feathered bristles on the inner margin of the sashes in its middle part, males of all species have the same bristles and additional bristles on the anterior-lower corner of the sashes. On the head of most species of females, a beak-like outgrowth, the rostrum, is developed. Under it, the first antennae (antennae I, or antennulae) are located - short outgrowths that have 9 olfactory bristles - aesthetasis, and one additional bristle on the lateral surface. In males, the first antennae are larger, more mobile, and have a large bristle ("flagellum") at the distal end in addition to the aesthetas. On the lateral surface of females' head there are projections of cuticle - fornixes. Their form, as well as form of hind margin of head shield is an important diagnostic sign of subgenera and groups of species. Under the fornixes, second antennae (antennae II, or antennae in the narrow sense, as opposed to antennulas) are attached to the head by a complex "joint". They are much larger than the antennulas and serve to float. They consist of a base and two branches - inner three-membered and outer four-membered. At the ends of the segments of the branches, flattened, hair-covered, bipartite swimming bristles are located, which form "oars" when swimming. There are five on the three-part branch (four only in D. cristata) and four on the four-part branch. There are several small sensitive bristles on the base. A large upper lip extends from the posterior surface of the female head. Inside it are several giant polyploid cells, which secrete a secret that glues food into a food clump. At the border between the head shield and the carapace under the flaps are the mandibles. They are complex in shape, asymmetrical and have strongly chitinized chewing surfaces covered with ridges and outgrowths. During feeding, the mandibles ferry food to the mouth opening. Under the carapace are small maxillae first (maxillulae), bearing four bristles. The maxillae second are reduced in daphnia. There are five pairs of bipartite thoracic legs of complex structure. Legs of the first and partly of the second pair differ by structure in males and females. The first pair of legs of males has hook-like outgrowths that allow them to cling to females during mating. The third and fourth pairs carry fans of filtering bristles. Each leg has a respiratory appendage, an epipodite. Behind the thoracic part is a reduced abdominal part, the presence of which is "marked" by dorsal abdominal outgrowths that close the exit from the brood chamber. They are usually four and are well developed in mature females and reduced in males of most species. The posterior part of the body is a large mobile postabdomen, which is homologous to the telson of other crustaceans. On its dorsal side there are two rows of denticles with an anal opening between them. In males of some species these denticles are partially or completely reduced. At the end of the postabdomen there are paired claws covered with spines. According to some data, they are homologous to the Furca, while according to others they represent a pair of large modified bristles. Spines are present on the outer and inner side of the claws, usually with three groups of spines on the outer side and two on the inner. The postabdomen serves to clean the filtration apparatus from large foreign particles.
     The cover system of daphnia is represented by a typical hypodermis. The hypoderma of the carpax consists of large cells forming rhombic-shaped cells. The central nervous system consists of the supraglottic ganglion (brain) and the ventral nerve chain with several paired ganglia. The cerebrum is clearly visible in living individuals, which is extremely rare. It consists of a large, bifurcated optic ganglion and the supraglottic ganglion itself. From the anterior part of the optic ganglion departs the optic nerve, which connects the brain to the compound faceted eye. The unpaired compound eye in daphnia is formed from a paired embryo (embryos have two eyes) and contains exactly 22 facets (ommatidia). It is located in a special cavity inside the head, to the walls of which it is suspended by two ligaments (ligaments) and is set in motion by three pairs of oculomotor muscles. In living individuals, eye tremors are noticeable, and occasionally larger eye jumps (saccades) are observed. Nerves also depart from the brain to the eye (simple eye), first antennae (at their base there is a sensitive ganglion whose cells innervate olfactory bristles - aesthetas), and nerves to a sensitive occipital organ of unknown purpose. A simple eye (oculus, nauplial eye) adjoins the lower part of the supraglottic ganglion. In most species, it contains pigment and is visible as a small black dot. There are four groups of sensitive cells around the pigment spot. Daphnia have a complex system of transverse striated muscles that drive the second antennae, postabdomen and thoracic limbs, as well as muscles that drive the eye, upper lip, etc. The muscles of the digestive tract are also transverse striated. The digestive tract begins with the mouth opening, which is covered by a large upper lip. Giant highly polyploid cells located inside the lip secrete a secret that glues food into a food clump. Through movements of the mandibles, it is ferried to the thin esophagus, whose dilator muscles create peristalsis, ensuring the transport of food down the esophagus. Inside the head, the esophagus passes into the broader midgut, which extends to the middle part of the postabdomen. Inside the head, two curved hepatic outgrowths branch from the midgut. In the posterior part of the postabdomen is the short posterior intestine. The heart is on the dorsal side of the body, in front of the edge of the brood chamber. Blood (hemolymph), the flow of which is clearly visible due to the presence of colorless cells - phagocytes, enters the heart through the ostia, two slit-shaped lateral openings. When the heart shrinks, the ostia close with valves, and blood is expelled through the anterior orifice to the head. Blood vessels are absent; the regular direction of blood flows is ensured by transparent septa between different parts of the myxocelium. Breathing occurs through the covers of the body, first of all thoracic legs, which have respiratory appendages - epipodites. The latter also take part in osmoregulation. An additional organ of osmoregulation in newborn individuals is a large occipital pore (occipital organ), which disappears after the first post-embryonic molt. The organs of excretion are complexly shaped maxillary glands, which are located on the inner surface of the flaps in their anterior part. The paired ovaries (in males - testes) are located on the sides of the intestine. At the posterior end is the oogonium breeding area, the rest of the ovary is filled with maturing oocytes. As the oocytes mature, they move to their posterior third, where there are narrow oviducts that open into the brooding chamber. In males, the oviducts open on the postabdomen in its distal part, in many species on special papillae.
