The body of a crustacean is composed of segments, which are grouped into three regions: thecephalon or head,[5] thepereon orthorax,[6] and thepleon orabdomen.[7] The head and thorax may be fused together to form acephalothorax,[8] which may be covered by a single largecarapace.[9] The crustacean body is protected by the hardexoskeleton, which must bemoulted for the animal to grow. The shell around each somite can be divided into a dorsaltergum, ventralsternum and a lateral pleuron. Various parts of the exoskeleton may be fused together.[10]: 289
Eachsomite, or body segment can bear a pair ofappendages: on the segments of the head, these include two pairs ofantennae, themandibles andmaxillae;[5] the thoracic segments bearlegs, which may be specialised aspereiopods (walking legs) andmaxillipeds (feeding legs).[6] Malacostraca and Remipedia (and the hexapods) have abdominal appendages. All other classes of crustaceans have a limbless abdomen, except from atelson andcaudal rami which is present in many groups.[11][12]The abdomen in malacostracans bearspleopods,[7] and ends in a telson, which bears theanus, and is often flanked by uropods to form atail fan.[13] The number and variety ofappendages in different crustaceans may be partly responsible for the group's success.[14]
Crustaceanappendages are typicallybiramous, meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is usuallyuniramous, the exception being in the Class Malacostraca where the antennules may be generally biramous or even triramous.[15][16] It is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups.Trilobites, for instance, also possessed biramous appendages.[17]
The main body cavity is anopen circulatory system, where blood is pumped into thehaemocoel by aheart located near the dorsum.[18] Malacostraca havehaemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods havehaemoglobins.[19] 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; this structure goes in a spiral format.[20] 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.[21]
In manydecapods, the first (and sometimes the second) pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans (such as theChristmas Island red crab) mate seasonally and return to the sea to release the eggs. Others, such aswoodlice, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs until they hatch into free-swimming larvae.[22]
Most crustaceans are aquatic, living in either marine orfreshwater environments, but a few groups haveadapted to life on land, such asterrestrial crabs,terrestrial hermit crabs, andwoodlice. Marine crustaceans are as ubiquitous in the oceans as insects are on land.[23][24] Most crustaceans are alsomotile, moving about independently, although a few taxonomic units areparasitic and live attached to their hosts (includingsea lice,fish lice,whale lice,tongue worms, andCymothoa exigua, all of which may be referred to as "crustacean lice"), and adult barnacles live asessile life – they are attached headfirst to the substrate and cannot move independently. Some branchiurans are able to withstand rapid changes ofsalinity and will also switch hosts from marine to non-marine species.[25]: 672 Krill are the bottom layer and most important part of the food chain inAntarctic animal communities.[26]: 64 Some crustaceans are significantinvasive species, such as the Chinese mitten crab,Eriocheir sinensis,[27] and the Asian shore crab,Hemigrapsus sanguineus.[28] Since the opening of theSuez Canal, close to 100 species of crustaceans from the Red Sea and the Indo-Pacific realm have established themselves in the eastern Mediterranean sub-basin, with often significant impact on local ecosystems.[29]
Most crustaceans haveseparate sexes, andreproduce sexually. In fact, a recent study explains how the maleT. californicus decide which females to mate with by dietary differences, preferring when the females are algae-fed instead of yeast-fed.[30] A small number arehermaphrodites, includingbarnacles,remipedes,[31] andCephalocarida.[32] Some may even change sex during the course of their life.[32]Parthenogenesis is also widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male.[30] This occurs in manybranchiopods, someostracods, someisopods, and certain "higher" crustaceans, such as theMarmorkrebs crayfish.
In many crustaceans, the fertilised eggs are released into thewater column, while others have developed a number of mechanisms for holding on to the eggs until they are ready to hatch. Mostdecapods carry the eggs attached to thepleopods, whileperacarids,notostracans,anostracans, and manyisopods form abrood pouch from thecarapace and thoracic limbs.[30] Female Branchiura do not carry eggs in external ovisacs but attach them in rows to rocks and other objects.[33]: 788 Mostleptostracans andkrill carry the eggs between their thoracic limbs; somecopepods carry their eggs in special thin-walled sacs, while others have them attached together in long, tangled strings.[30]
Crustaceans exhibit a number of larval forms, of which the earliest and most characteristic is thenauplius. This has three pairs ofappendages, all emerging from the young animal's head, and a single naupliar eye. In most groups, there are further larval stages, including thezoea (pl. zoeæ or zoeas[34]). This name was given to it when naturalists believed it to be a separate species.[35] It follows thenauplius stage and precedes thepost-larva. Zoea larvae swim with their thoracicappendages, as opposed to nauplii, which use cephalic appendages, and megalopa, which use abdominal appendages for swimming. It often has spikes on itscarapace, which may assist these small organisms in maintaining directional swimming.[36] In manydecapods, 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.
Providing camouflage against predators, the otherwise black eyes in several forms of swimming larvae are covered by a thin layer of crystallineisoxanthopterin that gives their eyes the same color as the surrounding water, while tiny holes in the layer allow light to reach the retina.[37] As the larvae mature into adults, the layer migrates to a new position behind the retina where it works as a backscattering mirror that increases the intensity of light passing through the eyes, as seen in many nocturnal animals.[38]
In an effort to understand whetherDNA repair processes can protect crustaceans againstDNA damage, basic research was conducted to elucidate the repair mechanisms used byPenaeus monodon (black tiger shrimp).[39] Repair of DNA double-strand breaks was found to be predominantly carried out by accuratehomologous recombinational repair. Another, less accurate process,microhomology-mediated end joining, is also used to repair such breaks. The expression pattern of DNA repair related and DNA damage response genes in the intertidal copepodTigriopus japonicus was analyzed after ultraviolet irradiation.[40] This study revealed increased expression of proteins associated with the DNA repair processes ofnon-homologous end joining,homologous recombination,base excision repair andDNA mismatch repair.
