Phylogeny of Malacostraca
Encyclopedia
The phylogeny of Malacostraca is the arrangement of the Malacostraca
classes
in the Crustacea subphylum
and the relationship of the malacostracan orders
.
. Recently molecular studies have attempted to infer the phylogeny of this clade
, resulting in phylogenies which have a limited amount of morphological support
, to resolve a well-supported eumalacostracan phylogeny,it will be necessary to look beyond the most commonly utilized sources of data (nuclear ribosomal and mitochondrial sequences) to obtain a robust tree in the future.
due to several common morphological traits which are present throughout the group and due to molecular studies that have also confirmed it.
William T. Calman in 1904 and 1909 described these common morphological features and introduced the major taxonomic subdivisions of the Malacostraca which are still in use today: he divided the Malacostraca in two subclasses
the Phyllocarida
and the Eumalacostraca
, which is further subdivided into four superorders: Eucarida
, Peracarida
, Hoplocarida
and Syncarida
.
W.T. Calman coined the term caridoid facies for the common eumalacostracan (shrimp-like) features; the most important of these is the constant number of segments in each tagma
: members of this class have five segments in the cephalon, eight thoracic segments (thoracomeres
) and six segments in the pleon
and possess a telson
, which forms a characteristic tail fan when the uropods are present. Many other characteristic features are present but their presence varies amongst lineages; one notable ancestral feature which varies is the carapace
, which may be absent, reduced or well developed covering the whole cephalothorax
. Furthermore, Richter, S., & Scholtz, G. (2001) list five separate unique eumalacostracan features which taken together form a strong argument in favour of the monophyly of the Eumalacostraca.
However debate arises in the relationship between the subdivisions of the Malacostraca, due to the presence of several contrasting features.
The Phyllocarida is a group of about 36 small marine species that are distributed across planet and possess a characteristic large bivalve carapace and an elongated abdomen with no uropods. This group is believed to be the most primitive malacostracan group, because they lack some of the caridoid facies, such as the presence of seven abdominal segments (eight if telson is included). Furthermore one study by Wills, M.A. places them as a sister branch to the Cephalocarida
and basal to a Maxillopoda
+ Eumalacostraca clade and therefore making the Malacostraca paraphyletic
.
The Eumalacostraca — the malacostraca minus the phyllocarida — were subdivided on the basis of several features, although which group is basal is unclear. Several authors, such as Siewin (1963), believe that the Syncarida is the most basal group due to the absence of morphological traits that are present in the remaining eumalacostracans; in addition, the Syncarida are distributed worldwide in reclusive habitats such as interstitial and groundwater, whereas their extensive fossil record shows that they were once marine, implying that the species present today are remnants of a more abundant group.
A second problematic group often attributed to be basal in the Eumalacostraca clade is the Hoplocarida
. This group is composed of 200 species commonly called mantis shrimps, which are found in shallow tropical and subtropical marine habitats that have adapted to a predatorial life thanks to their specialized large second pair of thoracopods (thoracic appendices), raptorial legs, which are used to capture prey, in fact their name is a combination of Greek words meaning “armed shrimp”. Its precise location amongst the Malacostraca is unclear and has been proposed to be a sister group to the remaining eumalacostracans due to its ancient fossil record
The phylogeny of Malacostraca is the arrangement of the Malacostraca
classes
in the Crustacea subphylum
and the relationship of the malacostracan orders
.
. Recently molecular studies have attempted to infer the phylogeny of this clade
,Jarman, S. N., Nicol, S., Elliott, N. G., & McMinn, A. (2000). 28S rDNA Evolution in the Eumalacostraca and the Phylogenetic Position of Krill. Molecular Phylogenetics and Evolution , 17(1), 26–36.Spears, T., DeBry, R., Abele, L., & Chodyla, K. (2005). Peracarid monophyly and interordinal phylogeny inferred from nuclear small-subunit ribosomal DNA sequences (Crustacea: Malacostraca: Peracarida). Proceedings of the biological society of Washinghton , 118(1) , 117-157.Meland, K., & Willassen, E. (2007). The disunity of “Mysidacea” (Crustacea). Molecular Phylogenetics and Evolution , 44, 1083–1104. resulting in phylogenies which have a limited amount of morphological support
,Poore, G. C. (2005). Peracarida: monophyly, relationships and evolutionary. Nauplius , 13(1), 1-27. to resolve a well-supported eumalacostracan phylogeny,it will be necessary to look beyond the most commonly utilized sources of data (nuclear ribosomal and mitochondrial sequences) to obtain a robust tree in the future.
due to several common morphological traits which are present throughout the group and due to molecular studies that have also confirmed it.
William T. Calman in 1904 and 1909 described these common morphological features and introduced the major taxonomic subdivisions of the Malacostraca which are still in use today: he divided the Malacostraca in two subclasses
the Phyllocarida
and the Eumalacostraca
, which is further subdivided into four superorders: Eucarida
, Peracarida
, Hoplocarida
and Syncarida
.Brusca, R., & Brusca, G. (2003). Invertebrates (2nd Edition ed.). Sunderland, Mass. : Sinauer Associates.
W.T. Calman coined the term caridoid facies for the common eumalacostracan (shrimp-like) features; the most important of these is the constant number of segments in each tagma
: members of this class have five segments in the cephalon, eight thoracic segments (thoracomeres
) and six segments in the pleon
and possess a telson
, which forms a characteristic tail fan when the uropods are present. Many other characteristic features are present but their presence varies amongst lineages; one notable ancestral feature which varies is the carapace
, which may be absent, reduced or well developed covering the whole cephalothorax
. Furthermore, Richter, S., & Scholtz, G. (2001)Richter, S., & Scholtz, G. (2001). Phylogenetic analysis of the Malacostraca (Crustacea). J. Zool. Syst. Evol. Research , 39, 113-136. list five separate unique eumalacostracan features which taken together form a strong argument in favour of the monophyly of the Eumalacostraca.
However debate arises in the relationship between the subdivisions of the Malacostraca, due to the presence of several contrasting features.
The Phyllocarida is a group of about 36 small marine species that are distributed across planet and possess a characteristic large bivalve carapace and an elongated abdomen with no uropods. This group is believed to be the most primitive malacostracan group, because they lack some of the caridoid facies, such as the presence of seven abdominal segments (eight if telson is included). Furthermore one study by Wills, M.A.Wills, M. A. (1998). A phylogeny of recent and fossil Crustacea derived from morphological characters. In R. F. Thomas, Arthropod Relatioships (Vol. 55). Chapman and Hall, London. places them as a sister branch to the Cephalocarida
and basal to a Maxillopoda
+ Eumalacostraca clade and therefore making the Malacostraca paraphyletic
.
The Eumalacostraca — the malacostraca minus the phyllocarida — were subdivided on the basis of several features, although which group is basal is unclear. Several authors, such as Siewin (1963), believe that the Syncarida is the most basal group due to the absence of morphological traits that are present in the remaining eumalacostracans; in addition, the Syncarida are distributed worldwide in reclusive habitats such as interstitial and groundwater, whereas their extensive fossil record shows that they were once marine, implying that the species present today are remnants of a more abundant group.
