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

superorder Order () 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 recognized ...

of small, mostly freshwater

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s, most of which feed on microscopic chunks of organic matter, though some forms are predatory. 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 143.1 Mya. ...

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

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

. Some have also adapted to a life in the ocean, the only members of Branchiopoda to do so, though 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 those of ocean water (3.5%, i.e. ). Specific microbial species can thrive i ...

s. Most are long, with a down-turned head with a single median

compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

, 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 unde ...

covering the apparently unsegmented

thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of 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 di ...

and abdomen. Most species show cyclical

parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

, 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 transparency and translucency, transparent Predation, predatory water fleas. They grow up to long, with two large antenna (biology), antennae used for swimming and a single compoun ...

'', which can be up to long. The body is not obviously segmented and bears a folded

which covers the

and

abdomen The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...

. 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 or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates. These eyes are called "simple" to distinguish the ...

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 (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

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 ** podium * 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 ...

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 Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

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

Carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

is lost, and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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 conception to reproduction *Life-cycle hypothesis, in economics *Erikson's stages of psy ...

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, Ireland * Clones, County Monaghan, a town in Ireland * Clones railway station, Ireland Biology * Clone (B-cell), a lymphocyte clone, the massiv ...

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

eggs An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...

. 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: Internationalism * World citizen, one who eschews traditional geopolitical divisions derived from national citizenship * Cosmopolitanism, the idea that all of humanity belongs to a single moral community * Cosmopolitan ...

–

distributions Distribution may refer to: Mathematics *Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations *Probability distribution, the probability of a particular value or value range of a varia ...

. 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 classif ...

, 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

. A Devonian to Carboniferous genus, '' Ebullitiocaris'', is tentatively placed as a diplostracan, however since it is only known from carapaces this is uncertain.

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 salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but 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. A lot of them are non-native species, many of which pose a great threat to aquatic ecosystems. Genera * ''Casp ...

, 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. Some Sididae have accidentally been introduced by huma ...

''. Some diplostracans inhabit

leaf litter Plant litter (also leaf litter, tree litter, soil litter, litterfall, or duff) is dead plant material (such as leaves, bark, needles, twigs, and cladodes) that has fallen to the ground. This detritus or dead organic material and its constituen ...

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, from Ancient Greek βράγχια (''bránkhia''), meaning "gill", and πούς (''poús''), meaning "foot", is a class (biology), class of crustaceans. It comprises Anostraca, fairy shrimp, clam shrimp, Diplostraca (or Cladocera), ...

. Both names are currently in use. The superorder forms a

monophyletic group In biology, a clade (), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy ...

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 (biology), order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their Saltation (gait), ...

'' alone contains around 150 species. Many groups of the water fleas are cryptic species or species flocks. 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 Branchiopoda, from Ancient Greek βράγχια (''bránkhia''), meaning "gill", and πούς (''poús''), m ...

G.O. Sars, 1865 :: Family Acantholeberidae 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. A lot of Chydoridae species are non-native species, many of which pose a great threat to aquatic ecosystems. Gen ...

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 ''Gondwanothrix halsei'' is a species of small crustaceans (ca. ) within the order Anomopoda, placed in its own family, Gondwanotrichidae. It exists in a macrotrichid habitus found in humic Humic substances (HS) are colored relatively recalci ...

Van Damme, Shiel & Dumont, 2007 :: Family

Ilyocryptidae Ilyocryptidae is a family of crustaceans belonging to the order Diplostraca 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 organ ...

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. Some Moinidae have acci ...

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

Ctenopoda Ctenopoda, from Ancient Greek κτείς (''kteís''), meaning "comb", and πούς (''poús''), meaning "foot", are an order of the superorder Diplostraca, comprising the three families Holopediidae, Pseudopenilidae, and Sididae. Its members ...

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 ''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. ...

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. Some Sididae have accidentally been introduced by humans to areas outside of their native range. Genera * '' Diaph ...

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 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 Onychopoda, from Ancient Greek ὄνυχος (''ónukhos''), meaning "claw", and πούς ''poús'', meaning "foot", are a specialised order of branchiopod crustaceans, belonging to the superorder Cladocera. The order Onychopoda is "one of the m ...

G.O. Sars, 1865 :: Family Cercopagididae Mordukhai-Boltovskoi, 1968 :: Family

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 classif ...

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 Leptestheriidae is a family of crustaceans in the order Spinicaudata. They are distinguished from the family Cyzicidae by the presence of a rostral spine. Members of Leptestheriidae are believed to graze on detritus. The family contains five ex ...

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 Antarctica () is Earth's southernmost and l ...

Burmeister, 1843 Daphnia Ephippia

Etymology

The word "Cladocera" derives via

Neo-Latin Neo-LatinSidwell, Keith ''Classical Latin-Medieval Latin-Neo Latin'' in ; others, throughout. (also known as New Latin and Modern Latin) is the style of written Latin used in original literary, scholarly, and scientific works, first in Italy d ...

from the

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