Daphnia magna

Daphnia magna is a small planktonic crustacean (adult length 1.5–5.0 mm) that belongs to the subclass Phyllopoda. 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. 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. Daphnia magna is widespread in the Northern Hemisphere and in particular in the holarctic.