|> 200 spp.|
Daphnia is a genus of small planktonic crustaceans, 0.2–6.0 mm (0.01–0.24 in) in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory (Wiktionary) swimming style resembles the movements of fleas. Daphnia spp. live in various aquatic environments ranging from acidic swamps to freshwater lakes and ponds.
The two most readily available species of Daphnia are D. pulex (small and most common) and D. magna (large). They are often associated with a related genus in the order Cladocera: Moina, which is in the Moinidae family instead of the Daphniidae, and is much smaller than D. pulex (roughly 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).
Appearance and characteristics
The body of a Daphnia species is usually 1–5 mm (0.039–0.197 in) long, and is divided into segments, although this division is not visible. The head is fused, and is generally bent down towards the body with a visible notch separating the two. In most species, the rest of the body is covered by a carapace, with a ventral gap in which the five or six pairs of legs lie. The most prominent features are the compound eyes, the second antennae, and a pair of abdominal setae. In many species, the carapace is translucent or nearly so, so they make excellent subjects for the microscope, as one can observe the beating heart.
Even under relatively low-power microscopy, the feeding mechanism can be observed, with immature young moving in the brood pouch; moreover, the eye being moved by the ciliary muscles can be seen, as well as blood cells being pumped around the circulatory system by the simple heart. The heart is at the top of the back, just behind the head, and the average heart rate is about 180 bpm under normal conditions. 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. They are tolerant of being observed live under a coverslip and appear to suffer no harm when returned to open water. This experiment can also be performed using caffeine, nicotine, or adrenaline, each producing an increase in the heart rate.
The blood/circulation system is an open circulatory system, with only the heart and blood without blood vessels, and when the heart spouts blood, since the shell of the water flea is attached to the back and head, and blood is transmitted all over the body along the water that enters the gap. Because the crust is transparent, you can see the heart beating if you observe it well under a microscope. Its heart beats fast because it's small.
Systematics and evolution
Daphnia is a large genus – comprising over 200 species – belonging to the cladoceran family Daphniidae. It is subdivided into several subgenera (Daphnia, Australodaphnia, Ctenodaphnia), but the division has been controversial and is still in development. Each subgenus has been further divided into a number of species complexes. The understanding of species boundaries has been hindered by phenotypic plasticity, hybridization, intercontinental introductions, and poor taxonomic descriptions.
Ecology and behaviour
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. In typical conditions, however, the lifecycle is much shorter, not usually exceeding 5–6 months.
Daphnia spp. are typically filter feeders, ingesting mainly unicellular algae and various sorts of organic detritus including protists and bacteria Beating of the legs produces a constant current through the carapace, which brings such material into the digestive tract. The trapped food particles are formed into a food bolus which then moves down the digestive tract until voided through the anus located on the ventral surface of the terminal appendage. The second and third pairs of legs are used in the organisms' filter-feeding, ensuring large, unabsorbable particles are kept out, while the other sets of legs create the stream of water rushing into the organism.
Daphnia spp. 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 have a larger size after hatching, along with developing neck-teeth at the back of the head, when in the presence of Chaoborus kairomones. These morphological defenses have shown to reduce mortality due to Chaoborus predation, which is a gape-limited predator. Chitin-related genes (deacetylases) are thought to play an important part in the expression/development of these morphological defenses in Daphnia. Chitin-modifying enzymes (chitin deacetylases) have been shown to catalyse the N-deacetylation of chitin to influence the protein-binding affinity of these chitin filaments. 
Most Daphnia species have a lifecycle based on "cyclical parthenogenesis", alternating between parthenogenetic (asexual) and sexual reproduction. For most of the growth season, females reproduce asexually. They produce a brood of diploid eggs every time they molt; these broods can contain as few as one or two eggs in smaller species, such as D. cucullata, but can be over 100 in larger species, such as D. magna. Under typical conditions, these eggs hatch after a day, and remain in the female's brood pouch for around three days (at 20°C). 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. The asexually produced offspring are typically female.
Towards the end of the growing season, however, the mode of reproduction changes, and the females produce tough "resting eggs" or "winter eggs". When environmental conditions deteriorate (e.g. crowding), some of the asexually produced offspring develop into males. The females start producing haploid sexual eggs, which the males fertilise. In species without males, resting eggs are also produced asexually and are diploid. In either case, the resting eggs are protected by a hardened coat 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) .
The diagram on the left shows the lifecycle of Pasteuria ramosa, a bacterial parasite of Daphnia. Susceptible hosts acquire the infection from spores in the sediment or in suspension. The parasite develops mainly in the host's body cavity and muscle tissue, increasing in density and eventually expanding to occupy the entire host. Typical effects on the host are sterility and gigantism. Spores are released mainly after the host dies and sinks to the substrate, and sometimes directly to the water via clumsy predation.
Several Daphnia species are considered threatened. These are listed as vulnerable by IUCN: Daphnia nivalis, Daphnia coronata, Daphnia occidentalis, and Daphnia jollyi. Some species are halophiles, and can be found in hypersaline lake environments, an example of which is the Makgadikgadi Pan.
Daphnia spp. 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. Because they are nearly transparent, their internal organs are easy to study in live specimens (e.g. to study the effect of temperature on the heart rate of these ectothermic organisms). Daphnia is also commonly used for experiments to test climate change aspects, as UVB that seriously damages zooplankton species (e.g. decrease feeding activity).
Because of their thin membranes, which allow drugs to be absorbed, they are used to monitor the effects of certain drugs, such as adrenaline or capsaicin, on the heart.
Some species have developed permanent defenses against fish eating them, such as spines and long hooks on the body, which also cause them to become entangled on fishing lines and cloud water with their high numbers. Species such as Daphnia lumholtzi (native to east Africa, the Asian subcontinent of India, and east Australia) have these characteristics and great care should be taken to prevent them from spreading further in North American waters.
Some species of Daphnia native to North America can develop sharp spines at the end of their bodies and helmet-like structures on their heads when they detect predators, but this is overall temporary for such species and they do not completely overwhelm or discourage native predators from eating them. While Daphnia spp. 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. end up outcompeted by the invasive ones. (This may not be the case, however, and the new invaders may mostly be a tangling and clogging nuisance.)
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