Difference between revisions of "Crustacean" - New World Encyclopedia

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{{Ebcompleted}}{{Copyedited}}{{Paid}}{{Approved}}{{Images OK}}{{Submitted}}{{2Copyedited}}
 
{{Taxobox_begin | color = pink | name = Crustaceans}}
 
{{Taxobox_begin | color = pink | name = Crustaceans}}
{{Taxobox_image | image = [[Image:Abludomelita obtusata.jpg|200px]]|Abludomelita obtusata | caption = ''Abludomelita obtusata'', an isopod}}
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{{Taxobox_image | image = [[Image:Abludomelita obtusata.jpg|200px]]|Abludomelita obtusata | caption = ''Abludomelita obtusata'', an amphipod}}
 
{{Taxobox_begin_placement | color = pink}}
 
{{Taxobox_begin_placement | color = pink}}
 
{{Taxobox_regnum_entry | taxon = [[Animal]]ia}}
 
{{Taxobox_regnum_entry | taxon = [[Animal]]ia}}
 
{{Taxobox_phylum_entry | taxon = [[Arthropod]]a}}
 
{{Taxobox_phylum_entry | taxon = [[Arthropod]]a}}
 
{{Taxobox_subphylum_entry | taxon = '''Crustacea'''}}
 
{{Taxobox_subphylum_entry | taxon = '''Crustacea'''}}
{{Taxobox_authority | author = [[Brünnich]] | date = [[1772]]}}
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{{Taxobox_authority | author = [[Brünnich]] | date = 1772}}
 
{{Taxobox_end_placement}}
 
{{Taxobox_end_placement}}
{{Taxobox_section_subdivision | color = pink | plural_taxon = [[Class (biology)|Classes]] & [[Subclass (biology)|Subclasses]]}}
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{{Taxobox_section_subdivision | color = pink | plural_taxon = Classes & Subclasses}}
 
Class [[Branchiopoda]]
 
Class [[Branchiopoda]]
 
:Subclass [[Phyllopoda]]
 
:Subclass [[Phyllopoda]]
 
:Subclass [[Fairy shrimp|Sarsostraca]]
 
:Subclass [[Fairy shrimp|Sarsostraca]]
Class [[Remipedia]]</br>
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Class [[Remipedia]]<br/>
Class [[Cephalocarida]]</br>
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Class [[Cephalocarida]]<br/>
 
Class [[Maxillopoda]]
 
Class [[Maxillopoda]]
 
:Subclass [[Thecostraca]]
 
:Subclass [[Thecostraca]]
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:Subclass [[Eumalacostraca]]
 
:Subclass [[Eumalacostraca]]
 
{{Taxobox_end}}
 
{{Taxobox_end}}
The '''crustaceans''' ('''Crustacea''') are a large group of [[arthropod]]s, comprising approximately 52,000 described [[species]]&nbsp;<ref name="M&D">{{cite book |url=http://www.nhm.org/research/publications/CrustaceaClassification.pdf |title=An Updated Classification of the Recent Crustacea |author=Martin, J. W. & Davis, G. E. |year=2001 |pages=132 pp. |publisher=Natural History Museum of Los Angeles County}}</ref>, and are usually treated as a [[subphylum]]&nbsp;<ref>{{ITIS |ID=83677 |taxon=Crustacea |date=1 May |year=2006}}</ref>.They include various familiar animals, such as [[lobster]]s, [[crab]]s, [[shrimp]] and [[barnacle]]s. The majority are aquatic, living in either [[fresh water]] or [[Marine (ocean)|marine]] environments, but a few groups have [[adaptation|adapted]] to terrestrial life, such as terrestrial [[crab]]s, [[Coenobita|terrestrial hermit crab]]s and [[woodlouse|woodlice]]. The majority are [[motility|motile]], moving about independently, although a few taxa are [[parasite|parasitic]] and live attached to their hosts (including [[sea louse|sea lice]], [[Branchiura|fish lice]], [[whale louse|whale lice]], [[Pentastomida|tongue worms]], and ''[[Cymothoa exigua]]''), and adult barnacles live a [[sessile]] life - they are attached head-first to the substrate and cannot move independently.
 
