Daphnia is a genus of small planktonic crustaceans, in length. Daphnia are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory 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 commonly found 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 group instead of the Daphniidae, and is much smaller than D. pulex (roughly half the maximum length). Another common species of Daphnia is D. pulicaria, often confused with D. pulex due to their similar appearances and ability to produce hybrids.

Appearance and characteristics

thumb|left|The beating heart of Daphnia under the microscope

The body of a Daphnia species is usually 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.

Systematics and evolution

Daphnia is a large genus – comprising over 200 species – belonging to the cladoceran family Daphniidae. Modern members of Daphnia belonging to the subgenera Daphnia and Ctenodaphnia are known since the Cretaceous, though the genus likely originated prior to the beginning of the Cretaceous. There are numerous poorly studied and cryptic species among crustaceans of the genus Daphnia.

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. 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 action of this second set of antennae is responsible for the jumping motion. In case of D. magna it was shown that the response to different kairomones also differ: While the presence of fish kairomones up-regulated one specific gene in the folding of proteins, whereas Chaoborus kairomone down-regulated the same gene. Based on this the response is a reduction of size at first reproduction in response to kairomones from fish whereas it shows increased size when confronted with larvae of Chaoborus. With the same publication it was shown that genes for glyceraldehyde 3-phosphate dehydrogenase and ubiquitin conjugating enzyme were up-regulated in the presence of microcystins in the food of D. magna and with this the enzymes of glycolysis and protein catabolism are significantly upgregulated when daphnids ingest these toxins.

Uses

Daphnia spp. are a popular live food in tropical and marine fish keeping. They are often fed to tadpoles or small species of amphibians such as the African dwarf frog (Hymenochirus boettgeri).

Daphnia spp. are used in scientific studies as a model organism. 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. 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). Environmental toxicological testing may be undertaken with researchers assessing mortality rates or metabolic perturbations to evaluate ecological impacts. Daphnia is also commonly used for experiments to test climate change aspects, as UVB that seriously damages zooplankton species (e.g. decrease feeding activity, impairment of growth).

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.

Invasive species

thumb|[[Fishhook waterflea (above) and Bythotrephes longimanus (spiny water flea) (below)]]

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.)

In the water bodies of the world, at least 15 species of Daphnia and hybrids are non-native species, many of which pose a great threat to aquatic ecosystems.

See also

  • List of Daphnia species
  • Rotifer

References

  • Daphnia Genomics Consortium
  • Daphnia: An Aquarist's Guide
  • Waterflea.org: a Community resource for cladoceran biology
  • Daphnia spp.: taxonomy, facts, life cycle, references at GeoChemBio