A second problematic group often attributed to be basal in the Eumalacostraca clade is the Hoplocarida
. This group is composed of 200 species commonly called mantis shrimps, which are found in shallow tropical and subtropical marine habitats that have adapted to a predatorial life thanks to their specialized large second pair of thoracopods (thoracic appendices), raptorial legs, which are used to capture prey, in fact their name is a combination of Greek words meaning “armed shrimp”. Its precise location amongst the Malacostraca is unclear and has been proposed to be a sister group to the remaining eumalacostracans due to its ancient fossil record Schram, F. R. (1986). Crustacea. Oxford University Press.
The phylogeny of Malacostraca is the arrangement of the Malacostraca
classes
in the Crustacea subphylum
and the relationship of the malacostracan orders
.
. Recently molecular studies have attempted to infer the phylogeny of this clade
,Jarman, S. N., Nicol, S., Elliott, N. G., & McMinn, A. (2000). 28S rDNA Evolution in the Eumalacostraca and the Phylogenetic Position of Krill. Molecular Phylogenetics and Evolution , 17(1), 26–36.Spears, T., DeBry, R., Abele, L., & Chodyla, K. (2005). Peracarid monophyly and interordinal phylogeny inferred from nuclear small-subunit ribosomal DNA sequences (Crustacea: Malacostraca: Peracarida). Proceedings of the biological society of Washinghton , 118(1) , 117-157.Meland, K., & Willassen, E. (2007). The disunity of “Mysidacea” (Crustacea). Molecular Phylogenetics and Evolution , 44, 1083–1104. resulting in phylogenies which have a limited amount of morphological support
,Poore, G. C. (2005). Peracarida: monophyly, relationships and evolutionary. Nauplius , 13(1), 1-27. to resolve a well-supported eumalacostracan phylogeny,it will be necessary to look beyond the most commonly utilized sources of data (nuclear ribosomal and mitochondrial sequences) to obtain a robust tree in the future.
due to several common morphological traits which are present throughout the group and due to molecular studies that have also confirmed it.
William T. Calman in 1904 and 1909 described these common morphological features and introduced the major taxonomic subdivisions of the Malacostraca which are still in use today: he divided the Malacostraca in two subclasses
the Phyllocarida
and the Eumalacostraca
, which is further subdivided into four superorders: Eucarida
, Peracarida
, Hoplocarida
and Syncarida
.Brusca, R., & Brusca, G. (2003). Invertebrates (2nd Edition ed.). Sunderland, Mass. : Sinauer Associates.
W.T. Calman coined the term caridoid facies for the common eumalacostracan (shrimp-like) features; the most important of these is the constant number of segments in each tagma
: members of this class have five segments in the cephalon, eight thoracic segments (thoracomeres
) and six segments in the pleon
and possess a telson
, which forms a characteristic tail fan when the uropods are present. Many other characteristic features are present but their presence varies amongst lineages; one notable ancestral feature which varies is the carapace
, which may be absent, reduced or well developed covering the whole cephalothorax
. Furthermore, Richter, S., & Scholtz, G. (2001)Richter, S., & Scholtz, G. (2001). Phylogenetic analysis of the Malacostraca (Crustacea). J. Zool. Syst. Evol. Research , 39, 113-136. list five separate unique eumalacostracan features which taken together form a strong argument in favour of the monophyly of the Eumalacostraca.
However debate arises in the relationship between the subdivisions of the Malacostraca, due to the presence of several contrasting features.
The Phyllocarida is a group of about 36 small marine species that are distributed across planet and possess a characteristic large bivalve carapace and an elongated abdomen with no uropods. This group is believed to be the most primitive malacostracan group, because they lack some of the caridoid facies, such as the presence of seven abdominal segments (eight if telson is included). Furthermore one study by Wills, M.A.Wills, M. A. (1998). A phylogeny of recent and fossil Crustacea derived from morphological characters. In R. F. Thomas, Arthropod Relatioships (Vol. 55). Chapman and Hall, London. places them as a sister branch to the Cephalocarida
and basal to a Maxillopoda
+ Eumalacostraca clade and therefore making the Malacostraca paraphyletic
.
The Eumalacostraca — the malacostraca minus the phyllocarida — were subdivided on the basis of several features, although which group is basal is unclear. Several authors, such as Siewin (1963), believe that the Syncarida is the most basal group due to the absence of morphological traits that are present in the remaining eumalacostracans; in addition, the Syncarida are distributed worldwide in reclusive habitats such as interstitial and groundwater, whereas their extensive fossil record shows that they were once marine, implying that the species present today are remnants of a more abundant group.
A second problematic group often attributed to be basal in the Eumalacostraca clade is the Hoplocarida
. This group is composed of 200 species commonly called mantis shrimps, which are found in shallow tropical and subtropical marine habitats that have adapted to a predatorial life thanks to their specialized large second pair of thoracopods (thoracic appendices), raptorial legs, which are used to capture prey, in fact their name is a combination of Greek words meaning “armed shrimp”. Its precise location amongst the Malacostraca is unclear and has been proposed to be a sister group to the remaining eumalacostracans due to its ancient fossil record Schram, F. R. (1986). Crustacea. Oxford University Press.Watling, L., Hof, C. H., & Schram, F. R. (2000). The Place of the Hoplocarida in the Malacostracan Pantheon. Journal of Crustacean Biology , (20)2, 1-11. but it has also been placed either sister to the Eucarida or even inside the Eucarida by molecular studies.
In fact, the Malacostracan has a well-documented fossil record, that, although patchy or missing entirely (ghost lineage
) for certain clades, offer a unique opportunity to analyse the morphology of the ancestral taxa of a clade or a dead-end sister taxa (plesion
), whose age (determined by its stratigraphy
) gives an estimate of how long has a group been around. However, the major limitation to fossilized samples is that typically the soft parts do not fossilize and are therefore lost, as a consequence a much more limited amount of information that can be gathered. Furthermore some taxa may not fossilize well, and therefore leave no trace even though they existed, when this occurs in the fossil record, the period were the taxa are expected to appear is called a ghost range.
and the Amphionidacea.
The members of the Euphausiacea are commonly called krill
and are all marine shrimp-like species whose pleopods (abdominal appendages) function as swimmerets, they swarm and mostly feed on plankton
, this group is composed of only 90 species, but some of these are one of the most abundant species on the planet, in fact, it is estimated that the biomass of the Antarctic krill
Euphausia superba is 500 million tons.
The Decapoda is a group with 18,000 species which have 5 pairs of thoracopods and a well-developed carapace that covers the gills (which are exposed in krill). Many of these species have common names and are often eaten. The decapods are further subdivided on the basis of the gill structure into two suborders Dendrobranchiata (prawns) and Pleocyemata
, which is further subdivided into several infraorders, such as the Caridea (true shrimps), the Stenopodidea
(boxer shrimp) and the Anomura
and the Brachyura (Crabs) and so forth, although some authors use alternative groupings for these three, Eukyphida, Euzygida and Reptantia (crabs and other decapods), respectively.
In addition, there is an enigmatic eucarid species, Amphionides reynaudii, which is the sole representative of its order, but due to the loss of several features resulting from its small size, its classification has been unclear.
, size and shape and contains 21,500 species, but this number is a gross underestimate as the number of described species has tripled in the past 20 years. Most authors studying morphological characters propose a monophyletic Peracarida which forms a well supported subtree
that is sister to a Eucarida subtree, one paper is an exception and proposes that the Peracarida is derived from a polyphyletic Eucarida.
