Branchiopoda is a class of

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. It comprises

fairy shrimp 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 mou ...

, clam shrimp, Diplostraca (or Cladocera), Notostraca and the Devonian '' Lepidocaris''. They are mostly small, freshwater animals that feed on plankton and detritus.

Description

Members of the Branchiopoda are unified by the presence of

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

s on many of the animals'

appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including ant ...

s, including some of the

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

. This is also responsible for the name of the group (from the grc, βράγχια, gills, akin to , windpipe; el, πούς, foot). They generally possess

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 ...

s 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 und ...

, which may be a shell of two valves enclosing the trunk (as in most Cladocera), broad and shallow (as in the Notostraca), or entirely absent (as in the Anostraca). In the groups where the carapace prevents the use of the trunk limbs for swimming (Cladocera, clam shrimp and the extinct

Lipostraca ''Lepidocaris rhyniensis'' is an extinct species of crustacean. It is the only species known from the order Lipostraca, and is the only abundant animal in the Pragian-aged Rhynie chert deposits. It resembles modern Anostraca, to which it is proba ...

), the antennae are used for locomotion, as they are in the nauplius. Male fairy shrimp have an enlarged pair of antennae with which they grasp the female during mating, while the bottom-feeding Notostraca, the antennae are reduced to vestiges. The trunk limbs are beaten in a metachronal rhythm, causing a flow of water along the midline of the animal, from which it derives

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 ...

food Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is ...

and, in the case of the Anostraca and Notostraca, movement.

Ecology

Among the branchiopods, only some

cladocera 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 species have been recognised so far, with many more ...

ns are marine; all the other groups are found in continental

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 incl ...

, including temporary pools and in salt lakes. Most branchiopodans eat floating

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commu ...

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...

, which they take using the

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. T ...

e on their appendages.

Taxonomy

In early taxonomic treatments, the current members of the Branchiopoda were all placed in a single genus, ''Monoculus''. The taxon Branchiopoda was erected by

Pierre André Latreille Pierre André Latreille (; 29 November 1762 – 6 February 1833) was a French zoology, zoologist, specialising in arthropods. Having trained as a Roman Catholic priest before the French Revolution, Latreille was imprisoned, and only regained hi ...

in 1817, initially at the

rank Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking, such as: Level or position in a hierarchical organization * Academic rank * Diplomatic rank * Hierarchy * H ...

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

. The current upper-level classification of Branchiopoda, according to the World Register of Marine Species (2021), is as follows: Class Branchiopoda Latreille, 1817 : Subclass Sarsostraca Tasch, 1969 ::Order

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 ...

Sars, 1867 :::Suborder Anostracina Weekers et al., 2002 :::Suborder Artemiina Weekers et al., 2002 : Subclass Phyllopoda Preuss, 1951 :: Superorder Diplostraca Gerstaecker, 1866 ::: Order Anomopoda G.O. Sars, 1865 ::: Order Ctenopoda G.O. Sars, 1865 ::: Order Cyclestherida Sars G.O., 1899 ::: Order Haplopoda G.O. Sars, 1865 ::: Order Laevicaudata Linder, 1945 ::: Order Onychopoda G.O. Sars, 1865 ::: Order Spinicaudata Linder, 1945 :: Order Notostraca G. O. Sars, 1867 ::Genus † Cryptocaris Barrande, 1872 ::Genus † Dithyrocaris In addition, the extinct genus '' Lepidocaris'' is generally placed in Branchiopoda.

Anostraca

The fairy shrimp of the order

are usually long (exceptionally up to ). Most species have 20 body segments, bearing 11 pairs of leaf-like ''phyllopodia'' (swimming legs), and the body lacks a

. They live in

vernal pool Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe ...

s and

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 across the world, including pools in deserts, in ice-covered mountain lakes and in

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest co ...

. They swim "upside-down" and feed by filtering organic particles from the water or by scraping

algae Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...

from surfaces. They are an important food for many birds and fish, and are cultured and harvested for use as fish food. There are 300 species spread across 8 families.

Lipostraca

Lipostraca contains a single extinct Early Devonian species, '' Lepidocaris rhyniensis'', which is the most abundant animal in the

Rhynie chert The Rhynie chert is a Lower Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is locate ...

deposits. It resembles modern Anostraca, to which it is probably closely related, although its relationships to other orders remain unclear. The body is long, with 23 body segments and 19 pairs of

s, but no

. It occurred chiefly among

charophyte Charophyta () is a group of freshwater green algae, called charophytes (), sometimes treated as a division, yet also as a superdivision or an unranked clade. The terrestrial plants, the Embryophyta emerged within Charophyta, possibly from terr ...

s, probably in alkaline temporary pools.

Notostraca

The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp or shield shrimp. The two genera, ''

Triops ''Triops'' is a genus of small crustaceans in the order Notostraca (tadpole shrimp). The long-lasting resting eggs of several species of ''Triops'' are commonly sold in kits as a pet. The animals hatch upon contact with fresh water. Most adult-st ...

'' and '' Lepidurus'', are considered

living fossil A living fossil is an extant taxon that cosmetically resembles related species known only from the fossil record. To be considered a living fossil, the fossil species must be old relative to the time of origin of the extant clade. Living foss ...

s, having not changed significantly in outward form since the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

. They have a broad, flat carapace, which conceals the head and bears a single pair of compound eyes. The abdomen is long, appears to be segmented and bears numerous pairs of flattened legs. The

telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...

is flanked by a pair of long, thin caudal rami.

Phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...

within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. The evidence of phenotypic plasticity of Arctic tadpole shrimp ('' Lepidurus arcticus'', Notostraca) has been observed in Svalbard. Notostracans are the largest branchiopodans and are

omnivore An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nut ...

s living on the bottom of

temporary pool Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe ...

s, ponds and shallow lakes.

Laevicaudata, Spinicaudata and Cyclestherida (once Conchostraca)

Clam shrimp are bivalved animals which have lived since at least the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

. The three groups are not believed to form a clade. They have 10–32 trunk segments, decreasing in size from front to back, and each bears a pair of

legs A leg is a weight-bearing and animal locomotion, locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single ...

which also carry

s. A strong muscle can close the two halves of the shell together.

Anomopoda, Ctenopoda, Onychopoda, and Haplopoda (once Cladocera)

These four

s make up a group of small

s commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed. They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are long, with a down-turned head, and a carapace covering the apparently unsegmented thorax and abdomen. There is a single median

. 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.

Evolution

The

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

of branchiopods extends back at least into the

Upper Cambrian The Furongian is the fourth and final epoch and series of the Cambrian. It lasted from to million years ago. It succeeds the Miaolingian series of the Cambrian and precedes the Lower Ordovician Tremadocian Stage. It is subdivided into three sta ...

and possibly further. The group is thought to be

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic ...

, with the

having been the first group to branch off. It is thought that the group evolved in the seas, but was forced into temporary pools and

s by the evolution of

bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartila ...

. Although they were previously considered the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to the remaining crustaceans, it is now widely accepted that crustaceans form a paraphyletic group, and Branchiopoda are thought to be sister to a clade comprising Xenocarida ( Remipedia and Cephalocarida) and

Hexapoda The subphylum Hexapoda (from Greek for 'six legs') comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered in ...

(

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs ...

s and their relatives).

References

{{Authority control Arthropod classes Extant Cambrian first appearances Taxa named by Pierre André Latreille