  
The scientific study of crustaceans is known as '''carcinology'''. Other names for carcinology are malacostracology, crustaceology and crustalogy, and a [[scientist]] who works in carcinology is a [[List of carcinologists|carcinologist]], crustaceologist or crustalogist.
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'''Crustaceans''' (subphylum ''Crustacea'') comprise the group of [[arthropod]]s (phylum Arthropoda) that includes the familiar [[crab]]s, [[lobster]]s, [[shrimp]]s, [[barnacle]]s, [[copepod]]s, [[krill]], water fleas, and [[crayfish]]. Crustaceans are characterized by having branched (biramous) appendages, an exoskeleton made up of [[chitin]] and [[calcium]], two pairs of antennae extending in front of the mouth, and paired appendages that act like jaws, with three pairs of biting mouthparts. They share with other arthropods the characteristic segmented body, a pair of jointed appendages on each segment, and a hard exoskeleton that must be periodically shed for growth.
 +
 
 +
Crustacea is the only subphylum of arthropoda to contain mostly aquatic [[species]] (Towle 1989), and most of these species, such as shrimps and barnacles, are marine organisms. Indeed, crustacea is the largest group of marine arthropods. However, Crustacea also includes many freshwater forms, such as crayfish; and also terrestrial forms, including sand fleas, land crabs, woodlice, and sow bugs, which generally need humid environments.
 +
 
 +
Crustaceans offer important economic, ecological, and aesthetic values and also can be appreciated from the perspective of bi-level functionality. Some larger crustaceans, including shrimp, lobsters, and crabs, are a major [[food]] commodity, while smaller crustaceans in their own way are integral to many [[food chain|food webs]]. Examples include krill and copepods, which can make up a considerable portion of [[plankton]], consuming phytoplankton and being consumed by [[whale]]s and [[fish]]. In playing their roles in the food chain, krill and copepods exemplify the common bi-level functionality of living systems: In this case the krill and copepods in pursuing their own needs serve also the higher order function of collecting and concentrating the marine nutrients, which makes these now available to whales and fish. Some crustaceans are detrivores, consuming dead organisms and recycling the nutrients. Crustaceans also can provide joy to [[human]]s in terms of their diverse colors and forms, and even their taste and texture when prepared as food.
 +
 
 +
There are more than 50,000 described [[species]] of crustaceans. The majority of these are motile, moving about independently, although a few taxa are [[parasite|parasitic]] and live attached to their hosts (including [[sea louse|sea lice]], [[Branchiura|fish lice]], [[whale louse|whale lice]], [[Pentastomida|tongue worms]], and ''Cymothoa exigua''), and adult barnacles live a sessile life&mdash;they are attached head-first to the substrate and cannot move independently.
 +
 
 +
Crustacea is sometimes considered a class or superclass rather than a subphylum. The scientific study of crustaceans is known as ''carcinology''. Other names for carcinology are malacostracology, crustaceology, and crustalogy, and a [[science|scientist]] who works in carcinology is a carcinologist, crustaceologist, or crustalogist.
  
 
==Structure of crustaceans==
 
==Structure of crustaceans==
Crustaceans have three distinct body parts: [[cephalon|head]], [[thorax]], and [[abdomen]] (or ''pleon''), although the head and thorax may fuse to form a [[cephalothorax]]. The head bears two pairs of [[antenna (biology)|antennae]], one pair of [[compound eye]]s and three pairs of [[mouthparts]]. The thorax and pleon bear a number of lateral [[appendage]]s, including the [[gill]]s, and the tail ends with a [[telson]]. Smaller crustaceans [[Respiration (physiology)|respire]] through their body surface by [[diffusion]]&nbsp;<ref>{{cite journal | quotes=no |author=Pirow R, F. Wollinger & R. J. Paul |year=1999 |title=The sites of respiratory gas exchange in the planktonic crustacean ''Daphnia magna'': An in vivo study employing blood haemoglobin as an internal oxygen probe |journal=Journal of experimental zoology |volume=202 |issue=22 |pages=3089&ndash;3099}}</ref>, and larger crustaceans respire with [[gill]]s or, as shown by ''[[Birgus latro]]'', with abdominal lungs&nbsp;<ref>{{cite journal | quotes=no |author=Farrelly, C. A. & P. Greenaway |year=2005 |title=The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (''Coenobita'' and ''Birgus''): gills, branchiostegal lungs and abdominal lungs |journal=Arthropod structure and development |volume=34 |issue=1 |pages=63&ndash;87}}</ref>. Both systems (diffusion and gills) were being used by various crustaceans as early as the [[Middle Cambrian]]&nbsp;<ref>{{cite journal | quotes=no |author = Vannier, J., M. Williams & D. J. Siveter |year=1997 |title=The Cambrian origin of the circulatory system of crustaceans |journal = Lethaia |volume=30 |issue=3 |pages=169&ndash;184}}</ref>.
+
Crustaceans have three distinct body parts: ''Head,'' ''thorax,'' and ''abdomen'' (or ''pleon''), although the head and thorax may fuse to form a ''cephalothorax''. The head bears two pairs of antennae, one pair of [[eye|compound eye]]s and three pairs of mouthparts. The thorax and pleon bear a number of lateral appendages, including the gills, and the tail ends with a telson (last division of the body of a crustacean, but not a true segment).  
  
In common with other [[arthropod]]s, crustaceans have a stiff [[exoskeleton]] which must be shed to allow the animal to grow ([[ecdysis]]). Various parts of the exoskeleton may be fused together; this is particularly noticeable in the [[carapace]], the thick dorsal shield seen on many crustaceans. Crustacean [[appendage]]s are typically [[biramous]], meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is [[uniramous]]. There is some doubt whether this is an advanced state, as had been traditionally assumed, or whether it may be a primitive state, with the branching of the limbs being lost in all extant arthoropod groups except the crustaceans. One piece of evidence supporting the latter view is the biramous nature of [[trilobite]] limbs&nbsp;<ref>{{cite journal | quotes=no |title=Trilobite tagmosis and body patterning from morphological and developmental perspectives |author=N. C. Hughes |year=2003 |journal=Integrative and Comparative Biology |volume=43 |issue=1 |pages=185-206 |url=http://www.earthscience.ucr.edu/people/hughes/HughesIntCoBio03.pdf}}</ref>.
+
Smaller crustaceans [[Respiration (physiology)|respire]] through their body surface by diffusion (Pirow et al. 1999), and larger crustaceans respire with gills or, as shown by ''Birgus latro'', with abdominal lungs (Farrelly and Greenaway 2005). Both systems (diffusion and gills) were being used by various crustaceans as early as the Middle [[Cambrian]] (Vannier et al. 1997).
  
[[Image:Shrimp nauplius.jpg|thumb|240px|The nauplius larva of a [[Dendrobranchiata|dendrobranchiate]]]]
+
[[Image:Shrimp nauplius.jpg|thumb|left|180px|The nauplius(larva) of a shrimp, an aquatic crustacean.]]
Despite their diversity of form, crustaceans are united by the special [[larva]]l form known as the [[nauplius (larva)|nauplius]].
+
Despite their diversity of forms as adults, crustaceans share the special larval form known as the nauplius.
 +
In common with other [[arthropod]]s, crustaceans grow larger through the process of ecdysis in which they periodically shed their stiff exoskeleton and replace it with a larger one. Various parts of the exoskeleton may be fused together; this is particularly noticeable in the carapace, the thick dorsal shield seen on many crustaceans. Crustacean appendages are typically ''biramous,'' meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is ''uniramous''. There is some doubt whether this is an advanced state, as had been traditionally assumed, or whether it may be a primitive state, with the branching of the limbs being lost in all extant arthropod groups except the crustaceans. One piece of evidence supporting the latter view is the biramous nature of [[trilobite]] limbs (Hughes 2003).
  