With the exception of thermosbaenacean species
, a characteristic of the members of this group is that they brood their young in a marsupium formed by branches (endites) of their thoracopods, called oostegites. The Peracarida is divided into nine orders (Isopoda
, Amphipoda
, Mysida, Lophogastrida
, Cumacea
, Tanaidacea
, Mictacea
, Thermosbaenacea
and Spelaeogriphacea
), although some authors prefer to unite two pairs of orders with similar organisms, which are the Mysidacea
, formed by the Mysida and the Lophogastrida, and the Edriophthalma
, formed by the Isopoda and the Amphipoda.
The members of the Mysida and the Lophogastrida have several common features: they are shrimp-like with compound stalked eyes and have a carapace which covers most of the thorax but does not fuse with the last four thoracic segments (as instead is seen in the Eucarida), they possess well-developed thoracopods (for swimming) and tail fan, they also have similar behaviour (swarming) and foregut structure; Pygocephalomorph, a fossil from the Permian
, appears similar to the Lophogastrida, which in addition to other factors, has traditionally allowed the Lophogastrida to be identified as the more primitive Mysidacea. However, the monophyly of the Mysidacea been recently disputed, due to molecular data and several differences, one profound difference between the two taxa is that in the Mysida the carapace acts as a respiratory surface due to the absence of gills, which are however present in the Lophogastrida. When they are considered together many authors have put Mysidacea basal to the remaining peracarids ((paraphyletic)Ruppert & Barnes, 1994; (monophyletic)), others have even either grouped them with Euphausiacea to form the Schizopoda
or made them basal to a eucarid subtree (paraphyletic).
The Isopoda and the Amphipoda are two of the largest pericarid groups, they both lack a carapace, possess sessile compound eyes and lack a sharp demarcation between thoracic and abdominal segments, but however differ in several features, such as gills.
The Isopoda contains 10,000 species, the organisms are dorsoventrally flattened and occupy not only marine and freshwater habitats, but even terrestrial (woodlice) for which they developed a thickened cuticle and gas exchange organs, allowing them to live even in arid regions.
The Amphipoda is a highly diverse group of 8,000 species, ranging from the Caprellida with a long and narrow body shape (skeleton shrimp
) to the shrimp-like Gammaridea
(scuds and sand hoppers).
The position of the Isopoda and the Amphipoda amongst the Peracarida is also debated, some authors support a derived united group (Edriophthalma) which is either monophyletic or paraphyletic (Wheeler, 1998), others support a basal paraphyletic Isopoda and Amphipoda group (Watling, 1999); however, other authors believe that several features that unite the Isopoda and the Amphipoda are homoplasious and that the two groups reside with different groups: one proposed a basal Amphipoda to a clade formed from Isopoda + Tanaidacea and Cumacea + Mictacea + Speleogriphacea, while some older phylogenetic trees Siewing, R. (1963). Studies in malacostracan morphology: results and problems. In H. B. Whittington, & W. D. Rolfe, Phylogeny and Evolution of Crustacea. (pp. 85-103). Cambridge, Mass: Museum of Comparative Zoology.Pires, A. M. (1987). Potiicoara brasiliensis: a new genus and species of Spelaeogriphacea (Crustacea: Peracarida) from Brasil with a phylogenetic analysis of the Peracarida. Journal of Natural History , 21, 225-238. place the Amphipoda and the Mysidacea basal to the Peracarida (without the Thermosbaenacea) either as a polyphyletic or a monophyletic group, whereas the Isopoda are in a derived clade with Tanaidacea.
Recent molecular studies by Jarman et al. (2000), Spears et al. (2005) and Meland & Willassen (2007) (which was derived from Spears et al., (2005) by adding 22 mysidacean taxa to those 26 taxa) suggest a phylogeny with some elements similar to that proposed by Richter & Scholtz (2001), but disprove the monophyly of both the Edriophthalma and the Mysidacea and do not possess a basal eumalacostracan taxa or a basal peracarid taxa (the Hoplocarida and the Syncarida, respectively in ): The Amphipoda (with Spleogriphacea) form a clade with the Lophogastrida, the Isopoda are in a derived clade with the Cumacea and Tanaidacea, while most importantly the Mysida in all three analyses falls basal to the non-Peracarida subtree, which however has a limited morphological support (Poore, 2005).
The remaining Peracarida orders are the cryptic and either moderately abundant, Cumacea and Tanaidacea, or are extremely rare and relictual, Mictacea, Spelaeogriphacea, and Thermosbaenacea.
There are about 1,600 members of the Cumacea, these are small burrowing crustaceans which have a characteristic large bulbous carapace (covering three thoracic segments) and an elongated abdomen which finishes in a pleotelson with stylus-like uropods, in fact due to their peculiar shape they are sometimes called hooded shrimp. Tanaidacea is a group of 1,500 species which are small burrowing or tube-dwelling crustaceans with a short carapace (covering two thoracic segments) that possess a pair of chelate second thoracopods (gnathopods).
Only three extant and two fossil speleogriphacean species have been found, these are blind cave-dwelling species
with a short carapace (one thoracic segment); while the Mictacea is a group erected only two decades ago, and to date, five species have been found, Mictocaris halope (cave dwelling) and four species in the Hursutiidae family, in the genera Hirsutia (at 1,000 meter depths
) and Thetispelecaris (submarine caves), these blind species lack a carapace but have a well-developed headshield, and have reduced pleopods. This enigmatic group is believed by some authors not to be monophyletic, in fact one author proposed that Mictocaris halope should be grouped with the Spelaeogriphacea, forming the Cosinzeneacea.
The Thermosbaenacea is a group of 11 species found in hot springs
, caves and groundwater
that has the peculiarity that its brooding pouch is formed by its extended carapace and not by modified thoracopod endites as occurs in the remaining peracarids, for this reason some authors have removed this group from the Peracarida and placed it in its own superorder, the Pancarida, basal to the Pericarida.
, molecular studies have not helped a particular evolutionary model based on morphology to become the accepted one, but rather they have added more uncertainty to the accepted phylogenetic relationships: in fact not only they contradicted several morphological studies, but they also questioned the monophyly of the Eucarida and the Peracarida, in particular regarding the positions of the Mysida the Syncarida and the Hoplocarida, furthermore the molecular studies to date are only three, two of which overlap and concentrate on the Peracarida, while the third uses a limited amount of taxa.
Analysis of the mitochondrial genome supports the monophyly of Brachyura (crabs), Caridea (shrimps), Euphausiacea (krill), Penaeidae
(prawns) and Stomatopoda (mantis shrimps).Shen X, Wang H, Wang M, Liu B (2011) The complete mitochondrial genome sequence of Euphausia pacifica (Malacostraca: Euphausiacea) reveals a novel gene order and unusual tandem repeats. Genome It also supports a close relationship between Euphausiacea and Decapoda
.
Malacostraca
Malacostraca is the largest of the six classes of crustaceans, containing over 25,000 extant species, divided among 16 orders. Its members display a greater diversity of body forms than any other class of animals, and include crabs, lobsters, shrimp, krill, woodlice, scuds , mantis shrimp and many...
classes
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
in the Crustacea subphylum
Subphylum
In life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
and the relationship of the malacostracan orders
Order (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...
.
Introduction
Although this class is united by a number of well-defined and documented features, which were recognised a century ago by Calman (1904), the phylogenetic relationship (the evolutionary tree) of the orders which compose this class is unclear due to the vast diversity present in their morphologyMorphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
. Recently molecular studies have attempted to infer the phylogeny of this clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, resulting in phylogenies which have a limited amount of morphological support
Long branch attraction
Long branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...