During molting, the cervical suture, the line between the head shield and carapace, splits apart and the animal emerges from the exuvium. Together with the carapace, the covers of the body and limbs are discarded. The molt repeats periodically throughout the life of the individual. Usually molting occurs in the water column, ephippial females of some species molt by sticking to the surface film of water from below. Several molts occur during embryonic development, in the brood chamber.
     The genus Daphnia has a worldwide distribution (including Antarctica, where Daphnia studeri, previously attributed to the genus Daphniopsis, was found in the relict salt lakes of the Vestfold Hills oasis). In the early 20th century, the prevailing view was that the distribution of most species was cosmopolitan, but it was later found that the faunas of different continents differed greatly. Some species, however, have very wide ranges and are distributed on several continents. The smallest number of species is characteristic of equatorial areas, where Daphnia are rare. The fauna of subtropics and temperate latitudes is the most diverse. In recent decades, the ranges of many species have changed due to their dispersal by humans. Thus, a species from the New World, Daphnia ambigua, has been introduced into Europe (England). In many reservoirs in the southern United States, D. lumholtzi, which had previously been found only in the Old World. In ponds and puddles of central Russia are common (and most popular among aquarists), the following crustaceans of the genus Daphnia: Daphnia magna, female - up to 6 mm, male - up to 2 mm, newborn - 0.7 mm. They mature within 10-14 days. Litter in 12-14 days. A clutch contains up to 80 eggs (usually 20-30). Life expectancy is up to 3 months. Daphnia pulex, female up to 3-4 mm, male 1-2 mm. Litter 3-5 days later. A clutch of up to 25 eggs (usually 10-12). Lives 26-47 days. Daphnia cucullata, Daphnia galeata, Daphnia cristata, and several other species are often found in lakes in the temperate zone of Eurasia.
     Daphnia are small crustaceans (adult body size from 0.6 to 6 mm). They inhabit all types of standing continental reservoirs; they are also found in many rivers with a slow current. In puddles, ponds and lakes they often have high numbers and biomass. Daphnia are typical planktonic crustaceans, spending most of their time in the water column. Different species inhabit shallow temporary reservoirs, littoral and pelagic lakes. Quite a few species, especially inhabiting arid areas, are halophiles inhabiting brackish, salty and hyperhaline continental reservoirs. Such species include, for example, Daphnia magna, Daphnia atkinsoni, Daphnia mediterranea, as well as most species previously assigned to the genus Daphniopsis.
     Daphnia spend most of their time in the water column, moving in sharp leaps due to their second antennae, which are covered with special plumed bristles (hence their common name, "water fleas", often referred to all branchivorous daphnia). Many daphnia are also able to crawl slowly along the bottom or vessel walls due to water currents created by their thoracic legs (the antennae are immobile in this mode of movement).
     Bacteria and unicellular algae are the main food for daphnia. Species that survive the winter in an active state (in deep, frost-free ponds) spend it in the bottom layers of water, feeding mainly on detritus. They feed by filtration, creating water currents by rhythmic movements of their thoracic legs. Food is filtered out by fans of filtering bristles located on endopodites III and IV pairs of thoracic legs. Large particles trapped in the filtration apparatus (e.g. filamentous algae) are removed by the postabdomen and its claws. From the filtering fans, food enters the abdominal food trough, passes to the maxillae of the first pair and then to the mandibles, whose movements ferry it to the gullet. In front, the mouth opening of daphnia is covered with a large upper lip, inside which there are salivary glands made of giant polyploid cells. Their secret glues food particles into a food clump. Filtration occurs continuously, even when the intestine becomes fully filled, which occurs after a time interval of 10 minutes to 4 hours, the particles continue to move along the abdominal groove to the mouth, but are thrown mouth appendages and the back of the body back into the water, and so - until there is room in the gut, then filtered food again begins to enter the gullet. With an average concentration of food in the water, adult daphnia of various species filter at a rate of 1 to 10 ml/day. The daily food intake of an adult Daphnia magna can reach 600% of its body weight. Daphnia eat from 5 to 41 million bacteria per day.
     There are sac-shaped respiratory appendages - gills - on the thoracic legs. Daphnia probably receive a significant portion of oxygen through the thin covers of the body and limbs, and the respiratory appendages, like the occipital organ of newborns, play an important role in osmoregulation. Some species (e.g., Daphnia pulex, Daphnia magna) start synthesizing hemoglobin when the oxygen content in water is reduced, so that their hemolymph and the whole body turn red.