[[Image:Porcellio scaber.jpg|thumb|240px|''[[Porcellio scaber]]'', the common rough woodlouse, a terrestrial crustacean]]
+
Although a few are [[hermaphrodite|hermaphroditic]], most crustaceans have separate sexes distinguished by appendages on the abdomen called swimmerets or, more technically, ''pleopods''. The first (and sometimes the second) pair of pleopods are specialized in the male for [[sperm]] transfer. Many terrestrial crustaceans (such as the Christmas Island red crab) mate seasonally and return to the sea to release the eggs. Others, such as [[woodlouse|woodlice]] lay their eggs on land, albeit in damp conditions. In many [[Decapoda|decapods]], the eggs are retained by the females until they hatch into free-swimming larvae.
Although a few are [[hermaphrodite|hermaphroditic]], most crustaceans have separate sexes, which are distinguished by appendages on the abdomen called swimmerets or, more technically, [[pleopod]]s. The first (and sometimes the second) pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans (such as the [[Christmas Island red crab]]) mate seasonally and return to the sea to release the eggs. Others, such as [[woodlouse|woodlice]] lay their eggs on land, albeit in damp conditions. In many [[Decapoda|decapods]], the eggs are retained by the females until they hatch into free-swimming larvae.
 
  
 
==Taxonomy==
 
==Taxonomy==
[[Image:Gooseneckbarnacles.jpg|thumb|240px|''Pollicipes polymerus'', the [[gooseneck barnacle]]]]
+
[[Image:Gooseneckbarnacles.jpg|thumb|240px|''Pollicipes polymerus'', the gooseneck barnacle.]]
Although the classification of crustaceans has been quite variable, the system used by Martin & Davis&nbsp;<ref name="M&D"/> is the most authoritative, and largely supersedes earlier works.
+
Carcinologists have long debated the taxonomic status of crustaceans, sometimes assigning the group to be at the phylum, subphylum, or superclass level, and with five, six, or even ten classes recognized (Hobbs 2003). Others have listed the Crustacea as a class.
 +
 
 +
The system used by Martin & Davis (2001) is one system considered by some to be authoritative and superceding earlier works.
 +
 
 +
Six classes of crustaceans are generally recognized:
 +
*''[[Branchiopoda]]''&mdash;small, freshwater crustaceans, including [[Artemia|brine shrimp]] ''(Artemia)'', water fleas (''Daphnia sp.), and ''Triops'' (Notostraca).
 +
*''[[Remipedia]]'' &mdash; a small class restricted to deep caves connected to salt water, called anchialine caves.
 +
*''[[Cephalocarida]]''&mdash;tiny, hermaphrodictic, shrimplike; horseshoe shrimp.
 +
*''[[Maxillopoda]]''&mdash;various groups, including [[barnacle]]s and [[copepod]]s. It contains ''[[Mystacocarida]]'' and ''[[Branchiura]],'' which are sometimes treated as their own classes.
 +
*''[[Ostracoda]]''&mdash;small marine and freshwater animals with bivalve shells that cover the body, with only antennae and parts of limbs extending.
 +
*''[[Malacostraca]]''&mdash;the largest class, with the largest and most familiar animals, such as [[crab]]s, [[lobster]]s, [[shrimp]], [[krill]], and [[woodlouse|woodlice]]. This class is sometimes arranged into the orders Decapoda, Amphipoda, and Isopoda.
  
Six classes of crustaceans are generally recognised:
+
In some [[taxonomy|taxonomic]] schemes that consider the Crustacea to be a "class," these divisions are considered to be "orders."
*[[Branchiopoda]] - including [[Artemia|brine shrimp]] (''Artemia'') and ''[[Triops]]'' (Notostraca)
 
*[[Remipedia]] - a small class restricted to deep caves connected to salt water, called anchialine caves
 
*[[Cephalocarida]] - horseshoe shrimp
 
*[[Maxillopoda]] - various groups, including [[barnacle]]s and [[copepod]]s. It contains [[Mystacocarida]] and [[Branchiura]], which are sometimes treated as their own classes.
 
*[[Ostracoda]] - small animals with bivalve shells
 
*[[Malacostraca]] - the largest class, with the largest and most familiar animals, such as [[crab]]s, [[lobster]]s, [[shrimp]], [[krill]] and [[woodlouse|woodlice]].
 
