Notostraca
Encyclopedia
The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp or shield shrimp. The two genera, Triops
and Lepidurus
, are considered living fossil
s, having not changed significantly in outward form since the Triassic
. 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
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
s living on the bottom of temporary pools and shallow lakes.
at the front end, and a long, slender abdomen. This gives them a similar overall shape to a tadpole
, from which the common name
tadpole shrimp derives. The carapace is dorso-ventrally flattened, smooth, and bears no rostrum
; it includes the head, and the two sessile compound eyes 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 comprise a pair of uniramous mandible
s and no maxillipeds.
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
and bear one pair of legs
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 telson
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 Triops
there is no such projection.
, some showing self-fertilisation of females, and some showing a mix of the two. The frequency of males in populations is therefore highly variable. In sexual population, the sperm
leave the male's body through simple pores, there being no penis
. The eggs are released by the female and then held in the cup-like brood pouch. The eggs are only retained by the female for a short time before being laid, and the larvae
develop directly, without passing through a metamorphosis
.
, eating small animals such as fish
es and fairy shrimp
. They are found worldwide in freshwater
, brackish water
or saline
pools, as well as in shallow lake
s, peat bogs and moorland
. In California
n rice paddies
, the species Triops longicaudatus
is considered a pest, because it stirs up sediment, preventing light from reaching the rice seedlings.
. The lack of major morphological change since has led to Notostraca being described as living fossil
s. Kazacharthra
, a group known only from Jurassic
fossils from Kazakhstan
, are closely related to notostracans, and may belong within the order Notostraca.
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 condition of having two separate compound eyes, which abut, but have not become united, as seen in other groups of Branchiopoda.
Notostraca is composed of a single family
, Triopsidae, and only two genera
, Triops
and Lepidurus
.
The phenotypic plasticity
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, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations.
Triops
Triops is a genus of small crustaceans in the order Notostraca . They are considered living fossils, with a fossil record that reaches back to the Carboniferous, .-Relatives and fossil record:...
and Lepidurus
Lepidurus
Lepidurus is the larger of the two genera of tadpole shrimp. It contains the following species:*Lepidurus apus *Lepidurus arcticus *Lepidurus batesoni Longhurst, 1955*Lepidurus bilobatus Packard, 1883...
, are considered living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
s, having not changed significantly in outward form since the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
. 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
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
is flanked by a pair of long, thin caudal rami. Phenotypic plasticity
Phenotypic plasticity
Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...
within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. Notostracans are omnivore
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
s living on the bottom of temporary pools and shallow lakes.
Description
Notostracans are 2–10 cm (0.78740157480315–3.9 ) long, with a broad carapaceCarapace
A carapace is a dorsal 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 underside is called the plastron.-Crustaceans:In crustaceans, the...
at the front end, and a long, slender abdomen. This gives them a similar overall shape to a tadpole
Tadpole
A tadpole or polliwog is the wholly aquatic larval stage in the life cycle of an amphibian, particularly that of a frog or toad.- Appellation :...
, from which the common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
tadpole shrimp derives. The carapace is dorso-ventrally flattened, smooth, and bears no rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
; it includes the head, and the two sessile compound eyes are located together on top of the head. The two pairs of antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
are much reduced, with the second pair sometimes missing altogether. The mouthparts comprise a pair of uniramous mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
s and no maxillipeds.
The trunk comprises a large number of "body rings", which appear to be body segments
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
, but do not always reflect the underlying segmentation. The first eleven body rings compose the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
and bear one pair of legs
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...
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 telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
and a pair of long, thin, multi-articulate caudal rami. The form of the telson varies between the two genera: in Lepidurus
Lepidurus
Lepidurus is the larger of the two genera of tadpole shrimp. It contains the following species:*Lepidurus apus *Lepidurus arcticus *Lepidurus batesoni Longhurst, 1955*Lepidurus bilobatus Packard, 1883...
