Daphnia magna is a small planktonic crustacean (adult length 1.5–5.0 mm) that belongs to the subclass Phyllopoda.
Description
Daphnia magna is a typical water flea of the genus Daphnia. The females reach up to 5 mm in size, the males about 2 mm, thus they are among the largest species in the genus. The body is protected by a translucent carapace made of chitin, a transparent polysaccharide. It has a ventral opening and five pairs of thoracic limbs, used to help the filtering process. Spike rows run along the back of the carapace. The intestine is hook-shaped and has two digestive ceca. The head has two antennae and a large compound eye.
Adult females can be distinguished from those of otherwise similar species such as D. pulex by the absence of a comb on the abdominal claw and the presence of two distinct combs on the abdomen. The males are smaller than the females and have larger first antennas, a diagnostic feature that distinguishes them from small females.
Ecology
Daphnia magna is a key species in many lentic habitats. It can be found in lakes and shallow ponds rich in organic matter sediment. Numerous natural predators are known and can lead to plastic phenotypic responses. In the presence of kairomones, Daphnia spp. develop conspicuous protective structures as an elongated spine and a large body size. In response to temperature differences, D. magna has shown both phenotypic plasticity and the ability to genetically evolve to deal with the heat stress of warmer, urban pond waters. Populations of Daphnia have also developed increased resistance to local pesticides. Another factor that affects both phenotypic and behavioural plasticity of D. magna is the salinity of its surroundings.
Daphnia magna is an important primary consumer and prey of many planktivorous fishes. Other invertebrate predators are the larvae of the phantom midge Chaoborus and hemipterans (Notonecta) and Triops. The large size of the adults protects them from predation from some planktivorous invertebrates.
Distribution and habitat
Daphnia magna is widespread in the Northern Hemisphere and in particular in the holarctic. It can be found in fresh and brackish water bodies of different sizes, from lakes to ponds and ephemeral rock pools near the sea. D. magna tolerates higher levels of salinity (up to one-fifth the salinity of sea water) than most other species of the genus.
Daphnia magna is mainly found in the pelagic zone of water bodies, as it feeds primarily on suspended particles in the water column (mainly algae, but also bacteria and detritus). Nevertheless, compared to other species of Daphnia, it is more often found in association with the substrate where it is able to exploit benthic food sources as periphyton and sediment.
The results of genetic research confirm the deep split between Eastern and Western mitochondrial haplotype super-clades of D. magna. A narrow contact zone has been found between these two super-clades in the eastern part of Western Siberia, with proven co-occurrence in a single lake in the Novosibirsk region. However, at present there is no evidence suggesting that the two mitochondrial super-clades represent cryptic species. Rather, they may be explained by secondary contact after expansion from different refugia. Interestingly, Central Siberia has previously been found to be an important contact zone also in other cladoceran species, and may thus be a crucial area for understanding the Eurasian phylogeography of freshwater invertebrates.
Nutrition
The main feeding strategy of D. magna is the filtering of suspended particles. A specialized filtering apparatus, formed by the thoracic appendices, generates a water current within the thoracic opening of the carapace, which permits the collection and the ingestion of unicellular algae, bacteria, and detritus. D. magna can also feed on periphyton and detritus, an ability that can offer a competitive advantage to this species over strictly pelagic filter feeders in some environments where suspended food sources might be temporally limited.
Reproduction
As most of the other species of the genus Daphnia, D. magna reproduces by cyclical parthenogenesis. This form of reproduction is characterised by the alternating production of asexual offspring (clonal reproduction) and at certain time sexual reproduction through haploid eggs that need to be fertilised.
The asexual eggs (up to few dozens per clutch) are diploid and usually develop into females, or in response to adverse environmental stimuli, into males.