  
The exact relationships of the Crustacea to other taxa are not yet entirely clear. Under the [[Pancrustacea]] hypothesis&nbsp;<ref>{{cite journal | quotes=no||author = J. Zrzavý & P. Štys |title=The basic body plan of arthropods: insights from evolutionary morphology and developmental biology |journal=Journal of Evolutionary Biology |year=1997 |volume=10 |pages=353-367}}</ref>, Crustacea and [[Hexapoda]] ([[insect]]s and allies) are [[sister group]]s. Studies using [[DNA sequence]]s tend to show a [[paraphyletic]] Crustacea, with the [[insect]]s (but not necessarily other hexapods) nested within that [[clade]].
+
Some biological classifications do not recognize Maxillopoda as a class, but recognize the barnacles, copepods, and others as separate classes:
 +
 
 +
*''[[Cirripedia]]''&mdash;sessile group of crustaceans that are marine and comprise the barnacles.
 +
*''[[Copepoda]]''&mdash;mostly marine and can occur in large numbers; millions of copepods may be found in a cubic meter of ocean.
 +
*''Branchiura''&mdash;flattened crustaceans that are temporary parasites on fish.
 +
*''Mystacocarida''&mdash;minute crustaceans restricted to waters of intertidal sand beached and subtidal sand stubstrates, and closely related to copepods (Hobbs 2003).
 +
 
 +
The exact relationships of the Crustacea to other taxa are not yet entirely clear. Under the Pancrustacea hypothesis (Zrzavý and Štys 1997), Crustacea and Hexapoda ([[insect]]s and allies) are sister groups. Studies using [[DNA]] sequences tend to show a paraphyletic Crustacea, with the insects (but not necessarily other hexapods) nested within that clade.
 +
 
 +
Crustaceans are sometimes viewed in terms of two groups. One, the entomostracans, comprising the smaller species, such as copepods, barnacles, and fairy shrimp; and the second group, the malacostaceans, comprising the larger forms, such as lobsters, crabs, and shrimp.
  
 
==Geological history==
 
==Geological history==
[[Image:Glyphea.png|thumb|240px|''Glyphea pseudastacus'', a fossil [[Glypheoidea|glypheoid]]]]
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[[Image:Glyphea.png|thumb|240px|''Glyphea pseudastacus,'' a fossil glypheoid]]
Those crustaceans that have hard [[exoskeleton]]s reinforced with [[calcium carbonate]], such as [[crab]]s and [[lobster]]s tend to preserve well as fossils, but many crustaceans have only thin exoskeletons. Most of the fossils known are from [[coral reef]] or shallow sea-floor environments, but many crustaceans live in open seas, on deep sea-floors or in [[burrow]]s. Crustaceans tend, therefore, to be rarer in the [[fossil record]] than [[trilobite]]s. Some crustaceans are reasonably common in [[Cretaceous]] and [[Caenozoic]] rocks, but barnacles have a particularly poor fossil record, with very few specimens from before the [[Mesozoic]] era.
+
Those crustaceans, such as [[crab]]s and [[lobster]]s, that have hard exoskeletons reinforced with calcium carbonate tend to preserve well as fossils, but many crustaceans have only thin exoskeletons. Most of the crustacean fossils known are from [[coral|coral reef]] or shallow sea-floor environments, but many crustaceans live in open seas, on deep sea-floors, or in burrows. Crustaceans tend, therefore, to be rarer in the [[fossil]] record than [[trilobite]]s. Some crustaceans are reasonably common in [[Cretaceous]] and [[Cenozoic]] rocks, but barnacles have a particularly poor fossil record, with very few specimens from before the [[Mesozoic]] era.
  
The [[Late Jurassic]] [[Solnhofen limestone|lithographic limestone]] of [[Solnhofen]], [[Bavaria]], which are famous as the home of ''[[Archaeopteryx]]'', are relatively rich in [[Decapoda|decapod]] crustaceans, such as ''Eryon'' (an [[Eryonoidea|eryonoid]]), ''Aeger'' (a [[prawn]]) or ''Pseudastacus'' (a [[lobster]]). The "lobster bed" of the [[Greensand]] formation from the [[Cretaceous]] period which occurs at [[Little Atherfield|Atherfield]] on the [[Isle of Wight]] contains many well preserved examples of the small [[Glypheoidea|glypheoid]] lobster ''Mecochirus magna''. Crabs have been found at a number of sites, such as the Cretacoues [[Gault clay]] and the [[Eocene]] [[London clay]].
+
The Late [[Jurassic]] lithographic [[limestone]]s of Solnhofen, Bavaria, which are famous as the home of ''[[Archaeopteryx]],'' are relatively rich in decapod crustaceans (five pairs of legs), such as ''Eryon'' (an eryonoid), ''Aeger'' (a prawn) or ''Pseudastacus'' (a [[lobster]]). The "lobster bed" of the Greensand formation from the [[Cretaceous]] period, which occurs at Atherfield on the Isle of Wight, contains many well preserved examples of the small glypheoid lobster ''Mecochirus magna''. Crabs have been found at a number of sites, such as the Cretacoues Gault clay and the [[Eocene]] London clay.
  