, a rounded projection extends between the caudal rami, while in Triops
Triops
Triops is a genus of small crustaceans in the order Notostraca . They are considered living fossils, with a fossil record that reaches back to the Carboniferous, .-Relatives and fossil record:...
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 sexuallySexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
, some showing self-fertilisation of females, and some showing a mix of the two. The frequency of males in populations is therefore highly variable. In sexual population, the sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
leave the male's body through simple pores, there being no penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...
. The eggs are released by the female and then held in the cup-like brood pouch. The eggs are only retained by the female for a short time before being laid, and the larvae
Crustacean larvae
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow...
develop directly, without passing through a metamorphosis
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
.
Ecology and distribution
Notostracans are omnivorousOmnivore
Omnivores are species that eat both plants and animals as their primary food source...
, eating small animals such as fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
es and fairy shrimp
Fairy shrimp
Anostraca is one of the four orders of crustaceans in the class Branchiopoda; its members are also known as fairy shrimp. They are usually long . Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia , and the body lacks a carapace...
. They are found worldwide in freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
, brackish water
Brackish water
Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...
or saline
Saline water
Saline water is a general term for water that contains a significant concentration of dissolved salts . The concentration is usually expressed in parts per million of salt....
pools, as well as in shallow lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
s, peat bogs and moorland
Moorland
Moorland or moor is a type of habitat, in the temperate grasslands, savannas, and shrublands biome, found in upland areas, characterised by low-growing vegetation on acidic soils and heavy fog...
. In California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
n rice paddies
Paddy field
A paddy field is a flooded parcel of arable land used for growing rice and other semiaquatic crops. Paddy fields are a typical feature of rice farming in east, south and southeast Asia. Paddies can be built into steep hillsides as terraces and adjacent to depressed or steeply sloped features such...
, the species Triops longicaudatus
Triops longicaudatus
Triops longicaudatus is a freshwater crustacean of the order Notostraca, resembling a miniature horseshoe crab. It is characterized by an elongated, segmented body, flattened shield-like brownish carapace covering two thirds of the thorax and two long filaments on the abdomen...
is considered a pest, because it stirs up sediment, preventing light from reaching the rice seedlings.
Evolution and fossil record
The fossil record of Notostraca is extensive, occurring in a wide range of geological deposits, and reaching back to the CarboniferousCarboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
. The lack of major morphological change since has led to Notostraca being described as living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
s. Kazacharthra
Kazacharthra
Kazacharthra is an extinct order of branchiopod crustaceans, closely related to the living order Notostraca . Kazacharthrans lived in marshes and ponds in the Upper Triassic of Western China and Mongolia, and in Lower Jurassic Kazakhstan...
, a group known only from Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
fossils from Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...
, are closely related to notostracans, and may belong within the order Notostraca.
The "central autapomorphy" of the Notostraca is the abandonment of filter feeding
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
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
Adduction
Adduction is a movement which brings a part of the anatomy closer to the middle sagittal plane of the body. It is opposed to abduction.-Upper limb:* of arm at shoulder ** Subscapularis** Teres major** Pectoralis major** Infraspinatus...
is missing. Notostracans retain the plesiomorphic condition of having two separate compound eyes, which abut, but have not become united, as seen in other groups of Branchiopoda.
Taxonomy
The orderOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Notostraca is composed of a single family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, Triopsidae, and only two genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, Triops
Triops
Triops is a genus of small crustaceans in the order Notostraca . They are considered living fossils, with a fossil record that reaches back to the Carboniferous, .-Relatives and fossil record:...
and Lepidurus
Lepidurus
Lepidurus is the larger of the two genera of tadpole shrimp. It contains the following species:*Lepidurus apus *Lepidurus arcticus *Lepidurus batesoni Longhurst, 1955*Lepidurus bilobatus Packard, 1883...
.
The phenotypic plasticity
Phenotypic plasticity
Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...
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
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
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, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations.