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Pancrustacea is the
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
that comprises all
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 and hexapods. This grouping is contrary to the
Atelocerata Atelocerata is a proposed clade of arthropods that includes Hexapoda (insects and a few related taxa) and Myriapoda (millipedes, centipedes, and similar taxa), but excludes Crustacea (such as shrimp and lobsters) and Chelicerata (such as spiders an ...
hypothesis, in which
Myriapoda Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. The fossil record of myriapods reaches back into the late Silurian, a ...
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 inse ...
are
sister taxa 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 t ...
, and
Crustacea 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 ...
are only more distantly related. As of 2010, the Pancrustacea taxon is considered well accepted, with most studies recovering Hexapoda within Crustacea. The clade has also been called Tetraconata, referring to having four cone cells in the
ommatidia The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The ou ...
. This name is preferred by some scientists as a means of avoiding confusion with the use of "pan-" to indicate a clade that includes a
crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
and all of its stem group representatives.


Molecular studies

A
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 gro ...
Pancrustacea has been supported by several molecular studies, in most of which the subphylum Crustacea is
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
with regard to hexapods (that is, that hexapods, including insects, are derived from crustacean ancestors). The evidence for this clade derives from molecular data and morphological characteristics. The molecular data consists of comparisons of
nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: * Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space *Nuclear ...
ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s,
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
l ribosomal RNA genes, and
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
coding genes. The morphological data consists of
ommatidia The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The ou ...
l structures (see ''
arthropod eye Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum. Some annelids and bivalves also h ...
''), the presence of
neuroblasts In vertebrates, a neuroblast or primitive nerve cell is a postmitotic cell that does not divide further, and which will develop into a neuron after a migration phase. In invertebrates such as ''Drosophila,'' neuroblasts are neural progenitor cells ...
, and the form and style of
axonogenesis Axon guidance (also called axon pathfinding) is a subfield of neural development concerning the process by which neurons send out axons to reach their correct targets. Axons often follow very precise paths in the nervous system, and how they mana ...
by pioneer
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
s.


Regier ''et al.'' (2005)

In a 2005 study of nuclear genomes Regier ''et al.'' suggest that
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 inse ...
is most closely related to
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. Description M ...
and
Cephalocarida The Cephalocarida are a class in the subphylum Crustacea comprising only 12 benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimp. They have been grouped together with the Remiped ...
+
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
, thereby hexapods are "terrestrial crustaceans", thus supporting the Pancrustacea hypothesis that maxillopods are not monophyletic (in the following cladograms Maxillopoda subclasses are highlighted). In addition, there appeared some evidence against the
Ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...
a monophyly: that Ostracoda subclass
Podocopa The Podocopa are a subclass of ostracods. Members of the subclass Podocopa can be differentiated from the other subclass of ostracods (Myodocopa) by the morphology of the second antenna: the Podocopa have a relatively long endopod, whereas the My ...
may form a clade with
Branchiura The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family, ...
.


Regier ''et al.'' (2010)