 
==References==
 
==References==
<div class="references-small"><references/></div>
+
* Farrelly, C. A., and P. Greenway. 2005. The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (''Coenobita'' and ''Birgus''): Gills, branchiostegal lungs and abdominal lungs. ''Arthropod Structure and Development'' 34(1): 63&ndash;87.
===General references===
+
* Hobbs, H. H. 2003. [http://www.routledge-ny.com/ref/caves/samples.html Crustacea.] In ''Encyclopedia of Caves and Karst Science''. Routledge. Retrieved December 5, 2006.
*{{cite book |author = [[Frederick Schram]] |title=Crustacea |id=ISBN 0-19-503742-1 |publisher = [[Oxford University Press]] |year=1986 |pages=620 pp.}}
+
* Hughes, N. C. 2003. Trilobite tagmosis and body patterning from morphological and developmental perspectives. ''Integrative and Comparative Biology'' 43(1):185-206.
 +
* Huys, R. 2003. An updated classification of the recent Crustacea. ''Journal of Crustacean Biology'' 23 (2): 495–497.
 +
* Martin, J. W., and G. E. Davis. 2001. [http://www.nhm.org/research/publications/CrustaceaClassification.pdf ''An Updated Classification of the Recent Crustacea.''] Natural History Museum of Los Angeles County.
 +
* Schram, F. 1986. ''Crustacea''. Oxford University Press. ISBN 0-19-503742-1.
 +
* Towle, A. 1989. ''Modern Biology''. Austin: Holt, Rinehart and Winston. ISBN 0-03-013919-8.
 +
* Vannier, J., M. Williams, and D. J. Siveter. 1997. The Cambrian origin of the circulatory system of crustaceans. ''Lethaia'' 30(3): 169&ndash;184.
 +
* Zrzavý, J. and P. Štys. 1997. The basic body plan of arthropods: insights from evolutionary morphology and developmental biology. ''Journal of Evolutionary Biology'' 10: 353-367.
  
==External links==
 
*[http://www.crustacea.net Crustacea.net, an online resource on the biology of crustaceans]
 
  
 
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{{credit|89460089}}
 
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
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[[Category:Animals]]
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[[Category:Invertebrates]]
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[[Category:Arthropods]]
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[[Category:Food]]

Latest revision as of 19:04, 4 June 2020

Crustaceans
Abludomelita obtusata.jpg
Abludomelita obtusata, an amphipod
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea

Brünnich, 1772

Classes & Subclasses

Class Branchiopoda

Subclass Phyllopoda
Subclass Sarsostraca

Class Remipedia
Class Cephalocarida
Class Maxillopoda

Subclass Thecostraca
Subclass Tantulocarida
Subclass Branchiura
Subclass Pentastomida
Subclass Mystacocarida
Subclass Copepoda

Class Ostracoda

Subclass Myodocopa
Subclass Podocopa

Class Malacostraca

Subclass Phyllocarida
Subclass Hoplocarida
Subclass Eumalacostraca

Crustaceans (subphylum Crustacea) comprise the group of arthropods (phylum Arthropoda) that includes the familiar crabs, lobsters, shrimps, barnacles, copepods, krill, water fleas, and crayfish. Crustaceans are characterized by having branched (biramous) appendages, an exoskeleton made up of chitin and calcium, two pairs of antennae extending in front of the mouth, and paired appendages that act like jaws, with three pairs of biting mouthparts. They share with other arthropods the characteristic segmented body, a pair of jointed appendages on each segment, and a hard exoskeleton that must be periodically shed for growth.

Crustacea is the only subphylum of arthropoda to contain mostly aquatic species (Towle 1989), and most of these species, such as shrimps and barnacles, are marine organisms. Indeed, crustacea is the largest group of marine arthropods. However, Crustacea also includes many freshwater forms, such as crayfish; and also terrestrial forms, including sand fleas, land crabs, woodlice, and sow bugs, which generally need humid environments.