, to resolve a well-supported eumalacostracan phylogeny,it will be necessary to look beyond the most commonly utilized sources of data (nuclear ribosomal and mitochondrial sequences) to obtain a robust tree in the future.
Features
The Malacostraca is assumed to be monophyleticMonophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
due to several common morphological traits which are present throughout the group and due to molecular studies that have also confirmed it.
William T. Calman in 1904 and 1909 described these common morphological features and introduced the major taxonomic subdivisions of the Malacostraca which are still in use today: he divided the Malacostraca in two subclasses
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
the Phyllocarida
Phyllocarida
Phyllocarida is a subclass of crustaceans, comprising the extant order Leptostraca and the extinct orders Hymenostraca and Archaeostraca....
and the Eumalacostraca
Eumalacostraca
The Eumalacostraca are a subclass of crustaceans, containing almost all living malacostracans, about 40,000 described species. The remaining subclasses are the Phyllocarida and possibly the Hoplocarida or mantis shrimps....
, which is further subdivided into four superorders: Eucarida
Eucarida
Eucarida is a superorder of the Malacostraca, a class of the crustacean subphylum, comprising the decapods, krill and Amphionides. They are characterised by having the carapace fused to all thoracic segments, and by the possession of stalked eyes....
, Peracarida
Peracarida
The superorder Peracarida is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or marsupium, formed from thin flattened plates borne on the basalmost segments of the legs Other...
, Hoplocarida
Hoplocarida
Hoplocarida is a subclass of crustaceans. The only extant members are the mantis shrimp , but two other orders existed in the Palaeozoic: Aeschronectida and Palaeostomatopoda....
and Syncarida
Syncarida
Syncarida is a superorder of crustaceans, comprising the two orders Anaspidacea and Bathynellacea. Fifty-nine genera are known, in six families:Anaspidacea Calman, 1904*Anaspididae Thomson, 1893*Koonungidae Sayce, 1908*Psammaspididae Schminke, 1974...
.
W.T. Calman coined the term caridoid facies for the common eumalacostracan (shrimp-like) features; the most important of these is the constant number of segments in each tagma
Tagma (biology)
In invertebrate biology, a tagma is a specialized grouping of arthropod segments, such as the head, the thorax, and the abdomen with a common function. The segments of a tagma may be either fused or moveable.-Tagmata:...
: members of this class have five segments in the cephalon, eight thoracic segments (thoracomeres
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
) and six segments in the pleon
Pleon
Pleon is a Europe based Public Relations and communications consultancy. It is part of the Brodeur Pleon Worldwide network and a subsidiary of BBDO Worldwide. Pleon belongs to the Omnicom Group. Pleon is headquartered in Düsseldorf, Germany.- Overview :...
and possess 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...
, which forms a characteristic tail fan when the uropods are present. Many other characteristic features are present but their presence varies amongst lineages; one notable ancestral feature which varies is the carapace
Carapace
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...
, which may be absent, reduced or well developed covering the whole cephalothorax
Cephalothorax
The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...
. Furthermore, Richter, S., & Scholtz, G. (2001) list five separate unique eumalacostracan features which taken together form a strong argument in favour of the monophyly of the Eumalacostraca.
However debate arises in the relationship between the subdivisions of the Malacostraca, due to the presence of several contrasting features.
The traditional basal malacostracans
The Phyllocarida is a group of about 36 small marine species that are distributed across planet and possess a characteristic large bivalve carapace and an elongated abdomen with no uropods. This group is believed to be the most primitive malacostracan group, because they lack some of the caridoid facies, such as the presence of seven abdominal segments (eight if telson is included). Furthermore one study by Wills, M.A. places them as a sister branch to the Cephalocarida
Cephalocarida
Cephalocarida is a class inside the subphylum Crustacea that comprises only twelve shrimp-like benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimps. They have been grouped together with the Remipedia in the Xenocarida...
and basal to a Maxillopoda
Maxillopoda
Maxillopoda is a diverse class of crustaceans including barnacles, copepods and a number of related animals. It does not appear to be a monophyletic group, and no single character unites all the members.-Description:...
+ Eumalacostraca clade and therefore making the Malacostraca paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
.
The Eumalacostraca — the malacostraca minus the phyllocarida — were subdivided on the basis of several features, although which group is basal is unclear. Several authors, such as Siewin (1963), believe that the Syncarida is the most basal group due to the absence of morphological traits that are present in the remaining eumalacostracans; in addition, the Syncarida are distributed worldwide in reclusive habitats such as interstitial and groundwater, whereas their extensive fossil record shows that they were once marine, implying that the species present today are remnants of a more abundant group.
A second problematic group often attributed to be basal in the Eumalacostraca clade is the Hoplocarida
Hoplocarida
Hoplocarida is a subclass of crustaceans. The only extant members are the mantis shrimp , but two other orders existed in the Palaeozoic: Aeschronectida and Palaeostomatopoda....
. This group is composed of 200 species commonly called mantis shrimps, which are found in shallow tropical and subtropical marine habitats that have adapted to a predatorial life thanks to their specialized large second pair of thoracopods (thoracic appendices), raptorial legs, which are used to capture prey, in fact their name is a combination of Greek words meaning “armed shrimp”. Its precise location amongst the Malacostraca is unclear and has been proposed to be a sister group to the remaining eumalacostracans due to its ancient fossil record
The phylogeny of Malacostraca is the arrangement of the Malacostraca
Malacostraca
Malacostraca is the largest of the six classes of crustaceans, containing over 25,000 extant species, divided among 16 orders. Its members display a greater diversity of body forms than any other class of animals, and include crabs, lobsters, shrimp, krill, woodlice, scuds , mantis shrimp and many...
classes
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
in the Crustacea subphylum
Subphylum
In life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
and the relationship of the malacostracan orders
Order (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...
.
Introduction
Although this class is united by a number of well-defined and documented features, which were recognised a century ago by Calman (1904),Calman, W. (1904). On the classification of the Crustacea Malacostraca. The Annals of Magazine of Natural History , (7)13, 144-158. the phylogenetic relationship (the evolutionary tree) of the orders which compose this class is unclear due to the vast diversity present in their morphologyMorphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
. Recently molecular studies have attempted to infer the phylogeny of this clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
,Jarman, S. N., Nicol, S., Elliott, N. G., & McMinn, A. (2000). 28S rDNA Evolution in the Eumalacostraca and the Phylogenetic Position of Krill. Molecular Phylogenetics and Evolution , 17(1), 26–36.Spears, T., DeBry, R., Abele, L., & Chodyla, K. (2005). Peracarid monophyly and interordinal phylogeny inferred from nuclear small-subunit ribosomal DNA sequences (Crustacea: Malacostraca: Peracarida). Proceedings of the biological society of Washinghton , 118(1) , 117-157.Meland, K., & Willassen, E. (2007). The disunity of “Mysidacea” (Crustacea). Molecular Phylogenetics and Evolution , 44, 1083–1104. resulting in phylogenies which have a limited amount of morphological support
Long branch attraction
Long branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...
,Poore, G. C. (2005). Peracarida: monophyly, relationships and evolutionary. Nauplius , 13(1), 1-27. to resolve a well-supported eumalacostracan phylogeny,it will be necessary to look beyond the most commonly utilized sources of data (nuclear ribosomal and mitochondrial sequences) to obtain a robust tree in the future.
Features
The Malacostraca is assumed to be monophyleticMonophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
due to several common morphological traits which are present throughout the group and due to molecular studies that have also confirmed it.
William T. Calman in 1904 and 1909 described these common morphological features and introduced the major taxonomic subdivisions of the Malacostraca which are still in use today: he divided the Malacostraca in two subclasses
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
the Phyllocarida
Phyllocarida
Phyllocarida is a subclass of crustaceans, comprising the extant order Leptostraca and the extinct orders Hymenostraca and Archaeostraca....
and the Eumalacostraca
Eumalacostraca
The Eumalacostraca are a subclass of crustaceans, containing almost all living malacostracans, about 40,000 described species. The remaining subclasses are the Phyllocarida and possibly the Hoplocarida or mantis shrimps....
, which is further subdivided into four superorders: Eucarida
Eucarida
Eucarida is a superorder of the Malacostraca, a class of the crustacean subphylum, comprising the decapods, krill and Amphionides. They are characterised by having the carapace fused to all thoracic segments, and by the possession of stalked eyes....
, Peracarida
Peracarida
The superorder Peracarida is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or marsupium, formed from thin flattened plates borne on the basalmost segments of the legs Other...
, Hoplocarida
Hoplocarida
Hoplocarida is a subclass of crustaceans. The only extant members are the mantis shrimp , but two other orders existed in the Palaeozoic: Aeschronectida and Palaeostomatopoda....
and Syncarida
Syncarida
Syncarida is a superorder of crustaceans, comprising the two orders Anaspidacea and Bathynellacea. Fifty-nine genera are known, in six families:Anaspidacea Calman, 1904*Anaspididae Thomson, 1893*Koonungidae Sayce, 1908*Psammaspididae Schminke, 1974...
.Brusca, R., & Brusca, G. (2003). Invertebrates (2nd Edition ed.). Sunderland, Mass. : Sinauer Associates.
W.T. Calman coined the term caridoid facies for the common eumalacostracan (shrimp-like) features; the most important of these is the constant number of segments in each tagma
Tagma (biology)
In invertebrate biology, a tagma is a specialized grouping of arthropod segments, such as the head, the thorax, and the abdomen with a common function. The segments of a tagma may be either fused or moveable.-Tagmata:...
: members of this class have five segments in the cephalon, eight thoracic segments (thoracomeres
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
) and six segments in the pleon
Pleon
Pleon is a Europe based Public Relations and communications consultancy. It is part of the Brodeur Pleon Worldwide network and a subsidiary of BBDO Worldwide. Pleon belongs to the Omnicom Group. Pleon is headquartered in Düsseldorf, Germany.- Overview :...
and possess 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...
, which forms a characteristic tail fan when the uropods are present. Many other characteristic features are present but their presence varies amongst lineages; one notable ancestral feature which varies is the carapace
Carapace
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...
, which may be absent, reduced or well developed covering the whole cephalothorax
Cephalothorax
The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...
. Furthermore, Richter, S., & Scholtz, G. (2001)Richter, S., & Scholtz, G. (2001). Phylogenetic analysis of the Malacostraca (Crustacea). J. Zool. Syst. Evol. Research , 39, 113-136. list five separate unique eumalacostracan features which taken together form a strong argument in favour of the monophyly of the Eumalacostraca.
However debate arises in the relationship between the subdivisions of the Malacostraca, due to the presence of several contrasting features.
The traditional basal malacostracans
The Phyllocarida is a group of about 36 small marine species that are distributed across planet and possess a characteristic large bivalve carapace and an elongated abdomen with no uropods. This group is believed to be the most primitive malacostracan group, because they lack some of the caridoid facies, such as the presence of seven abdominal segments (eight if telson is included). Furthermore one study by Wills, M.A.Wills, M. A. (1998). A phylogeny of recent and fossil Crustacea derived from morphological characters. In R. F. Thomas, Arthropod Relatioships (Vol. 55). Chapman and Hall, London. places them as a sister branch to the Cephalocarida
Cephalocarida
Cephalocarida is a class inside the subphylum Crustacea that comprises only twelve shrimp-like benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimps. They have been grouped together with the Remipedia in the Xenocarida...
and basal to a Maxillopoda
Maxillopoda
Maxillopoda is a diverse class of crustaceans including barnacles, copepods and a number of related animals. It does not appear to be a monophyletic group, and no single character unites all the members.-Description:...
+ Eumalacostraca clade and therefore making the Malacostraca paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
.
The Eumalacostraca — the malacostraca minus the phyllocarida — were subdivided on the basis of several features, although which group is basal is unclear. Several authors, such as Siewin (1963), believe that the Syncarida is the most basal group due to the absence of morphological traits that are present in the remaining eumalacostracans; in addition, the Syncarida are distributed worldwide in reclusive habitats such as interstitial and groundwater, whereas their extensive fossil record shows that they were once marine, implying that the species present today are remnants of a more abundant group.
A second problematic group often attributed to be basal in the Eumalacostraca clade is the Hoplocarida
Hoplocarida
Hoplocarida is a subclass of crustaceans. The only extant members are the mantis shrimp , but two other orders existed in the Palaeozoic: Aeschronectida and Palaeostomatopoda....
. This group is composed of 200 species commonly called mantis shrimps, which are found in shallow tropical and subtropical marine habitats that have adapted to a predatorial life thanks to their specialized large second pair of thoracopods (thoracic appendices), raptorial legs, which are used to capture prey, in fact their name is a combination of Greek words meaning “armed shrimp”. Its precise location amongst the Malacostraca is unclear and has been proposed to be a sister group to the remaining eumalacostracans due to its ancient fossil record Schram, F. R. (1986). Crustacea. Oxford University Press.
The phylogeny of Malacostraca is the arrangement of the Malacostraca
Malacostraca
Malacostraca is the largest of the six classes of crustaceans, containing over 25,000 extant species, divided among 16 orders. Its members display a greater diversity of body forms than any other class of animals, and include crabs, lobsters, shrimp, krill, woodlice, scuds , mantis shrimp and many...
classes
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
in the Crustacea subphylum
Subphylum
In life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
and the relationship of the malacostracan orders
Order (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...
.
Introduction
Although this class is united by a number of well-defined and documented features, which were recognised a century ago by Calman (1904),Calman, W. (1904). On the classification of the Crustacea Malacostraca. The Annals of Magazine of Natural History , (7)13, 144-158. the phylogenetic relationship (the evolutionary tree) of the orders which compose this class is unclear due to the vast diversity present in their morphologyMorphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
. Recently molecular studies have attempted to infer the phylogeny of this clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
,Jarman, S. N., Nicol, S., Elliott, N. G., & McMinn, A. (2000). 28S rDNA Evolution in the Eumalacostraca and the Phylogenetic Position of Krill. Molecular Phylogenetics and Evolution , 17(1), 26–36.Spears, T., DeBry, R., Abele, L., & Chodyla, K. (2005). Peracarid monophyly and interordinal phylogeny inferred from nuclear small-subunit ribosomal DNA sequences (Crustacea: Malacostraca: Peracarida). Proceedings of the biological society of Washinghton , 118(1) , 117-157.Meland, K., & Willassen, E. (2007). The disunity of “Mysidacea” (Crustacea). Molecular Phylogenetics and Evolution , 44, 1083–1104. resulting in phylogenies which have a limited amount of morphological support
Long branch attraction
Long branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...