A 2010 study of nuclear genomes (Regier ''et al.'') strongly supports Pancrustacea and strongly favour
Mandibulata Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda (millipedes and others), Crustacea and Hexapoda (insects and others). Mandibulata is currently believed to be the sister group of the clade ...
(
Myriapoda Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. The fossil record of myriapods reaches back into the late Silurian, a ...
+ Pancrustacea) over
Paradoxopoda The Myriochelata or Paradoxopoda, is a proposed grouping of arthropods comprising the Myriapoda (including millipedes and centipedes) and Chelicerata (including spiders and scorpions). If this proposition holds true, the Myriochelata are the sister ...
(Myriapoda +
Chelicerata The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including Opiliones, harvestmen, scorpions, spiders, Solifugae, solif ...
). According to this study, Pancrustacea is divided into four lineages: Oligostraca (
Ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...
a,
Mystacocarida Derocheilocarididae is a family of crustaceans that form part of the meiobenthos. It is the only family in the monotypic order Mystacocaridida, and the monotypic subclass Mystacocarida. These mystacocarids are less than long, and live interstiti ...
,
Branchiura The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family, ...
,
Pentastomida The Pentastomida are an enigmatic group of parasitic arthropods commonly known as tongue worms due to the resemblance of the species of the genus ''Linguatula'' to a vertebrate tongue; molecular studies point to them being degenerate crustaceans. ...
), Vericrustacea (
Malacostraca Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobst ...
,
Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constit ...
,
Copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
a,
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. Description M ...
),
Xenocarida Xenocarida (from the Greek for ''strange shrimp'') is a proposed clade inside the subphylum Crustacea that comprises two classes that were discovered in the 20th century: Remipedia and Cephalocarida. The clade was recovered as the sister groups ...
(
Cephalocarida The Cephalocarida are a class in the subphylum Crustacea comprising only 12 benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimp. They have been grouped together with the Remiped ...
,
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
) 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 inse ...
, with Xenocarida as a sister group to the Hexapoda (comprising "Miracrustacea"). New clades proposed by Regier ''et al.'' are: *Vericrustacea (''"true crustaceans"'') —
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. Description M ...
,
Copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
a,
Malacostraca Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobst ...
,
Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constit ...
; *Multicrustacea (''"numerous crustaceans"'') —
Copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
a,
Malacostraca Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobst ...
,
Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constit ...
; *Communostraca (''"common shelled ones"'') —
Malacostraca Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobst ...
,
Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constit ...
; *Miracrustacea (''"surprising crustaceans"'') —
Cephalocarida The Cephalocarida are a class in the subphylum Crustacea comprising only 12 benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimp. They have been grouped together with the Remiped ...
,
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
,
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 inse ...
; *
Xenocarida Xenocarida (from the Greek for ''strange shrimp'') is a proposed clade inside the subphylum Crustacea that comprises two classes that were discovered in the 20th century: Remipedia and Cephalocarida. The clade was recovered as the sister groups ...
(''"strange shrimp"'') —
Cephalocarida The Cephalocarida are a class in the subphylum Crustacea comprising only 12 benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimp. They have been grouped together with the Remiped ...
,
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
. Of these proposed clades, only Multicrustacea was confirmed in later molecular studies.


von Reumont ''et al.'' (2012)

In a 2012 molecular study, von Reumont ''et al.'' challenge the monophyly of Vericrustacea: they present four versions of Pancrustacea cladogram (figures 1–4), and in all four figures
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
is a sister group to
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 inse ...
, and
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. Description M ...
is a sister group to (Remipedia + Hexapoda). Thus, their data strongly suggest that Branchiopoda is more closely related to Hexapoda and Remipedia than to Multicrustacea. Based on these data, they propose the following scenario of evolution of Branchiopoda, Remipedia and Hexapoda: under the impact of predatory fishes their common ancestors go to the
littoral zone The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal areas ...
, then ancestors of Branchiopoda go to the ephemeral freshwater habitat, whereas ancestors of Remipedia go to the
anchialine cave An anchialine system (, from Greek ''ankhialos'', "near the sea") is a landlocked body of water with a subterranean connection to the ocean. Depending on its formation, these systems can exist in one of two primary forms: pools or caves. The primary ...
, and ancestors of Hexapoda go to the
land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other bodies of water. It makes up 29% of Earth's surface and includes the continents and various islan ...
.


Jondeung ''et al.'' (2012)

Another molecular study (of mitochondrial genomes), conducted in 2012 by Jondeung ''et al.'', strongly support monophyletic Pancrustacea and places
Malacostraca Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobst ...
+
Entomostraca Entomostraca is a historical subclass of crustaceans, no longer in technical use. It was originally considered one of the two major lineages of crustaceans (the other being the class Malacostraca), combining all other classes—Branchiopoda, Cepha ...
and
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. Description M ...
as the sister clade to Hexapoda and places
Cirripedia A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive ...
+
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
as a basal lineage of Pancrustacea.


Oakley ''et al.'' (2013)

In 2013 combined study of morphology, including fossils, and molecular data, including expressed sequence tag, mitochondrial genome, nuclear genome, and ribosomal DNA data Oakley ''et al.'' obtained support for three pancrustacean clades: Oligostraca (Ostracoda, Mystacocarida, Branchiura, Pentastomida), Multicrustacea (Copepoda, Thecostraca, Malacostraca) and a clade they refer to as Allotriocarida (Branchiopoda, Cephalocarida, Remipedia, Hexapoda), as well as for monophyly of Ostracoda. Within Multicrustacea they obtained support for a clade they suggest the name
Hexanauplia The Hexanauplia is a clade proposed by Oakley et al. (2013) that constitutes a class of crustaceans, comprising the Copepoda and Thecostraca. A number of recent phylogenomic studies have not found support for this clade. Classification This t ...
: Thecostraca + Copepoda. Relations within Allotriocarida remain uncertain: sister taxon to Hexapoda is either Remipedia, or the clade Branchiopoda + Cephalocarida, however, authors are inclined to the first version (see "Conclusion", 4), which is also consistent with von Reumont ''et al.'' (2012) results. New proposed by Oakley ''et al.'' clades are: *Hexanauplia (refers to six (''"hexa-"'') naupliar molts) —
Copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
a,
Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constit ...
; *Allotriocarida (''"allotrios"'' is ''"strange"'', ''"carida"'' is ''"shrimp"'') —
Cephalocarida The Cephalocarida are a class in the subphylum Crustacea comprising only 12 benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimp. They have been grouped together with the Remiped ...
,
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. Description M ...
,
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
,
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 inse ...
. Note: the Allotriocarida clade was also recovered in 2005 by Regier ''et al.'' as Clade #33, but relations within it were different, and they did not choose a name for it.


