The Diplostraca or Cladocera, commonly known as water fleas, are a superorder of small

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

s that feed on microscopic chunks of organic matter (excluding some predatory forms). Over 1000 species have been recognised so far, with many more undescribed. The oldest fossils of diplostracans date to the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

, though their modern morphology suggests that they originated substantially earlier, during the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

. Some have also adapted to a life in the ocean, the only members of Branchiopoda to do so, even if several

anostraca Anostraca is one of the four orders of crustaceans in the class Branchiopoda; its members are referred to as fairy shrimp. They live in vernal pools and hypersaline lakes across the world, and they have even been found in deserts, ice-covered ...

ns live in hypersaline lakes. Most are long, with a down-turned head with a single median

compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which dis ...

, and a carapace covering the apparently unsegmented thorax and abdomen. Most species show cyclical

parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...

, where asexual reproduction is occasionally supplemented by

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...

, which produces resting eggs that allow the species to survive harsh conditions and disperse to distant habitats.

Description

They are mostly long, with the exception of '' Leptodora'', which can be up to long. The body is not obviously segmented and bears a folded carapace which covers the

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the ...

and

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the tors ...

. 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". The

mouthparts Mouthparts may refer to: * The parts of a mouth ** Arthropod mouthparts *** Insect mouthparts {{disambig ...

are small, and consist of an unpaired labrum, a pair of mandibles, a pair of maxillae, and an unpaired labium. They are used to eat "organic detritus of all kinds" and

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

. The thorax bears five or six pairs of lobed, leaf-like appendages, each with numerous hairs or setae.

Carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

is lost, and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

taken up, through the body surface.

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 Cycle, cycles, or cyclic may refer to: Anthropology and social sciences * Cyclic history, a theory of history * Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr. * Social cycle, various cycles in soc ...

. 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. Except for the genus Leptodora, which has a metanauplius stage, a nauplius larval stage is absent in Diplostraca.

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

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

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

Ecology

Most diplostracan 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 diplostracans inhabit leaf litter.

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. The genus ''

Daphnia ''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 resemb ...

'' alone contains around 150 species. The following families are recognised: Superorder Diplostraca Gerstaecker, 1866 (=Cladocera) : Order Anomopoda G.O. Sars, 1865 :: Family

Acantholeberidae ''Acantholeberis'' is a genus of crustaceans belonging to the monotypic family Acantholeberidae. Species: *''Acantholeberis curvirostris'' *''Acantholeberis dentata ''Acantholeberis'' is a genus of crustaceans belonging to the monotypic famil ...

Smirnov, 1976 :: Family Bosminidae Baird, 1845 :: Family

Chydoridae Chydoridae is a family of water fleas in the order Anomopoda. There are more than 50 genera and 520 described species in Chydoridae. Genera These 52 genera belong to the family Chydoridae: * ''Acroperus'' Baird, 1843 * '' Alona'' Baird, 1850 * ...

Dybowski & Grochowski, 1894 :: Family

Daphniidae Daphniidae is a family of water fleas in the order Anomopoda. Description Members of the family Daphniidae differ from other, similar diplostracans, such as the Macrotrichidae and Moinidae, in that the antennae of females are short and immobi ...

Straus, 1820 :: Family Dumontiidae Santos-Flores & Dodson, 2003 :: Family Eurycercidae Kurz, 1875 :: Family Gondwanothrichidae Van Damme, Shiel & Dumont, 2007 :: Family

Ilyocryptidae Ilyocryptidae is a family of crustaceans belonging to the order Diplostraca. Genera: * '' Ilyocryptus'' Sars, 1861 References Diplostraca Crustacean families * {{branchiopoda-stub ...

Smirnov, 1976 :: Family Macrothricidae Norman & Brady, 1867 :: Family

Moinidae Moinidae is a crustacean family within the order Cladocera. Species within this family are widely occurring, including North America and Africa. In newer classifications, it is sometimes included in the family Daphniidae Daphniidae is a famil ...

Goulden, 1968 :: Family Ophryoxidae Smirnov, 1976 : Order Ctenopoda G.O. Sars, 1865 :: Family Holopediidae G.O. Sars, 1865 :: Family Pseudopenilidae Korovchinsky & Sergeeva, 2008 :: Family Sididae Baird, 1850 : Order Cyclestherida Sars G.O., 1899 :: Family Cyclestheriidae Sars G.O., 1899 : Order Haplopoda G.O. Sars, 1865 :: Family Leptodoridae Lilljeborg, 1861 : Order Laevicaudata Linder, 1945 :: Family

Lynceidae Lynceidae is a family of clam shrimp in the order Laevicaudata. There are about 5 genera and more than 20 described species in Lynceidae. Genera These five genera belong to the family Lynceidae: * '' Leptestheriella'' * '' Limnetis'' Loven, 18 ...

Stebbing, 1902 : Order Onychopoda G.O. Sars, 1865 :: Family

Cercopagididae Cercopagididae is a family of crustaceans belonging to the order Diplostraca. Genera: * '' Blythotrephes'' Leydig, 1860 (e.g. '' Bythotrephes longimanus'' * '' Cercopagis'' Sars, 1897 (e.g. ''Cercopagis pengoi ''Cercopagis pengoi'', or the ...

Mordukhai-Boltovskoi, 1968 :: Family Podonidae Mordukhai-Boltovskoi, 1968 :: Family Polyphemidae Baird, 1845 : Order Spinicaudata Linder, 1945 :: Family Cyzicidae Stebbing, 1910 :: Family Eocyzicidae Schwentner, et al., 2020 :: Family Leptestheriidae Daday, 1913: 44 :: Family

Limnadiidae Limnadiidae is a family of crustaceans in the order Spinicaudata that live in seasonal wetlands, inland saline pools and lakes. They are found on all the world's continents except Antarctica, and are distinguished from other families in the s ...

Burmeister, 1843

Etymology

The word "Cladocera" derives via

New Latin New Latin (also called Neo-Latin or Modern Latin) is the revival of Literary Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoological and botanical taxonomy ...

from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

(', "branch") and (', "horn").

References

* 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 * * {{Taxonbar, from1=Q768012, from2=Q391240 Freshwater crustaceans Crustacean orders