Crustaceans offer important economic, ecological, and aesthetic values and also can be appreciated from the perspective of bi-level functionality. Some larger crustaceans, including shrimp, lobsters, and crabs, are a major food commodity, while smaller crustaceans in their own way are integral to many food webs. Examples include krill and copepods, which can make up a considerable portion of plankton, consuming phytoplankton and being consumed by whales and fish. In playing their roles in the food chain, krill and copepods exemplify the common bi-level functionality of living systems: In this case the krill and copepods in pursuing their own needs serve also the higher order function of collecting and concentrating the marine nutrients, which makes these now available to whales and fish. Some crustaceans are detrivores, consuming dead organisms and recycling the nutrients. Crustaceans also can provide joy to humans in terms of their diverse colors and forms, and even their taste and texture when prepared as food.

There are more than 50,000 described species of crustaceans. The majority of these are motile, moving about independently, although a few taxa are parasitic and live attached to their hosts (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua), and adult barnacles live a sessile life—they are attached head-first to the substrate and cannot move independently.

Crustacea is sometimes considered a class or superclass rather than a subphylum. The scientific study of crustaceans is known as carcinology. Other names for carcinology are malacostracology, crustaceology, and crustalogy, and a scientist who works in carcinology is a carcinologist, crustaceologist, or crustalogist.

Structure of crustaceans

Crustaceans have three distinct body parts: Head, thorax, and abdomen (or pleon), although the head and thorax may fuse to form a cephalothorax. The head bears two pairs of antennae, one pair of compound eyes and three pairs of mouthparts. The thorax and pleon bear a number of lateral appendages, including the gills, and the tail ends with a telson (last division of the body of a crustacean, but not a true segment).

Smaller crustaceans respire through their body surface by diffusion (Pirow et al. 1999), and larger crustaceans respire with gills or, as shown by Birgus latro, with abdominal lungs (Farrelly and Greenaway 2005). Both systems (diffusion and gills) were being used by various crustaceans as early as the Middle Cambrian (Vannier et al. 1997).

The nauplius(larva) of a shrimp, an aquatic crustacean.

Despite their diversity of forms as adults, crustaceans share the special larval form known as the nauplius. In common with other arthropods, crustaceans grow larger through the process of ecdysis in which they periodically shed their stiff exoskeleton and replace it with a larger one. Various parts of the exoskeleton may be fused together; this is particularly noticeable in the carapace, the thick dorsal shield seen on many crustaceans. Crustacean appendages are typically biramous, meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is uniramous. There is some doubt whether this is an advanced state, as had been traditionally assumed, or whether it may be a primitive state, with the branching of the limbs being lost in all extant arthropod groups except the crustaceans. One piece of evidence supporting the latter view is the biramous nature of trilobite limbs (Hughes 2003).

Although a few are hermaphroditic, most crustaceans have separate sexes distinguished by appendages on the abdomen called swimmerets or, more technically, pleopods. The first (and sometimes the second) pair of pleopods are specialized in the male for sperm transfer. Many terrestrial crustaceans (such as the Christmas Island red crab) mate seasonally and return to the sea to release the eggs. Others, such as woodlice lay their eggs on land, albeit in damp conditions. In many decapods, the eggs are retained by the females until they hatch into free-swimming larvae.

Taxonomy

Pollicipes polymerus, the gooseneck barnacle.

Carcinologists have long debated the taxonomic status of crustaceans, sometimes assigning the group to be at the phylum, subphylum, or superclass level, and with five, six, or even ten classes recognized (Hobbs 2003). Others have listed the Crustacea as a class.

The system used by Martin & Davis (2001) is one system considered by some to be authoritative and superceding earlier works.

Six classes of crustaceans are generally recognized:

  • Branchiopoda—small, freshwater crustaceans, including brine shrimp (Artemia), water fleas (Daphnia sp.), and Triops (Notostraca).
  • Remipedia — a small class restricted to deep caves connected to salt water, called anchialine caves.
  • Cephalocarida—tiny, hermaphrodictic, shrimplike; horseshoe shrimp.
  • Maxillopoda—various groups, including barnacles and copepods. It contains Mystacocarida and Branchiura, which are sometimes treated as their own classes.
  • Ostracoda—small marine and freshwater animals with bivalve shells that cover the body, with only antennae and parts of limbs extending.
  • Malacostraca—the largest class, with the largest and most familiar animals, such as crabs, lobsters, shrimp, krill, and woodlice. This class is sometimes arranged into the orders Decapoda, Amphipoda, and Isopoda.

