Crustaceans (Crustacea, ) form a large, diverse
arthropod taxon which includes such animals as
decapods,
seed shrimp
Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typica ...
,
branchiopods,
fish lice
The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family, ...
,
krill,
remipede
Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
s,
isopods,
barnacles,
copepods,
amphipods and
mantis shrimp. The crustacean group can be treated as a
subphylum under the
clade Mandibulata. It is now well accepted that the
hexapods emerged deep in the Crustacean group, with the completed group referred to as
Pancrustacea.
Some crustaceans (
Remipedia,
Cephalocarida,
Branchiopoda) are more closely related to
insects and the other hexapods than they are to certain other crustaceans.
The 67,000 described species range in size from ''
Stygotantulus stocki'' at , to the
Japanese spider crab with a leg span of up to and a mass of . Like other
arthropods, crustaceans have an
exoskeleton, which they
moult to grow. They are distinguished from other groups of arthropods, such as
insects,
myriapods and
chelicerates, by the possession of
biramous (two-parted) limbs, and by their
larval forms, such as the
nauplius stage of
branchiopods and
copepods.
Most crustaceans are free-living
aquatic animals, but some are
terrestrial (e.g.
woodlice,
sandhoppers), some are
parasitic (e.g.
Rhizocephala,
fish lice
The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family, ...
,
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, consisting mostly of
shrimp and prawn
Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are referre ...
s.
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
A carcinologist is a scientist who studies crustaceans or is otherwise involved in carcinology
Carcinology is a branch of zoology that consists of the study of crustaceans, a group of arthropods that includes lobsters, crayfish, shrimp, krill, ...
.
Structure
The body of a crustacean is composed of segments, which are grouped into three regions: the ''cephalon'' or
head,
the ''pereon'' or
thorax,
and the ''pleon'' or
abdomen.
The head and thorax may be fused together to form a
cephalothorax, which may be covered by a single large
carapace. 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
A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or ' ...
, ventral
sternum and a lateral pleuron. Various parts of the exoskeleton may be fused together.
Each
somite, or body segment can bear a pair of
appendages: on the segments of the head, these include two pairs of
antennae, the
mandible
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
s and
maxillae;
the thoracic segments bear
legs
A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element c ...
, which may be specialised as
pereiopods (walking legs) and
maxilliped
An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body.
In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including ante ...
s (feeding legs).
The abdomen bears
pleopods,
and ends in a
telson, which bears the
anus, and is often flanked by uropods to form a
tail fan.
The number and variety of
appendages in different crustaceans may be partly responsible for the group's success.
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.
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.
The main body cavity is an
open circulatory system
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...
, where blood is pumped into the
haemocoel by a
heart
The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
located near the dorsum. Malacostraca have
haemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods have
haemoglobins. 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. 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.
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.
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. 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
The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family, ...
,
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.
Krill are the bottom layer and the most important part of the food chain in
Antarctic animal communities. Some crustaceans are significant
invasive species, such as the Chinese mitten crab, ''
Eriocheir sinensis
The Chinese mitten crab ('; , "big sluice crab"), also known as the Shanghai hairy crab (, p ''Shànghǎi máoxiè''), is a medium-sized burrowing crab that is named for its furry claws, which resemble mittens. It is native to river ...
'', and the Asian shore crab, ''
Hemigrapsus sanguineus
''Hemigrapsus sanguineus'', the Japanese shore crab or Asian shore crab, is a species of crab from East Asia. It has been introduced to several other regions, and is now an invasive species in North America and Europe. It was introduced to these ...
''. 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.
Life cycle
Mating system
Most crustaceans have
separate sexes, and
reproduce sexually. In fact, a recent study explains how the male crustaceans,T. Californicus decide which females to mate with by dietary differences, preferring when the females are algae-fed instead of yeast-fed.
A small number are
hermaphrodites, including
barnacles,
remipede
Remipedia is a class of blind crustaceans found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The ...
s, and
Cephalocarida.
Some may even change sex during the course of their life.
Parthenogenesis is also widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male.
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.
Female Branchiura do not carry eggs in external ovisacs but attach them in rows to rocks and other objects.
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.
Larvae
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). This name was given to it when naturalists believed it to be a separate species. 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. 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.
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
''Penaeus monodon'', commonly known as the giant tiger prawn, Asian tiger shrimp, black tiger shrimp, and other names, is a marine crustacean that is widely reared for food.
Taxonomy
''Penaeus monodon'' was first described by Johan Christian ...
'' (black tiger shrimp). 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. This study revealed increased expression of proteins associated with the DNA repair processes of
non-homologous end joining,
homologous recombination,
base excision repair
Base excision repair (BER) is a cellular mechanism, studied in the fields of biochemistry and genetics, that repairs damaged DNA throughout the cell cycle. It is responsible primarily for removing small, non-helix-distorting base lesions from ...
and
DNA mismatch repair.
Classification and phylogeny
The name "crustacean" dates from the earliest works to describe the animals, including those of
Pierre Belon and
Guillaume Rondelet, but the name was not used by some later authors, including
Carl Linnaeus
Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, ...
