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, 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 mo ...

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 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 tortoises, the under ...

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 f ...

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 ''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 ...

'', 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 crea ...

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 to ...

. 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 A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-le ...

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 Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * R ...

or "beak". The mouthparts 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. 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 tr ...

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 ...

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. When conditions are favourable, reproduction occurs by parthenogenesis for several generations, producing only female

clones Clone or Clones or Cloning or Cloned or The Clone may refer to: Places * Clones, County Fermanagh * Clones, County Monaghan, a town in Ireland Biology * Clone (B-cell), a lymphocyte clone, the massive presence of which may indicate a patholog ...

. As the conditions deteriorate, males are produced, and sexual reproduction occurs. This results in the production of long-lasting dormant

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

. 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 Metanauplius is an early larval stage of some crustaceans such as krill. It follows the nauplius stage. In sac-spawning krill, there is an intermediary phase called pseudometanauplius, a newly hatched form distinguished from older metanauplii by ...

stage, a nauplius larval stage is absent in Diplostraca.

Evolutionary history

Diplostraca are nested within the

clam shrimp 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 ...

, 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 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

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 includ ...

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, 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 ...

''. 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 Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca (or Cladocera), Notostraca and the Devonian '' Lepidocaris''. They are mostly small, freshwater animals that feed on plankton and detritus. Descriptio ...

. 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 resembl ...

'' alone contains around 150 species. The following families are recognised: Superorder Diplostraca Gerstaecker, 1866 (=Cladocera) : Order

Anomopoda Anomopoda is an order of the superorder Diplostraca. These crustaceans, a type of water flea, are members of the class Branchiopoda. The Anomopoda typically have five pairs of thoracic limbs, but sometimes have six pairs. The head of the Anomopo ...

G.O. Sars, 1865 :: Family Acantholeberidae Smirnov, 1976 :: Family Bosminidae Baird, 1845 :: Family Chydoridae 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 immobil ...

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 Goulden, 1968 :: Family Ophryoxidae Smirnov, 1976 : Order

Ctenopoda The Ctenopoda are an order of the superorder Diplostraca, comprising the three families Holopediidae, Pseudopenilidae, and Sididae. Its members mostly live in fresh water, but ''Penilia ''Penilia'' is a genus of ctenopods in the family Sidi ...

G.O. Sars, 1865 :: Family

Holopediidae ''Holopedium'' is the sole genus of water fleas in the family Holopediidae. There are about seven described species in ''Holopedium''. Species These seven species belong to the genus ''Holopedium'': * '' Holopedium acidophilum'' Rowe, Adamowicz ...

G.O. Sars, 1865 :: Family Pseudopenilidae Korovchinsky & Sergeeva, 2008 :: 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 In ancient Greek mythology and religion, Leto (; grc-gre, ...

Baird, 1850 : Order

Cyclestherida 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 :: Family Cyclestheriidae Sars G.O., 1899 : Order

Haplopoda ''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 ...

G.O. Sars, 1865 :: Family Leptodoridae Lilljeborg, 1861 : Order

Laevicaudata 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 ...

Linder, 1945 :: Family Lynceidae Stebbing, 1902 : Order

Onychopoda Onychopoda are a specialised order of branchiopod crustaceans, belonging to the superorder Cladocera. The order Onychopoda is "one of the most morphologically distinctive groups of cladocerans". They have only four pairs of legs, compared to fiv ...

G.O. Sars, 1865 :: Family Cercopagididae Mordukhai-Boltovskoi, 1968 :: Family Podonidae Mordukhai-Boltovskoi, 1968 :: Family Polyphemidae Baird, 1845 : Order

Spinicaudata 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 ...

Linder, 1945 :: Family Cyzicidae Stebbing, 1910 :: Family Eocyzicidae Schwentner, et al., 2020 :: Family Leptestheriidae Daday, 1913: 44 :: Family Limnadiidae 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 a ...

from the Ancient Greek (', "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