Eucladocera (no evidence for grouping together all other cladocerans as the sister taxon to the monotypic Haplopoda (Leptodora))
The Diplostraca or Cladocera, commonly known as water fleas, is a
superorder of small, mostly freshwater
crustaceans, most of which feed on microscopic chunks of organic matter, though some forms are predatory.[1]
Over 1000 species have been recognised so far, with many more
undescribed.[1][2][3][4][5] The oldest fossils of diplostracans date to the
Jurassic, though their modern morphology suggests that they originated substantially earlier, during the
Paleozoic. Some have also adapted to a life in the ocean, the only members of Branchiopoda to do so, though several
anostracans live in
hypersaline lakes.[6] 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.
Description
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.[7] The body is not obviously segmented and bears a folded
carapace which covers the
thorax and
abdomen.[8]
The head is angled downwards, and may be separated from the rest of the body by a "cervical sinus" or notch.[8] 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.[9] 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.[8] The first antennae bear olfactory
setae, while the second are used for swimming by most species.[9] The pattern of setae on the second antennae is useful for identification.[8] The part of the head which projects in front of the first antennae is known as the
rostrum or "beak".[8]
The
mouthparts are small, and consist of an unpaired labrum, a pair of mandibles, a pair of maxillae, and an unpaired labium.[8] They are used to eat "organic detritus of all kinds" and
bacteria.[8]
The thorax bears five or six pairs of lobed, leaf-like appendages, each with numerous hairs or setae.[8]Carbon dioxide is lost, and
oxygen taken up, through the body surface.[8]
Lifecycle
With the exception of a few purely asexual species, the
lifecycle of diplostracans is dominated by asexual reproduction, with occasional periods of sexual reproduction; this is known as
cyclical parthenogenesis.[10] 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.[8] Except for the genus Leptodora, which has a
metanauplius stage, a
nauplius larval stage is absent in Diplostraca.[11]
Evolutionary history
Diplostraca are nested within the
clam shrimp, being most closely related to the order Cyclestherida, the only living genus of which is Cyclestheria. Though several fossils from the
Paleozoic have been claimed to represent fossils of diplostracans, none of these records can be confirmed. The oldest confirmed records of diplostracans are from the Early
Jurassic of Asia. Fossils from the Jurassic are assignable to modern as well as extinct groups, indicating that the initial radiation of the group occurred prior to the beginning of the Jurassic, likely during the late
Paleozoic.[12]
Ecology
Most diplostracan species live in
fresh water and other inland water bodies, with only eight species being truly
oceanic.[9] The marine species are all in the family
Podonidae, except for the genus Penilia.[9] Some diplostracans inhabit
leaf litter.[13]
Taxonomy
According to the World Registry of Marine Species, Cladocera is a synonym of the superorder Diplostraca, which is included in the class
Branchiopoda. Both names are currently in use. The superorder forms a
monophyletic group of 7 orders, about 24 families, and more than 11,000 species. Many more species
remain undescribed.[1][7] The genus Daphnia alone contains around 150 species.[10] Many groups of the water fleas are cryptic species or species flocks.[14]
^Kotov, Alexey A.; Garibian, Petr G.; Bekker, Eugeniya I.; Taylor, Derek J.; Karabanov, Dmitry P. (2020-06-17). "A new species group from the Daphnia curvirostris species complex (Cladocera: Anomopoda) from the eastern Palaearctic: taxonomy, phylogeny and phylogeography". Zoological Journal of the Linnean Society. 191 (3): 772–822.
doi:
10.1093/zoolinnean/zlaa046.
ISSN0024-4082.
^K. Van Damme; R. J. Shiel; H. J. Dumont (2007). "Notothrix halsei gen. n., sp. n., representative of a new family of freshwater cladocerans (Branchiopoda, Anomopoda) from SW Australia, with a discussion of ancestral traits and a preliminary molecular phylogeny of the order". Zoologica Scripta. 36 (5): 465–487.
doi:
10.1111/j.1463-6409.2007.00292.x.
S2CID83893469.
Brusca, R.C.; Brusca, G.J. (1990). Invertebrates. Sinauer Associates: Sunderland, MA (USA). ISBN 0-87893-098-1. 922 pp
Martin, J.W., & Davis, G.E. (2001). An updated classification of the recent Crustacea. Science Series, 39. Natural History Museum of Los Angeles County. Los Angeles, CA (USA). 124 pp.
Norambuena, J., J. Farías & P. De los Ríos. (2019). he water flea Daphnia pulex (Cladocera, Daphniidae), a possible model organism to evaluate aspects of freshwater ecosystems. Crustaceana, (11-12): 1415-1426.
External links
Cladocera – Guide to the Marine Zooplankton of South Eastern Australia