, who included crustaceans among the "
Aptera" in his '.
The earliest nomenclaturally valid work to use the name "Crustacea" was
Morten Thrane Brünnich's ' in 1772, although he also included
chelicerates in the group.
The subphylum Crustacea comprises almost 67,000 described
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
, which is thought to be just to of the total number as most species remain as yet
undiscovered
''Undiscovered'' is a 2005 American drama film directed by Meiert Avis. The plot is about a group of aspiring entertainers who intend to establish their careers in Los Angeles. Released on August 26, 2005, the film received a largely negative r ...
. 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 – and the smallest, the 100-
micrometre
The micrometre (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...
-long (0.004 in) ''
Stygotantulus stocki''. 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 . Studies based on morphology led to the
Pancrustacea hypothesis, 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.
Although the classification of crustaceans has been quite variable, the system used by Martin and Davis
largely supersedes earlier works.
Mystacocarida and
Branchiura
The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family, ...
, here treated as part of
Maxillopoda, are sometimes treated as their own classes. Eleven classes are recognised:
The following cladogram shows the updated relationships between the different extant groups of the paraphyletic Crustacea in relation to the class
Hexapoda.
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.
Fossil record
Crustaceans have a rich and extensive
fossil record, which begins with animals such as ''
Canadaspis
''Canadaspis'' ("Shield of Canada") is an extinct genus of bivalved Cambrian arthropod, known from North America and China. They are thought to have been benthic feeders that moved mainly by walking and possibly used its biramous appendages to st ...
'' and ''
Perspicaris
''Perspicaris'' (from the Latin ''perspicax'', meaning “sharp-sighted,” and ''caris'', “crab/shrimp”) an extinct genus of bivalved arthropod from the Cambrian period. Fossils have been found in North America, primarily the Burgess Shale ...
'' from the
Middle Cambrian
Middle or The Middle may refer to:
* Centre (geometry), the point equally distant from the outer limits.
Places
* Middle (sheading), a subdivision of the Isle of Man
* Middle Bay (disambiguation)
* Middle Brook (disambiguation)
* Middle Creek ( ...
age
Burgess Shale.
Most of the major groups of crustaceans appear in the fossil record before the end of the Cambrian, namely the
Branchiopoda,
Maxillopoda (including
barnacles and
tongue worms) and
Malacostraca
Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobs ...
; there is some debate as to whether or not Cambrian animals assigned to Ostracoda are truly
ostracods, which would otherwise start in the
Ordovician. The only classes to appear later are the
Cephalocarida, which have no fossil record, and the
Remipedia, which were first described from the fossil ''
Tesnusocaris goldichi'', but do not appear until the
Carboniferous. Most of the early crustaceans are rare, but fossil crustaceans become abundant from the
Carboniferous period
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonif ...
onwards.
Within the Malacostraca, no fossils are known for
krill, 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; Phyllopoda:
Canadaspidida are extinct, while
Leptostraca are extant
).
Cumacea
Cumacea is an order of small marine crustaceans of the superorder Peracarida, occasionally called hooded shrimp or comma shrimp. Their unique appearance and uniform body plan makes them easy to distinguish from other crustaceans. They live in so ...
and
Isopoda are both known from the
Carboniferous, as are the first true mantis shrimp. In the
Decapoda,
prawns and
polychelids appear in the Triassic, and
shrimp and
crabs appear in the
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
.
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.
However, the great radiation of crustaceans occurred in the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
, particularly in crabs, and may have been driven by the
adaptive radiation
In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic in ...
of their main predators,
bony fish.
The first true
lobsters also appear in the Cretaceous.
Consumption by humans
Many crustaceans are consumed by humans, and nearly 10,700,000
ton
Ton is the name of any one of several units of measure. It has a long history and has acquired several meanings and uses.
Mainly it describes units of weight. Confusion can arise because ''ton'' can mean
* the long ton, which is 2,240 pounds
...
s were harvested in 2007; the vast majority of this output is of
decapod crustaceans:
crabs,
lobsters,
shrimp,
crawfish, and
prawns.
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.
Non-decapod crustaceans are not widely consumed, with only 118,000 tons of
krill being caught,
despite krill having one of the greatest
biomasses on the planet.
See also
*
Pain in crustaceans
References
Sources
*
Powers, M., Hill, G., Weaver, R., & Goymann, W. (2020). An experimental test of mate choice for red carotenoid coloration in the marine copepod Tigriopus californicus. Ethology., 126(3), 344–352
An experimental test of mate choice for red carotenoid coloration in the marine copepod Tigriopus californicus
External links
*
Crustacea.net, an online resource on the biology of crustaceansCrustacea Natural History Museum of Los Angeles CountyCrustacea Tree of Life Web ProjectThe Crustacean SocietyNatural History Collections: Crustacea University of EdinburghCrustaceans (Crustacea) on the shore of SingaporeCrustacea(crabs, lobsters, shrimps, prawns, barnacles): Biodiversity Explorer
{{Good article
Cambrian Series 2 first appearances
Extant Cambrian first appearances
Paraphyletic groups
Pancrustacea