In some taxonomic schemes that consider the Crustacea to be a "class," these divisions are considered to be "orders."

Some biological classifications do not recognize Maxillopoda as a class, but recognize the barnacles, copepods, and others as separate classes:

  • Cirripedia—sessile group of crustaceans that are marine and comprise the barnacles.
  • Copepoda—mostly marine and can occur in large numbers; millions of copepods may be found in a cubic meter of ocean.
  • Branchiura—flattened crustaceans that are temporary parasites on fish.
  • Mystacocarida—minute crustaceans restricted to waters of intertidal sand beached and subtidal sand stubstrates, and closely related to copepods (Hobbs 2003).

The exact relationships of the Crustacea to other taxa are not yet entirely clear. Under the Pancrustacea hypothesis (Zrzavý and Štys 1997), Crustacea and Hexapoda (insects and allies) are sister groups. Studies using DNA sequences tend to show a paraphyletic Crustacea, with the insects (but not necessarily other hexapods) nested within that clade.

Crustaceans are sometimes viewed in terms of two groups. One, the entomostracans, comprising the smaller species, such as copepods, barnacles, and fairy shrimp; and the second group, the malacostaceans, comprising the larger forms, such as lobsters, crabs, and shrimp.

Geological history

Glyphea pseudastacus, a fossil glypheoid

Those crustaceans, such as crabs and lobsters, that have hard exoskeletons reinforced with calcium carbonate tend to preserve well as fossils, but many crustaceans have only thin exoskeletons. Most of the crustacean fossils known are from coral reef or shallow sea-floor environments, but many crustaceans live in open seas, on deep sea-floors, or in burrows. Crustaceans tend, therefore, to be rarer in the fossil record than trilobites. Some crustaceans are reasonably common in Cretaceous and Cenozoic rocks, but barnacles have a particularly poor fossil record, with very few specimens from before the Mesozoic era.

The Late Jurassic lithographic limestones of Solnhofen, Bavaria, which are famous as the home of Archaeopteryx, are relatively rich in decapod crustaceans (five pairs of legs), such as Eryon (an eryonoid), Aeger (a prawn) or Pseudastacus (a lobster). The "lobster bed" of the Greensand formation from the Cretaceous period, which occurs at Atherfield on the Isle of Wight, contains many well preserved examples of the small glypheoid lobster Mecochirus magna. Crabs have been found at a number of sites, such as the Cretacoues Gault clay and the Eocene London clay.

References
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  • Farrelly, C. A., and P. Greenway. 2005. The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (Coenobita and Birgus): Gills, branchiostegal lungs and abdominal lungs. Arthropod Structure and Development 34(1): 63–87.
  • Hobbs, H. H. 2003. Crustacea. In Encyclopedia of Caves and Karst Science. Routledge. Retrieved December 5, 2006.
  • Hughes, N. C. 2003. Trilobite tagmosis and body patterning from morphological and developmental perspectives. Integrative and Comparative Biology 43(1):185-206.
  • Huys, R. 2003. An updated classification of the recent Crustacea. Journal of Crustacean Biology 23 (2): 495–497.
  • Martin, J. W., and G. E. Davis. 2001. An Updated Classification of the Recent Crustacea. Natural History Museum of Los Angeles County.
  • Schram, F. 1986. Crustacea. Oxford University Press. ISBN 0-19-503742-1.
  • Towle, A. 1989. Modern Biology. Austin: Holt, Rinehart and Winston. ISBN 0-03-013919-8.
  • Vannier, J., M. Williams, and D. J. Siveter. 1997. The Cambrian origin of the circulatory system of crustaceans. Lethaia 30(3): 169–184.
  • Zrzavý, J. and P. Štys. 1997. The basic body plan of arthropods: insights from evolutionary morphology and developmental biology. Journal of Evolutionary Biology 10: 353-367.


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