The subphylum Crustacea comprises almost 67,000 describedspecies,[43] which is thought to be just1⁄10 to1⁄100 of the total number as most species remain as yetundiscovered.[44] Although most crustaceans are small, their morphology varies greatly and includes both the largest arthropod in the world – theJapanese spider crab with a leg span of 3.7 metres (12 ft)[45] – and the smallest, the 100-micrometre-long (0.004 in)Stygotantulus stocki.[46] Despite their diversity of form, crustaceans are united by the speciallarval form known as thenauplius.
The exact relationships of the Crustacea to other taxa are not completely settled as of April 2012[update]. Studies based on morphology led to thePancrustacea hypothesis,[47] in which Crustacea andHexapoda (insects and allies) aresister groups. More recent studies usingDNA sequences suggest that Crustacea isparaphyletic, with the hexapods nested within a larger Pancrustaceaclade.[48][49]
The traditional classification of Crustacea based on morphology recognised four to six classes.[50] Bowman and Abele (1982) recognised 652 extant families and 38 orders, organised into six classes:Branchiopoda,Remipedia,Cephalocarida,Maxillopoda,Ostracoda, andMalacostraca.[50] Martin and Davis (2001) updated this classification, retaining the six classes but including 849 extant families in 42 orders. Despite outlining the evidence that Maxillopoda was non-monophyletic, they retained it as one of the six classes, although did suggest that Maxillipoda could be replaced by elevating its subclasses to classes.[51] Since then phylogenetic studies have confirmed the polyphyly of Maxillipoda and the paraphyletic nature of Crustacea with respect to Hexapoda.[52][53][54][55] Recent classifications recognise ten to twelve classes in Crustacea or Pancrustacea, with several former maxillopod subclasses now recognised as classes (e.g.Thecostraca,Tantulocarida,Mystacocarida,Copepoda,Branchiura andPentastomida).[56][57]
The following cladogram shows the updated relationships between the different extant groups of the paraphyleticCrustacea in relation to the classHexapoda.[53]
According to this diagram, the Hexapoda are deep in the Crustacea tree, and any of the Hexapoda is distinctly closer to e.g. a Multicrustacean than an Oligostracan is.
Crustaceans have a rich and extensivefossil record, most of the major groups of crustaceans appear in the fossil record before the end of the Cambrian, namely theBranchiopoda,Maxillopoda (includingbarnacles andtongue worms) andMalacostraca; there is some debate as to whether or not Cambrian animals assigned to Ostracoda are trulyostracods, which would otherwise start in theOrdovician.[58] The only classes to appear later are theCephalocarida,[59] which have no fossil record, and theRemipedia, which were first described from the fossilTesnusocaris goldichi, but do not appear until theCarboniferous.[60] Most of the early crustaceans are rare, but fossil crustaceans become abundant from theCarboniferous period onwards.[61]
Within the Malacostraca, no fossils are known forkrill,[62] while bothHoplocarida andPhyllopoda contain important groups that are now extinct as well as extant members (Hoplocarida:mantis shrimp are extant, whileAeschronectida are extinct;[63] Phyllopoda:Canadaspidida are extinct, whileLeptostraca are extant[64]).Cumacea andIsopoda are both known from theCarboniferous,[65][66] as are the first true mantis shrimp.[67] In theDecapoda,prawns andpolychelids appear in the Triassic,[68][69] andshrimp andcrabs appear in theJurassic.[70][71] The fossil burrowOphiomorpha is attributed to ghost shrimps, whereas the fossil burrowCamborygma is attributed to crayfishes. The Permian–Triassic deposits of Nurra preserve the oldest (Permian: Roadian) fluvial burrows ascribed to ghost shrimps (Decapoda: Axiidea, Gebiidea) and crayfishes (Decapoda: Astacidea, Parastacidea), respectively.[72]
However, the great radiation of crustaceans occurred in theCretaceous, particularly in crabs, and may have been driven by theadaptive radiation of their main predators,bony fish.[71] The first truelobsters also appear in the Cretaceous.[73]
Many crustaceans are consumed by humans, and nearly 10,700,000 tons were harvested in 2007; the vast majority of this output is ofdecapod crustaceans:crabs,lobsters,shrimp,crawfish, andprawns.[74] Over 60% by weight of all crustaceans caught for consumption are shrimp and prawns, and nearly 80% is produced in Asia, with China alone producing nearly half the world's total.[74] Non-decapod crustaceans are not widely consumed, with only 118,000 tons ofkrill being caught,[74] despite krill having one of the greatestbiomasses on the planet.[75]
^Burkenroad, M. D. (1963). "The evolution of the Eucarida (Crustacea, Eumalacostraca), in relation to the fossil record".Tulane Studies in Geology.2 (1):1–17.
^Rhee, J. S.; Kim, B. M.; Choi, B. S.; Lee, J. S. (2012). "Expression pattern analysis of DNA repair-related and DNA damage response genes revealed by 55K oligomicroarray upon UV-B irradiation in the intertidal copepod, Tigriopus japonicus".Comparative Biochemistry and Physiology. Toxicology & Pharmacology.155 (2):359–368.doi:10.1016/j.cbpc.2011.10.005.PMID22051804.
^Jerome C. Regier; Jeffrey W. Shultz; Andreas Zwick; April Hussey; Bernard Ball; Regina Wetzer; Joel W. Martin; Clifford W. Cunningham (February 25, 2010). "Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences".Nature.463 (7284):1079–1083.Bibcode:2010Natur.463.1079R.doi:10.1038/nature08742.PMID20147900.S2CID4427443.
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