Rota-Stabelli ''et al.'' (2013)

In 2013 Rota-Stabelli ''et al.'' used the signal in the 62 protein-coding genes assembled by Regier ''et al.'' in 2010 to improve the knowledge of the internal relationship in the Pancrustacea group. This data set infers a highly supported
nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...
tree that is substantially different from the corresponding, but poorly supported,
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
one. The discrepancy between the nucleotide-based and the amino acids-based trees is caused by substitutions within synonymous
codon The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
families (especially those of
serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
-TCN and AGY): different arthropod lineages are differentially biased in their usage of
serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
,
arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) and both the am ...
, and
leucine Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α- ca ...
synonymous codons, and the serine bias is correlated with the topology derived from the nucleotides, but not the amino acids. The authors suggest that a parallel, partially compositionally driven, synonymous codon-usage bias affects the nucleotide topology. As substitutions between serine codon families can proceed through
threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COOâ ...
or
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...
intermediates, amino acid data sets might also be affected by the serine codon-usage bias. The analyses suggests that a '' Dayhoff recoding strategy'' would partially ameliorate the effects of such bias. Although amino acids provide an alternative hypothesis of pancrustacean relationships, neither the nucleotides nor the amino acids version of this data set bring enough genuine phylogenetic information to robustly resolve the relationships within group, which should still be considered unresolved. However the amino acid tree seems to be more likely since it appears to be free from the synonymous codon-family bias affecting the nucleotide one. Most of the inferences based on amino acids sequences support a clade which includes
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. Description M ...
,
Remipedia Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
,
Copepoda Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...
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 inse ...
(group A). Using the best amino acids substitution model, ''CATGTR'', also
Cephalocarida The Cephalocarida are a class in the subphylum Crustacea comprising only 12 benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimp. They have been grouped together with the Remiped ...
falls inside this group. In all the analyses group A (with or without Cephalocarida) is sister-group of a clade composed by
Malacostraca Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobst ...
, Oligostraca and
Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constit ...
(group B). The following image shows the tree resulting from the ''Dayhoff recoding''.


Lozano-Fernandez ''et al.'' (2019)

The relationship of Hexapoda and the crustacean classes is shown in the following phylogenetic tree, which shows Allotriocarida, along with
Oligostraca Oligostraca is a superclass of crustaceans. It includes the seed shrimp, mystacocarids, branchiura The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is ...
and
Multicrustacea The clade Multicrustacea constitutes the largest superclass of crustaceans, containing approximately four-fifths of all described crustacean species, including crabs, lobsters, crayfish, shrimp, krill, prawns, woodlice, barnacles, copepods, amp ...
, as the three main divisions of subphylum Pancrustacea, embracing the traditional crustaceans and the hexapods (including insects).


Position of Tantulocarida

According to Petrunina A.S. and Kolbasov G.A., the sixth subclass of Maxillopoda
Tantulocarida Tantulocarida is a highly specialised group of parasitic crustaceans that consists of about 33 species, treated as a class in superclass Multicrustacea. They are typically ectoparasites that infest copepods, isopods, tanaids, amphipods and ostr ...
may lie within
Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constit ...
, forming a clade with thecostracan infraclass
Cirripedia A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive ...
(if so, Thecostraca excluding Tantulocarida is paraphyletic):


See also

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Crustaceomorpha Crustaceomorpha is a proposed clade of arthropods that includes crustaceans and numerous extinct groups. Synapomorphies for the clade are that the larval antenna is a feeding or locomotory organ, and there are six endopodal podomeres in post- ...
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Mandibulata Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda (millipedes and others), Crustacea and Hexapoda (insects and others). Mandibulata is currently believed to be the sister group of the clade ...


References

{{Taxonbar, from=Q671280 Arthropod taxonomy