The Cladocera are an order of small crustaceans commonly called water fleas. Over 650 species have been recognised so far, with many more undescribed. They first appeared in the Oligocene period, and have since invaded most freshwater habitats. Some have also adapted to a life in the ocean, the only members of Branchiopoda to do so, even if several anostracans live in hypersaline lakes. Most are 0.2–6.0 mm (0.01–0.24 in) long, with a down-turned head with a single median compound eye, and a carapace covering the apparently unsegmented thorax and abdomen. Most species show cyclical parthenogenesis, where asexual reproduction is occasionally supplemented by sexual reproduction, which produces resting eggs that allow the species to survive harsh conditions and disperse to distant habitats.
They are mostly 0.2–6.0 mm (0.01–0.24 in) long, with the exception of Leptodora, which can be up to 18 mm (0.71 in) long. The body is not obviously segmented and bears a folded carapace which covers the thorax and abdomen.
The head is angled downwards, and may be separated from the rest of the body by a "cervical sinus" or notch. It bears a single black compound eye, located on the animal's midline, in all but two genera, and often, a single ocellus is present. The head also bears two pairs of antennae – the first antennae are small, unsegmented appendages, while the second antennae are large, segmented, and branched, with powerful muscles. The first antennae bear olfactory setae, while the second are used for swimming by most species. The pattern of setae on the second antennae is useful for identification. The part of the head which projects in front of the first antennae is known as the rostrum or "beak".
With the exception of a few purely asexual species, the lifecycle of cladocerans is dominated by asexual reproduction, with occasional periods of sexual reproduction; this is known as cyclical parthenogenesis. The system evolved in the Permian, when the Cladocera arose. When conditions are favourable, reproduction occurs by parthenogenesis for several generations, producing only female clones. As the conditions deteriorate, males are produced, and sexual reproduction occurs. This results in the production of long-lasting dormant eggs. These ephippial eggs can be transported over land by wind, and hatch when they reach favourable conditions, allowing many species to have very wide – even cosmopolitan – distributions.
Most cladoceran species live in fresh water and other inland water bodies, with only eight species being truly oceanic. The marine species are all in the family Podonidae, except for the genus Penilia. Some cladocerans inhabit leaf litter.
The order Cladocera is included in the class Branchiopoda, and forms a monophyletic group, which is currently divided into four suborders. Around 620 species have been described, but many more species remain undescribed. The genus Daphnia alone contains around 150 species.
These families are recognised:
Order Cladocera Latreille, 1829
- Suborder Ctenopoda Sars, 1865
- Suborder Anomopoda Stebbing, 1902
- Suborder Onychopoda Sars, 1865
- Suborder Haplopoda Sars, 1865
- Leptodoridae Lilljeborg, 1900
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