Crustacean
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{{For|the Dutch band|Crustacean (band)}}
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{{Taxobox | name = Crustacea
| fossil_range= [[Cambrian]] to recent
| image = Abludomelita obtusata.jpg
| image_width = 240px
| image_caption = ''Abludomelita obtusata'', an [[amphipod]]
| regnum = [[Animal]]ia
| phylum = [[Arthropod]]a
| subphylum = '''Crustacea'''
| subphylum_authority = [[Morten Thrane Brünnich|Brünnich]], 1772
| subdivision_ranks = [[Class (biology)|Classes]] & [[Subclass (biology)|Subclasses]]
| subdivision =
[[Branchiopoda]]
:[[Phyllopoda]]
:[[Fairy shrimp|Sarsostraca]]
[[Remipedia]]<br/>
[[Cephalocarida]]<br/>
[[Maxillopoda]]
:[[Thecostraca]]
:[[Tantulocarida]]
:[[Branchiura]]
:[[Pentastomida]]
:[[Mystacocarida]]
:[[Copepoda]]
[[Ostracod]]a
:[[Myodocopa]]
:[[Podocopa]]
[[Malacostraca]]
:[[Phyllocarida]]
:[[Hoplocarida]]
:[[Eumalacostraca]]
}}
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[[Image:Shrimp nauplius.jpg|thumb|240px|The nauplius larva of a [[Dendrobranchiata|dendrobranchiate]]]]
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[[Image:Porcellio scaber.jpg|thumb|240px|''[[Porcellio scaber]]'', the common rough woodlouse, a terrestrial crustacean]]
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[[Image:Gooseneckbarnacles.jpg|thumb|240px|''Pollicipes polymerus'', the [[gooseneck barnacle]]]]
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[[Image:Glyphea.png|thumb|240px|''Glyphea pseudastacus'', a fossil [[Glypheoidea|glypheoid]]]]
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The '''crustaceans''' ('''Crustacea''') are a large group of [[arthropod]]s, comprising almost 52,000 described [[species]] ,<ref name="M&D">{{cite book |url=http://www.nhm.org/research/publications/CrustaceaClassification.pdf |title=An Updated Classification of the Recent Crustacea |author=J. W. Martin & G. E. Davis |year=2001 |pages=132 pp |publisher=[[Natural History Museum of Los Angeles County]]}}</ref> and are usually treated as a [[subphylum]] .<ref>{{ITIS|ID=83677|taxon=Crustacea|date=1 May|year=2006}}</ref> They include various familiar animals, such as [[crab]]s, [[lobster]]s, [[crayfish]], [[shrimp]] and [[barnacle]]s. The majority of them are aquatic, living in either [[Marine (ocean)|marine]] or [[fresh water]] environments, but a few groups have [[adaptation|adapted]] to life on land, such as terrestrial [[crab]]s, [[Coenobita|terrestrial hermit crab]]s and [[woodlouse|woodlice]]. Crustaceans are among the most successful animals, and are as abundant in the oceans as much as insects are on land. Over half of animals in the world are marine [[copepod]] crustaceans. The majority of crustaceans are also [[motility|motile]], moving about independently, although a few taxonomic units 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]]'', all of which may be referred to as "crustacean lice"), and adult barnacles live a [[sessile]] life — they are attached headfirst to the substrate and cannot move independently. Although most crustaceans are small, their morphology varies greatly and they include such large animals as lobsters 70 [[Centimetre|cm]] long and spider crabs with a leg span of nearly 4 [[Metre|m]].<ref name="Encarta">[[Encarta]] Encyclopedia 2005. Article — Crustacean, by [[Michael Ghiselin|Michael T. Ghiselin]].</ref>
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.
