The order Notostraca, containing the single family Triopsidae, is a group of

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

s known as 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, with similar forms having existed since 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, wh ...

. They have a broad, flat

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 ...

, 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 acco ...

is flanked by a pair of long, thin caudal rami. Phenotypic plasticity within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. Notostracans 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 nu ...

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 saf ...

s and shallow lakes.

Description

Notostracans are long, with a broad

at the front end, and a long, slender abdomen. This gives them a similar overall shape to a

tadpole A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found ...

, from which the

common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...

''tadpole shrimp'' derives. The carapace is dorso-ventrally flattened, smooth, and bears no rostrum; it includes the head, and the two sessile

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 ...

s are located together on top of the head. The two pairs of antennae are much reduced, with the second pair sometimes missing altogether. The

mouthparts Mouthparts may refer to: * The parts of a mouth ** Arthropod mouthparts The mouthparts of arthropods have evolved into a number of forms, each adapted to a different style or mode of feeding. Most mouthparts represent modified, paired append ...

comprise a pair of uniramous mandibles and no maxillipeds. Triops australiensis belly

The trunk comprises a large number of "body rings", which appear to be body segments, but do not always reflect the underlying segmentation. The first eleven body rings compose 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 c ...

and bear one pair of

legs A leg is a weight-bearing and 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, linear element c ...

each, the last of which also bears the genital opening. In the female, it is modified to form a "brood pouch". The first one or two pairs of legs differ from the remainder, and probably function as sensory organs. The remaining segments form the abdomen. The number of body rings is variable both within and between species, and the number of pairs of legs per body ring can rise as high as six. The legs become progressively smaller along the abdomen, with the last segments being legless. The abdomen ends in a

and a pair of long, thin, multi-articulate caudal rami. The form of the telson varies between the two genera: in '' Lepidurus'', a rounded projection extends between the caudal rami, while in ''

'' there is no such projection.

Life cycle

Within the Notostraca, and even within species, there is variation in the mode of reproduction, with some populations reproducing sexually, some showing

self-fertilisation Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species ...

of females, and some showing a mix of the two. The frequency of males in populations is therefore highly variable. In sexual populations, the

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...

leave the male's body through simple pores, there being no

penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males d ...

. The eggs are released by the female and then held in the cup-like brood pouch. The eggs are retained by the female only for a short time before being laid, and the

larvae A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. T ...

develop directly, without passing through a

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

Ecology and distribution

Notostracans are

omnivorous 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 ...

, eating small animals such as

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

es and

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 mo ...

. They are found worldwide in

freshwater 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 ...

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

, or saline pools, as well as in shallow

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

s, peat bogs, and

moorland Moorland or moor is a type of habitat found in upland areas in temperate grasslands, savannas, and shrublands and montane grasslands and shrublands biomes, characterised by low-growing vegetation on acidic soils. Moorland, nowadays, generall ...

. The species ''

Triops longicaudatus ''Triops longicaudatus'' (commonly called American tadpole shrimp or longtail tadpole shrimp) is a freshwater crustacean of the order Notostraca, resembling a miniature horseshoe crab. It is characterized by an elongated, segmented body, a flatten ...

'' is considered an agricultural pest in

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

rice paddies, because it prevents light from reaching the rice seedlings by stirring up sediment.

Evolution and fossil record

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 Notostraca is extensive, occurring in a wide range of geological deposits. The oldest known notostracan is the species '' Strudops goldenbergi'' from the Late

(

Famennian The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used ...

~ 365 million years ago) of Belgium. The lack of major morphological change since has led to Notostraca being described as

s. Kazacharthra, a group known only from

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 per ...

and

Jurassic The Jurassic ( ) is a geologic period and 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 Jurassic constitutes the middle period of ...

fossils from

Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a transcontinental country located mainly in Central Asia and partly in Eastern Europe. It borders Russia to the north and west, China to the east, Kyrgyzstan to the southeast, Uzbeki ...

and Western

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

, are closely related to notostracans, and may belong within the order Notostraca, or alternatively are placed as their sister group within the clade Calmanostraca. The "central autapomorphy" of the Notostraca is the abandonment of filter feeding in open water, and the development of a benthic lifestyle in muddy waters, taking up food from particles of sediment and preying on small animals. A number of other characteristics are correlated with this change, including the increased size of the animal compared to its relatives, and the loss of the ability to hinge the carapace; although a central keel marks the former separation into two valves, the adductor muscle is missing. Notostracans retain the

plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

condition of having two separate compound eyes, which abut, but have not become united, as seen in other groups of Branchiopoda.

Taxonomy

The extant members of

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 ...

Notostraca composed a single

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

, Triopsidae, with only two

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

, ''

'' and '' Lepidurus''. The problematic Middle Ordovician fossil '' Douglasocaris'' has been erected and placed in its own family Douglasocaridae by Caster & Brooks 1956, and may be ancestral to Notostraca. The

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 ...

shown by notostracan species make identification to the species level difficult. Many putative species have been described based on morphological variation, such that by the 1950s, as many as 70 species were recognised. Two important revisions – those of Linder in 1952 and Longhurst in 1955 – synonymised many taxa, and resulted in the recognition of only 11 species in the two genera. This taxonomy was accepted for decades, "even attaining the status of dogma". More recent studies, especially those employing

molecular phylogenetics Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations.

Genera list

* '' Strudops'' Strud locality, Belgium, late Devonian (Fammenian) * '' Chenops''

Yixian Formation The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its exq ...

, China, Early Cretaceous (

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ag ...

) * '' Jeholops'' Yixian Formation, China, Early Cretaceous (Aptian) * '' Weichangiops'' Dabeigou Formation, China, Early Cretaceous * '' Brachygastriops'' Dabeigou Formation, China, Early Cretaceous * '' Lynceites'' Germany, Canada, Carboniferous * '' Xinjiangiops'' Kelamayi Formation, China, Middle Triassic ''Incertae sedis'' species * ''"Notostraca" minor'' (often referred to as ''Triops cancriformis minor,'' or "''Triops" minor'' in historic literature)

Hassberge Formation The Hassberge Formation is a geologic formation in Bavaria, Germany. It preserves fossils dating back to the Carnian stage of the Triassic period. Hassberge Formationat Fossilworks.org Fossil content Insects ;Coleoptera * '' Carabilarva t ...

, Germany, Late Triassic (

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227 m ...

) * ''"Notostraca" oleseni'' Yixian Formation, China, Early Cretaceous (Aptian) * ''"Calmanostraca" hassbergella'' Hassberge Formation, Germany, Late Triassic (Carnian)

References

External links

* * * {{Taxonbar, from=Q3344541 Freshwater crustaceans Extant Carboniferous first appearances Pennsylvanian taxonomic orders Cisuralian taxonomic orders Guadalupian taxonomic orders Lopingian taxonomic orders Early Triassic taxonomic orders Middle Triassic taxonomic orders Late Triassic taxonomic orders Early Jurassic taxonomic orders Middle Jurassic taxonomic orders Late Jurassic taxonomic orders Early Cretaceous taxonomic orders Late Cretaceous taxonomic orders Paleocene taxonomic orders Eocene taxonomic orders Oligocene taxonomic orders Miocene taxonomic orders Pliocene taxonomic orders Pleistocene taxonomic orders Holocene taxonomic orders Crustacean orders