,Poore, G. C. (2005). Peracarida: monophyly, relationships and evolutionary. Nauplius , 13(1), 1-27. to resolve a well-supported eumalacostracan phylogeny,it will be necessary to look beyond the most commonly utilized sources of data (nuclear ribosomal and mitochondrial sequences) to obtain a robust tree in the future.
Features
The Malacostraca is assumed to be monophyleticMonophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
due to several common morphological traits which are present throughout the group and due to molecular studies that have also confirmed it.
William T. Calman in 1904 and 1909 described these common morphological features and introduced the major taxonomic subdivisions of the Malacostraca which are still in use today: he divided the Malacostraca in two subclasses
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
the Phyllocarida
Phyllocarida
Phyllocarida is a subclass of crustaceans, comprising the extant order Leptostraca and the extinct orders Hymenostraca and Archaeostraca....
and the Eumalacostraca
Eumalacostraca
The Eumalacostraca are a subclass of crustaceans, containing almost all living malacostracans, about 40,000 described species. The remaining subclasses are the Phyllocarida and possibly the Hoplocarida or mantis shrimps....
, which is further subdivided into four superorders: Eucarida
Eucarida
Eucarida is a superorder of the Malacostraca, a class of the crustacean subphylum, comprising the decapods, krill and Amphionides. They are characterised by having the carapace fused to all thoracic segments, and by the possession of stalked eyes....
, Peracarida
Peracarida
The superorder Peracarida is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or marsupium, formed from thin flattened plates borne on the basalmost segments of the legs Other...
, Hoplocarida
Hoplocarida
Hoplocarida is a subclass of crustaceans. The only extant members are the mantis shrimp , but two other orders existed in the Palaeozoic: Aeschronectida and Palaeostomatopoda....
and Syncarida
Syncarida
Syncarida is a superorder of crustaceans, comprising the two orders Anaspidacea and Bathynellacea. Fifty-nine genera are known, in six families:Anaspidacea Calman, 1904*Anaspididae Thomson, 1893*Koonungidae Sayce, 1908*Psammaspididae Schminke, 1974...
.Brusca, R., & Brusca, G. (2003). Invertebrates (2nd Edition ed.). Sunderland, Mass. : Sinauer Associates.
W.T. Calman coined the term caridoid facies for the common eumalacostracan (shrimp-like) features; the most important of these is the constant number of segments in each tagma
Tagma (biology)
In invertebrate biology, a tagma is a specialized grouping of arthropod segments, such as the head, the thorax, and the abdomen with a common function. The segments of a tagma may be either fused or moveable.-Tagmata:...
: members of this class have five segments in the cephalon, eight thoracic segments (thoracomeres
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
) and six segments in the pleon
Pleon
Pleon is a Europe based Public Relations and communications consultancy. It is part of the Brodeur Pleon Worldwide network and a subsidiary of BBDO Worldwide. Pleon belongs to the Omnicom Group. Pleon is headquartered in Düsseldorf, Germany.- Overview :...
and possess 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...
, which forms a characteristic tail fan when the uropods are present. Many other characteristic features are present but their presence varies amongst lineages; one notable ancestral feature which varies is the carapace
Carapace
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...
, which may be absent, reduced or well developed covering the whole cephalothorax
Cephalothorax
The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...
. Furthermore, Richter, S., & Scholtz, G. (2001)Richter, S., & Scholtz, G. (2001). Phylogenetic analysis of the Malacostraca (Crustacea). J. Zool. Syst. Evol. Research , 39, 113-136. list five separate unique eumalacostracan features which taken together form a strong argument in favour of the monophyly of the Eumalacostraca.
However debate arises in the relationship between the subdivisions of the Malacostraca, due to the presence of several contrasting features.
The traditional basal malacostracans
The Phyllocarida is a group of about 36 small marine species that are distributed across planet and possess a characteristic large bivalve carapace and an elongated abdomen with no uropods. This group is believed to be the most primitive malacostracan group, because they lack some of the caridoid facies, such as the presence of seven abdominal segments (eight if telson is included). Furthermore one study by Wills, M.A.Wills, M. A. (1998). A phylogeny of recent and fossil Crustacea derived from morphological characters. In R. F. Thomas, Arthropod Relatioships (Vol. 55). Chapman and Hall, London. places them as a sister branch to the Cephalocarida
Cephalocarida
Cephalocarida is a class inside the subphylum Crustacea that comprises only twelve shrimp-like benthic species. They were discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimps. They have been grouped together with the Remipedia in the Xenocarida...
and basal to a Maxillopoda
Maxillopoda
Maxillopoda is a diverse class of crustaceans including barnacles, copepods and a number of related animals. It does not appear to be a monophyletic group, and no single character unites all the members.-Description:...
+ Eumalacostraca clade and therefore making the Malacostraca paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
.
The Eumalacostraca — the malacostraca minus the phyllocarida — were subdivided on the basis of several features, although which group is basal is unclear. Several authors, such as Siewin (1963), believe that the Syncarida is the most basal group due to the absence of morphological traits that are present in the remaining eumalacostracans; in addition, the Syncarida are distributed worldwide in reclusive habitats such as interstitial and groundwater, whereas their extensive fossil record shows that they were once marine, implying that the species present today are remnants of a more abundant group.
A second problematic group often attributed to be basal in the Eumalacostraca clade is the Hoplocarida
Hoplocarida
Hoplocarida is a subclass of crustaceans. The only extant members are the mantis shrimp , but two other orders existed in the Palaeozoic: Aeschronectida and Palaeostomatopoda....
. This group is composed of 200 species commonly called mantis shrimps, which are found in shallow tropical and subtropical marine habitats that have adapted to a predatorial life thanks to their specialized large second pair of thoracopods (thoracic appendices), raptorial legs, which are used to capture prey, in fact their name is a combination of Greek words meaning “armed shrimp”. Its precise location amongst the Malacostraca is unclear and has been proposed to be a sister group to the remaining eumalacostracans due to its ancient fossil record Schram, F. R. (1986). Crustacea. Oxford University Press.Watling, L., Hof, C. H., & Schram, F. R. (2000). The Place of the Hoplocarida in the Malacostracan Pantheon. Journal of Crustacean Biology , (20)2, 1-11. but it has also been placed either sister to the Eucarida or even inside the Eucarida by molecular studies.
In fact, the Malacostracan has a well-documented fossil record, that, although patchy or missing entirely (ghost lineage
Ghost lineage
A ghost lineage is a phylogenetic lineage that is inferred to exist but has no fossil record....
) for certain clades, offer a unique opportunity to analyse the morphology of the ancestral taxa of a clade or a dead-end sister taxa (plesion
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
), whose age (determined by its stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
) gives an estimate of how long has a group been around. However, the major limitation to fossilized samples is that typically the soft parts do not fossilize and are therefore lost, as a consequence a much more limited amount of information that can be gathered. Furthermore some taxa may not fossilize well, and therefore leave no trace even though they existed, when this occurs in the fossil record, the period were the taxa are expected to appear is called a ghost range.
Eucarida
Eucarida is a diverse and abundant group, whose members have a carapace which is fused to the thoracic segments to form a cephalothorax. The Eucarida is divided into three orders, the Euphausiacea, the DecapodaDecapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...
and the Amphionidacea.