==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]], an excellent example of [[tagmatization]]. The head bears two pairs of [[antenna (biology)|antennae]], three pairs of [[mouthparts]], and usually eyes (two [[compound eye]]s, an unpaired eye, or both). The thorax and pleon bear a number of lateral [[appendage]]s, including the [[gill]]s, and the tail ends with a [[telson]]. Crustacean appendages are used for swimming, crawling, and feeding. They may be highly modified as jaws and other structures, or may be lost. Smaller crustaceans [[Respiration (physiology)|respire]] through their body surface by [[diffusion]] ,<ref>{{cite journal | quotes=no |author=R. F. Pirow, 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=3089–3099 |url=http://jeb.biologists.org/cgi/content/abstract/202/22/3089 |pmid=10539957}}</ref> and larger crustaceans respire with [[gill]]s or, as shown by ''[[Birgus latro]]'', with abdominal lungs .<ref>{{cite journal | quotes=no |author=C. A. Farrelly & 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=63–87 |doi=10.1016/j.asd.2004.11.002}}</ref> Both systems (diffusion and gills) were being used by various crustaceans as early as the [[Middle Cambrian]] .<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=169–184}}</ref>
As [[arthropod]]s, crustaceans have a stiff [[exoskeleton]], which must be shed to allow the animal to grow ([[ecdysis]] or molting). Various parts of the exoskeleton may be fused together; this is particularly noticeable in the [[carapace]], the thick dorsal shield seen on many crustaceans that often forms a protective chamber for the gills. 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 a derived 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 .<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= |doi=10.1093/icb/43.1.185 }}</ref>
The main body cavity is an expanded circulatory system, through which blood is pumped by a heart located near the dorsum. The alimentary canal consists of a straight tube that often has a gizzard-like gastric mill for grinding food and a pair of digestive glands that absorb food. Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, and a collection of major ganglia is found below the gut.<ref name="Encarta"/>
Despite their diversity of form, crustaceans are united by the special [[larva]]l form known as the [[nauplius (larva)|nauplius]].
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 females retain the eggs until they hatch into free-swimming larvae.
==Life Cycle==
===Larva Stage===
[[Image:Homarus gammarus megalope.jpg|thumb|Zoea larva of the [[European lobster]], ''Homarus gammarus''.]]
[[Image:Gnathophausia zoea.jpg|thumb|''Gnathophausia zoea'', a [[Mysidacea|lophogastrid]].]]
The [[larva]]l stage of a crustacean's life cycle is called a '''zoea''' (pl. zoeal)s. It follows the [[nauplius (larva)|nauplius]] stage and precedes the [[post-larva]]. Zoea larvae swim with their thoracic [[appendage]]s, as opposed to nauplii, which use cephalic appendages, and megalopa, which use abdominal appendages for swimming. It often has spikes on its [[carapace]], which may assist these small organisms in maintaining directional swimming.<ref>{{cite journal | quotes=no |author=Weldon, W.F.R. |year=1889 |title=Note on the function of the spines of the Crustacean zoea |journal=Journal of the Marine Biological Association of the United Kingdom |volume=1 |issue=2 |pages=169–172}}</ref> In many [[Decapoda|decapods]], due to their accelerated development, the zoea is the first larval stage. In some cases, the zoea stage is followed by the mysis stage, and in others, by the megalopa stage, depending on the crustacean group involved.
==Taxonomy==
Although the classification of crustaceans has been quite variable, the system used by Martin and Davis <ref name="M&D"/> is the most authoritative, and largely supersedes earlier works.
Six classes of crustaceans are generally recognised:
*[[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 ,<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 |doi=10.1046/j.1420-9101.1997.10030353.x}}</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]].
==Fossil record==
Those crustaceans that have soft [[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 more rare 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.
The [[Late Jurassic]] [[Solnhofen limestone|lithographic limestones]] 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 Cretaceous [[Gault clay]] and the [[Eocene]] [[London clay]].
==Consumption==
[[Image:2005crustacean.PNG|thumb|Crustacean output in 2005]]
Many crustaceans are consumed by humans, and nearly 10,000,000 [[ton]]s were produced in 2005 .<ref name="FAO">{{cite web |url=http://www.fao.org/figis/servlet/TabLandArea?tb_ds=Production&tb_mode=TABLE&tb_act=SELECT&tb_grp=COUNTRY |publisher=[[Food and Agriculture Organization]] |accessdate=2007-06-18 |title=FIGIS: Global Production Statistics 1950–2005}}</ref> The vast majority of this output is of [[Decapoda|decapod crustaceans]]: [[crab]]s, [[lobster]]s, [[shrimp]] and [[prawn]]s. Over 70% by weight of all crustaceans caught for consumption are shrimp and prawns, and over 80% is produced in Asia, with China alone producing nearly half the world's total. Non-decapod crustaceans are not widely consumed, with only 130,000 tons of [[krill]] being caught, despite krill having one of the greatest [[biomass]]es on the planet.
==References==
{{Commonscat|Crustacea}}
{{Wikispecies|Crustacea}}
{{reflist}}
===General references===
*{{cite book |author = [[Frederick Schram]] |title=Crustacea |id=ISBN 0-19-503742-1 |publisher = [[Oxford University Press]] |year=1986 |pages=620 pp}}
==External links==
*[http://www.crustacea.net Crustacea.net, an online resource on the biology of crustaceans]
{{Arthropods}}
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