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

superorder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

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 group ...

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 m ...

ns live in

hypersaline lake A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in ...

s. 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 disti ...

, and a

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

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 development ...

, where

asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...

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 tha ...

, 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

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 cre ...

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 torso. ...

. 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

seta In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. Th ...

e, 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 among ...

. 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 transpar ...

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 wel ...

taken up, through the body surface.

Lifecycle

With the exception of a few purely asexual species, the

lifecycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia * Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis ...

of diplostracans is dominated by asexual reproduction, with occasional periods of sexual reproduction; this is known as cyclical parthenogenesis. 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 Ephippia (singular: ephippium) are winter or dry-season eggs of the various species of small crustacean in the order Cladocera (within the Branchiopoda); they are provided with an extra shell layer, which preserves and protects the resting ...

eggs can be transported over land by wind, and hatch when they reach favourable conditions, allowing many species to have very wide – even

cosmopolitan Cosmopolitan may refer to: Food and drink * Cosmopolitan (cocktail), also known as a "Cosmo" History * Rootless cosmopolitan, a Soviet derogatory epithet during Joseph Stalin's anti-Semitic campaign of 1949–1953 Hotels and resorts * Cosmopoli ...

– 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 ''Cyclestheria'' is a genus of shrimps Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some cru ...

''. 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 Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

and other inland water bodies, with only eight species being truly

oceanic Oceanic may refer to: *Of or relating to the ocean *Of or relating to Oceania **Oceanic climate **Oceanic languages **Oceanic person or people, also called "Pacific Islander(s)" Places * Oceanic, British Columbia, a settlement on Smith Island, ...

. The marine species are all in the family

Podonidae Podonidae is a family of onychopods in the order Diplostraca. There are about 8 genera and at least 20 described species in Podonidae. Genera * '' Caspievadne'' * '' Cornigerius'' * ''Evadne'' Lovén, 1836 * '' Pleopis'' * '' Pleopsis'' Dana, 18 ...

, except for the genus ''

Penilia ''Penilia'' is a genus of ctenopods in the family Sididae Sididae is a family of ctenopods in the order Diplostraca. There are about 6 genera and at least 20 described species in Sididae. Genera * ''Diaphanosoma'' Fischer, 1850 * ''Latona'' St ...

''. 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'' 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 Bosminidae is a family of anomopods in the order Diplostraca. There are at least 3 genera and 40 described species in Bosminidae. Genera * ''Bosmina ''Bosmina'' is a genus in the order Cladocera, the water fleas. Its members can be distinguish ...

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 Straus, 1820 :: Family Dumontiidae Santos-Flores & Dodson, 2003 :: Family Eurycercidae Kurz, 1875 :: Family Gondwanothrichidae Van Damme, Shiel & Dumont, 2007 :: Family Ilyocryptidae 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. Genera These two genera ...

Goulden, 1968 :: Family Ophryoxidae Smirnov, 1976 : Order Ctenopoda G.O. Sars, 1865 :: Family

Holopediidae ''Holopedium'' is the sole genus of water fleas The Diplostraca or Cladocera, commonly known as water fleas, are a superorder of small crustaceans that feed on microscopic chunks of organic matter (excluding some predatory forms). Over 1000 ...

G.O. Sars, 1865 :: Family

Pseudopenilidae ''Pseudopenilia bathyalis'' is a species of diplostracan, described in 2004, that lives at depths of in the anoxic zone of the Black Sea. Originally described in the family Sididae Sididae is a family of ctenopods in the order Diplostraca. The ...

Korovchinsky & Sergeeva, 2008 :: Family Sididae Baird, 1850 : Order Cyclestherida Sars G.O., 1899 :: Family

Cyclestheriidae Clam shrimp are a group of bivalved branchiopod crustaceans that resemble the unrelated bivalved molluscs. They are extant and also known from the fossil record, from at least the Devonian period and perhaps before. They were originally classifi ...

Sars G.O., 1899 : Order Haplopoda G.O. Sars, 1865 :: Family

Leptodoridae ''Leptodora'' is a genus containing two species of large, nearly transparent predatory water fleas. They grow up to long, with two large antennae used for swimming and a single compound eye. The legs are used to catch copepods that it comes in ...

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, 1846 ...

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

Mordukhai-Boltovskoi, 1968 :: Family Polyphemidae Baird, 1845 : Order Spinicaudata Linder, 1945 :: Family

Cyzicidae Cyzicidae is a family of crustaceans in the order Spinicaudata. They are distinguished from the family Leptestheriidae by the absence of a rostral spine. Some members of Cyzicidae are known to burrow through mud, leading to speculation that they ...

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 same ...

Burmeister, 1843

Etymology

The word "Cladocera" derives via New Latin 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 peri ...

(', "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