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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 County
Crustacea
Tree of Life Web Project
The Crustacean Society

Natural History Collections: Crustacea
University of Edinburgh
Crustaceans (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