The members of the Euphausiacea are commonly called krill
Krill
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
and are all marine shrimp-like species whose pleopods (abdominal appendages) function as swimmerets, they swarm and mostly feed on plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, this group is composed of only 90 species, but some of these are one of the most abundant species on the planet, in fact, it is estimated that the biomass of the Antarctic krill
Antarctic krill
Antarctic krill, Euphausia superba, is a species of krill found in the Antarctic waters of the Southern Ocean. It is a shrimp-like crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic metre...
Euphausia superba is 500 million tons.
The Decapoda is a group with 18,000 species which have 5 pairs of thoracopods and a well-developed carapace that covers the gills (which are exposed in krill). Many of these species have common names and are often eaten. The decapods are further subdivided on the basis of the gill structure into two suborders Dendrobranchiata (prawns) and Pleocyemata
Pleocyemata
Pleocyemata is a sub-order of decapod crustaceans, erected by Martin Burkenroad in 1963. Burkenroad's classification replaced the earlier sub-orders of Natantia and Reptantia with the monophyletic groups Dendrobranchiata and Pleocyemata...
, which is further subdivided into several infraorders, such as the Caridea (true shrimps), the Stenopodidea
Stenopodidea
The Stenopodidea is a small group of decapod crustaceans. Often confused with shrimp or prawns, they are neither, but belong in a group closer to the reptant decapods, such as lobsters and crabs. They may be easily recognised by their third pereiopod , which is greatly enlarged...
(boxer shrimp) and the Anomura
Anomura
Anomura is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans includes the word crab, all true crabs are in the sister group to the Anomura, the Brachyura .-Description:The name Anomala reflects the unusual variety of forms in this group;...
and the Brachyura (Crabs) and so forth, although some authors use alternative groupings for these three, Eukyphida, Euzygida and Reptantia (crabs and other decapods), respectively.
In addition, there is an enigmatic eucarid species, Amphionides reynaudii, which is the sole representative of its order, but due to the loss of several features resulting from its small size, its classification has been unclear.
Peracarida
The other major malacostracan superorder, the Peracarida, is highly diverse in habitHabit (biology)
Habit, when used in the context of biology, refers to the instinctive actions of animals and the natural tendencies of plants.In zoology, this term most often refers to specific behavioral characteristics, even when directly related to physiology...
, size and shape and contains 21,500 species, but this number is a gross underestimate as the number of described species has tripled in the past 20 years. Most authors studying morphological characters propose a monophyletic Peracarida which forms a well supported subtree
Dendrogram
A dendrogram is a tree diagram frequently used to illustrate the arrangement of the clusters produced by hierarchical clustering...
that is sister to a Eucarida subtree, one paper is an exception and proposes that the Peracarida is derived from a polyphyletic Eucarida.
With the exception of thermosbaenacean species
Thermosbaenacea
Thermosbaenacea is a group of crustaceans that live in thermal springs in fresh water, brackish water and anchialine habitats. They have occasionally been treated as a distinct superorder , but are generally considered to belong to the Peracarida...
, a characteristic of the members of this group is that they brood their young in a marsupium formed by branches (endites) of their thoracopods, called oostegites. The Peracarida is divided into nine orders (Isopoda
Isopoda
Isopods are an order of peracarid crustaceans, including familiar animals such as woodlice and pill bugs. The name Isopoda derives from the Greek roots and...
, Amphipoda
Amphipoda
Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. The name amphipoda means "different-footed", and refers to the different forms of appendages, unlike isopods, where all the legs are alike. Of the 7,000 species, 5,500 are classified...
, Mysida, Lophogastrida
Lophogastrida
Lophogastrida is an order of malacostracan crustaceans in the superorder Peracarida. They are shrimp-like animals that mostly inhabit the relatively deep pelagic waters of the oceans throughout the world....
, Cumacea
Cumacea
Cumacea is an order of small marine crustaceans, occasionally called hooded shrimp. Their unique appearance and uniform body plan makes them easy to distinguish from other crustaceans.-Anatomy:...
, Tanaidacea
Tanaidacea
The crustacean order Tanaidacea make up a minor group within the class Malacostraca. There are about 940 species in this order.-Description:...
, Mictacea
Mictacea
Mictacea is the most recently established order of crustaceans, erected for five species of small shrimp-like animals of the deep sea and anchialine caves.-Description:...
, Thermosbaenacea
Thermosbaenacea
Thermosbaenacea is a group of crustaceans that live in thermal springs in fresh water, brackish water and anchialine habitats. They have occasionally been treated as a distinct superorder , but are generally considered to belong to the Peracarida...
and Spelaeogriphacea
Spelaeogriphacea
Spelaeogriphacea is an order of crustaceans that grow to no more than . Little is known about the ecology of the order.Only four species, all subterranean, have been described...
), although some authors prefer to unite two pairs of orders with similar organisms, which are the Mysidacea
Mysidacea
Mysida is a group of small, shrimp-like crustaceans, an order in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch, or marsupium, in females. Mysids are mostly found in marine waters throughout the world, but are also important in...
, formed by the Mysida and the Lophogastrida, and the Edriophthalma
Edriophthalma
Edriophthalma is a disused peracarid classification comprising Isopoda and Amphipoda, first proposed by William Elford Leach in 1815. They have several common features, such as the fact that they both lack a carapace, possess sessile compound eyes, and thoracic coxae fused to their pleurites. Some...
, formed by the Isopoda and the Amphipoda.
The members of the Mysida and the Lophogastrida have several common features: they are shrimp-like with compound stalked eyes and have a carapace which covers most of the thorax but does not fuse with the last four thoracic segments (as instead is seen in the Eucarida), they possess well-developed thoracopods (for swimming) and tail fan, they also have similar behaviour (swarming) and foregut structure; Pygocephalomorph, a fossil from the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
, appears similar to the Lophogastrida, which in addition to other factors, has traditionally allowed the Lophogastrida to be identified as the more primitive Mysidacea. However, the monophyly of the Mysidacea been recently disputed, due to molecular data and several differences, one profound difference between the two taxa is that in the Mysida the carapace acts as a respiratory surface due to the absence of gills, which are however present in the Lophogastrida. When they are considered together many authors have put Mysidacea basal to the remaining peracarids ((paraphyletic)Ruppert & Barnes, 1994; (monophyletic)), others have even either grouped them with Euphausiacea to form the Schizopoda
Schizopoda
Schizopoda is a former taxonomical classification of a division of the class Malacostraca. Although it was split in 1883 by Johan Erik Vesti Boas into the two distinct orders Mysidacea and Euphausiacea, the order Schizopoda continued to be in use until the 1930s....
or made them basal to a eucarid subtree (paraphyletic).
The Isopoda and the Amphipoda are two of the largest pericarid groups, they both lack a carapace, possess sessile compound eyes and lack a sharp demarcation between thoracic and abdominal segments, but however differ in several features, such as gills.
The Isopoda contains 10,000 species, the organisms are dorsoventrally flattened and occupy not only marine and freshwater habitats, but even terrestrial (woodlice) for which they developed a thickened cuticle and gas exchange organs, allowing them to live even in arid regions.
The Amphipoda is a highly diverse group of 8,000 species, ranging from the Caprellida with a long and narrow body shape (skeleton shrimp
Skeleton shrimp
Skeleton shrimp are marine crustaceans of the infraorder Caprellida. The name denotes the threadlike slender body which allows them to virtually disappear among the fine filaments of seaweed, hydroids and bryozoans.-Ecology:...
