Most crustaceans are free-living
aquatic animals, but some are
terrestrial (e.g.
woodlice,
sandhoppers), some are
parasitic (e.g.
Rhizocephala,
fish lice,
tongue worms) and some are
sessile (e.g.
barnacles). The group has an extensive
fossil record, reaching back to the
Cambrian. More than 7.9 million tons of crustaceans per year are harvested by fishery or farming for human consumption,[4] consisting mostly of
shrimp and prawns.
Krill and
copepods are not as widely fished, but may be the animals with the greatest
biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as
carcinology (alternatively, malacostracology, crustaceology or crustalogy), and a scientist who works in carcinology is a
carcinologist.
Structure
The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or
head,[5] the pereon or
thorax,[6] and the pleon or
abdomen.[7] The head and thorax may be fused together to form a
cephalothorax,[8] which may be covered by a single large
carapace.[9] The crustacean body is protected by the hard
exoskeleton, which must be
moulted for the animal to grow. The shell around each somite can be divided into a dorsal
tergum, ventral
sternum and a lateral pleuron. Various parts of the exoskeleton may be fused together.[10]: 289
Each
somite, or body segment can bear a pair of
appendages: on the segments of the head, these include two pairs of
antennae, the
mandibles and
maxillae;[5] the thoracic segments bear
legs, which may be specialised as
pereiopods (walking legs) and
maxillipeds (feeding legs).[6] Malacostraca and Remipedia (and the hexapods) have abdominal appendages. All other classes of crustaceans have a limbless abdomen, except from a
telson and
caudal rami which is present in many groups.[11][12]
The abdomen in malacostracans bears
pleopods,[7] and ends in a telson, which bears the
anus, and is often flanked by uropods to form a
tail fan.[13] The number and variety of
appendages in different crustaceans may be partly responsible for the group's success.[14]
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 usually
uniramous, 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 an
open circulatory system, where blood is pumped into the
haemocoel by a
heart located near the dorsum.[18] Malacostraca have
haemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods have
haemoglobins.[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 many
decapods, 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
woodlice, 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]
Ecology
Most crustaceans are aquatic, living in either marine or
freshwater environments, but a few groups have
adapted to life on land, such as
terrestrial crabs,
terrestrial hermit crabs, and
woodlice. Marine crustaceans are as ubiquitous in the oceans as insects are on land.[23][24] Most crustaceans are also
motile, moving about independently, although a few taxonomic units are
parasitic and live attached to their hosts (including
sea lice,
fish lice,
whale lice,
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. Some branchiurans are able to withstand rapid changes of
salinity and will also switch hosts from marine to non-marine species.[25]: 672 Krill are the bottom layer and the most important part of the food chain in
Antarctic animal communities.[26]: 64 Some crustaceans are significant
invasive species, such as the Chinese mitten crab, Eriocheir sinensis,[27] and the Asian shore crab, Hemigrapsus sanguineus.[28] Since the piercing of the
Suez 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]
Life cycle
Mating system
Most crustaceans have
separate sexes, and
reproduce sexually. In fact, a recent study explains how the male T. 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 are
hermaphrodites, including
barnacles,
remipedes,[31] and
Cephalocarida.[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 many
branchiopods, some
ostracods, some
isopods, and certain "higher" crustaceans, such as the Marmorkrebs crayfish.
Eggs
In many crustaceans, the fertilised eggs are released into the
water column, while others have developed a number of mechanisms for holding on to the eggs until they are ready to hatch. Most
decapods carry the eggs attached to the
pleopods, while
peracarids,
notostracans,
anostracans, and many
isopods form a
brood pouch from the
carapace 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 Most
leptostracans and
krill carry the eggs between their thoracic limbs; some
copepods 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 the
nauplius. This has three pairs of
appendages, all emerging from the young animal's head, and a single naupliar eye. In most groups, there are further larval stages, including the
zoea (pl. zoeæ or zoeas[34]). This name was given to it when naturalists believed it to be a separate species.[35] It follows the
nauplius stage and precedes the
post-larva. Zoea larvae swim with their thoracic
appendages, 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.[36] In many
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.
Providing camouflage against predators, the otherwise black eyes in several forms of swimming larvae are covered by a thin layer of crystalline
isoxanthopterin 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]
DNA repair
In an effort to understand whether
DNA repair processes can protect crustaceans against
DNA damage, basic research was conducted to elucidate the repair mechanisms used by Penaeus monodon (black tiger shrimp).[39] Repair of DNA double-strand breaks was found to be predominantly carried out by accurate
homologous 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 copepod Tigriopus japonicus was analyzed after ultraviolet irradiation.[40] This study revealed increased expression of proteins associated with the DNA repair processes of
non-homologous end joining,
homologous recombination,
base excision repair and
DNA mismatch repair.
The subphylum Crustacea comprises almost 67,000 described
species,[43] which is thought to be just 1⁄10 to 1⁄100 of the total number as most species remain as yet
undiscovered.[44] Although most crustaceans are small, their morphology varies greatly and includes both the largest arthropod in the world – the
Japanese 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 special
larval form known as the
nauplius.
The exact relationships of the Crustacea to other taxa are not completely settled as of April 2012[update]. Studies based on morphology led to the
Pancrustacea hypothesis,[47] in which Crustacea and
Hexapoda (
insects and allies) are
sister groups. More recent studies using
DNA sequences suggest that Crustacea is
paraphyletic, with the hexapods nested within a larger Pancrustacea
clade.[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, and
Malacostraca.[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 and
Pentastomida).[56][57]
The following cladogram shows the updated relationships between the different extant groups of the paraphyletic Crustacea in relation to the class
Hexapoda.[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.
Within the Malacostraca, no fossils are known for
krill,[63] while both
Hoplocarida and
Phyllopoda contain important groups that are now extinct as well as extant members (Hoplocarida:
mantis shrimp are extant, while
Aeschronectida are extinct;[64] Phyllopoda:
Canadaspidida are extinct, while
Leptostraca are extant[59]).
Cumacea and
Isopoda are both known from the
Carboniferous,[65][66] as are the first true mantis shrimp.[67] In the
Decapoda,
prawns and
polychelids appear in the Triassic,[68][69] and
shrimp and
crabs appear in the
Jurassic.[70][71] The fossil burrow Ophiomorpha is attributed to ghost shrimps, whereas the fossil burrow Camborygma 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 the
Cretaceous, particularly in crabs, and may have been driven by the
adaptive radiation of their main predators,
bony fish.[71] The first true
lobsters also appear in the Cretaceous.[73]
Consumption by humans
Many crustaceans are consumed by humans, and nearly 10,700,000
tons were harvested in 2007; the vast majority of this output is of
decapod crustaceans:
crabs,
lobsters,
shrimp,
crawfish, and
prawns.[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 of
krill being caught,[74] despite krill having one of the greatest
biomasses 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.
^Bernot, James P.; Owen, Christopher L.; Wolfe, Joanna M.; Meland, Kenneth; Olesen, Jørgen; Crandall, Keith A. (2023). "Major Revisions in Pancrustacean Phylogeny and Evidence of Sensitivity to Taxon Sampling". Molecular Biology and Evolution. 40 (8): msad175.
doi:
10.1093/molbev/msad175.
PMC 10414812.
PMID37552897.
^Brusca, Richard C. (2016). Invertebrates (3rd ed.). Sunderland, MA: Sinauer Associates. p. 222.
ISBN9781605353753.