) to the shrimp-like Gammaridea
Gammaridea
Gammaridea is a suborder of small, shrimp-like crustaceans in the order Amphipoda. It contains about 7,275 of the 7,900 described species of amphipods, in approximately 1,000 genera, divided among around 125 families. Gammaridea includes almost all freshwater amphipods , as well as many marine...
(scuds and sand hoppers).
The position of the Isopoda and the Amphipoda amongst the Peracarida is also debated, some authors support a derived united group (Edriophthalma) which is either monophyletic or paraphyletic (Wheeler, 1998), others support a basal paraphyletic Isopoda and Amphipoda group (Watling, 1999); however, other authors believe that several features that unite the Isopoda and the Amphipoda are homoplasious and that the two groups reside with different groups: one proposed a basal Amphipoda to a clade formed from Isopoda + Tanaidacea and Cumacea + Mictacea + Speleogriphacea, while some older phylogenetic trees Siewing, R. (1963). Studies in malacostracan morphology: results and problems. In H. B. Whittington, & W. D. Rolfe, Phylogeny and Evolution of Crustacea. (pp. 85-103). Cambridge, Mass: Museum of Comparative Zoology.Pires, A. M. (1987). Potiicoara brasiliensis: a new genus and species of Spelaeogriphacea (Crustacea: Peracarida) from Brasil with a phylogenetic analysis of the Peracarida. Journal of Natural History , 21, 225-238. place the Amphipoda and the Mysidacea basal to the Peracarida (without the Thermosbaenacea) either as a polyphyletic or a monophyletic group, whereas the Isopoda are in a derived clade with Tanaidacea.
Recent molecular studies by Jarman et al. (2000), Spears et al. (2005) and Meland & Willassen (2007) (which was derived from Spears et al., (2005) by adding 22 mysidacean taxa to those 26 taxa) suggest a phylogeny with some elements similar to that proposed by Richter & Scholtz (2001), but disprove the monophyly of both the Edriophthalma and the Mysidacea and do not possess a basal eumalacostracan taxa or a basal peracarid taxa (the Hoplocarida and the Syncarida, respectively in ): The Amphipoda (with Spleogriphacea) form a clade with the Lophogastrida, the Isopoda are in a derived clade with the Cumacea and Tanaidacea, while most importantly the Mysida in all three analyses falls basal to the non-Peracarida subtree, which however has a limited morphological support (Poore, 2005).
The remaining Peracarida orders are the cryptic and either moderately abundant, Cumacea and Tanaidacea, or are extremely rare and relictual, Mictacea, Spelaeogriphacea, and Thermosbaenacea.
There are about 1,600 members of the Cumacea, these are small burrowing crustaceans which have a characteristic large bulbous carapace (covering three thoracic segments) and an elongated abdomen which finishes in a pleotelson with stylus-like uropods, in fact due to their peculiar shape they are sometimes called hooded shrimp. Tanaidacea is a group of 1,500 species which are small burrowing or tube-dwelling crustaceans with a short carapace (covering two thoracic segments) that possess a pair of chelate second thoracopods (gnathopods).
Only three extant and two fossil speleogriphacean species have been found, these are blind cave-dwelling species
Troglobite
Troglobites are small cave-dwelling animals that have adapted to their dark surroundings. Troglobite species include spiders, insects, fish and others. They live permanently underground and cannot survive outside the cave environment. Troglobite adaptations and characteristics include a heightened...
with a short carapace (one thoracic segment); while the Mictacea is a group erected only two decades ago, and to date, five species have been found, Mictocaris halope (cave dwelling) and four species in the Hursutiidae family, in the genera Hirsutia (at 1,000 meter depths
Deep sea communities
Deep sea communities currently remain largely unexplored, due the technological and logististical challenges and expense involved in visiting these remote biomes. Because of the unique challenges , it was long believed that little life existed in this hostile environment...
) and Thetispelecaris (submarine caves), these blind species lack a carapace but have a well-developed headshield, and have reduced pleopods. This enigmatic group is believed by some authors not to be monophyletic, in fact one author proposed that Mictocaris halope should be grouped with the Spelaeogriphacea, forming the Cosinzeneacea.
The Thermosbaenacea is a group of 11 species found in hot springs
Hot Springs
Hot Springs may refer to:* Hot Springs, Arkansas** Hot Springs National Park, Arkansas*Hot Springs, California**Hot Springs, Lassen County, California**Hot Springs, Modoc County, California**Hot Springs, Placer County, California...
, caves and groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
that has the peculiarity that its brooding pouch is formed by its extended carapace and not by modified thoracopod endites as occurs in the remaining peracarids, for this reason some authors have removed this group from the Peracarida and placed it in its own superorder, the Pancarida, basal to the Pericarida.
Molecular studies
The major feature that emerges from the picture of the phylogeny of the Malacostraca is that their diversity has resulted in several studies proposing dramatically different phylogenetic trees. With the advent of DNA sequencingDNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....
, molecular studies have not helped a particular evolutionary model based on morphology to become the accepted one, but rather they have added more uncertainty to the accepted phylogenetic relationships: in fact not only they contradicted several morphological studies, but they also questioned the monophyly of the Eucarida and the Peracarida, in particular regarding the positions of the Mysida the Syncarida and the Hoplocarida, furthermore the molecular studies to date are only three, two of which overlap and concentrate on the Peracarida, while the third uses a limited amount of taxa.
Analysis of the mitochondrial genome supports the monophyly of Brachyura (crabs), Caridea (shrimps), Euphausiacea (krill), Penaeidae
Penaeidae
Penaeidae is a family of prawns, although they are often referred to as penaeid shrimp. It contains many species of economic importance, such as the tiger prawn , whiteleg shrimp, Atlantic white shrimp and Indian prawn. Many prawns are the subject of commercial fishery, and farming, both in marine...
(prawns) and Stomatopoda (mantis shrimps).Shen X, Wang H, Wang M, Liu B (2011) The complete mitochondrial genome sequence of Euphausia pacifica (Malacostraca: Euphausiacea) reveals a novel gene order and unusual tandem repeats. Genome It also supports a close relationship between Euphausiacea and Decapoda
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...
.
External links
- Malacostraca, Tree of Life Web ProjectTree of Life Web ProjectThe Tree of Life Web Project is an ongoing Internet project providing information about the diversity and phylogeny of life on Earth. This collaborative peer reviewed project began in 1995, and is written by biologists from around the world....
- Introduction to the Malacostraca, University of California, BerkeleyUniversity of California, BerkeleyThe University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
- "Malacostraca" by Thomas Roscoe Rede StebbingThomas Roscoe Rede StebbingThe Reverend Thomas Roscoe Rede Stebbing F.R.S., F.L.S. was a British zoologist, who described himself as "a serf to natural history, principally employed about Crustacea". Educated in London and Oxford, he only took to natural history in his thirties, having worked as a teacher until then...
, Encyclopædia Britannica Eleventh EditionEncyclopædia Britannica Eleventh EditionThe Encyclopædia Britannica Eleventh Edition is a 29-volume reference work, an edition of the Encyclopædia Britannica. It was developed during the encyclopaedia's transition from a British to an American publication. Some of its articles were written by the best-known scholars of the time... - Malacostraca, The Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...
- Malacostraca image key - Guide to the marine zooplankton of south eastern Australia, Tasmanian Aquaculture and Fisheries Institute