Crustaceans (
Crustacea) form a very large group of
arthropods, usually treated as a
subphylum, which includes such familiar animals as
crabs,
lobsters,
crayfish,
shrimp,
krill and
barnacles. The 50,000 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 the
nauplius form of the
larvae.
Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. woodlice), some are parasitic (e.g. fish lice, tongue worms) and some are sessile (e.g. barnacles). The group has an extensive fossil record, reaching back to the Cambrian, and includes living fossils such as Triops cancriformis, which has existed apparently unchanged since the Triassic period. More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimps 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
of a
lady crab, part of the hard
exoskeleton|alt=A convex oval-shaped piece of shell, covered with fine orange-pink markings: the front edge is lined with 13 coarse serrations, while the rear edge is smooth.]]
The body of a crustacean is composed of
body segments, which are grouped into three regions: the
cephalon or
head, the
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, 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 mandibles and maxillae; 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 uniramous. 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, where blood is pumped into the haemocoel 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; 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
The majority of crustaceans are aquatic, living in either
marine or
fresh water 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. The majority of 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.
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 and the
Asian shore crab.
Life cycle
, a freshwater crab|alt=Seven round translucent spheres: inside some of them, a pair of compound eyes can be seen.]]
larva of the
European lobster,
Homarus gammarus|alt=A grey-green translucent animal is seen from the side. The eye is large and shining, and is in a recess of the large carapace and its long rostrum. An abdomen, similar in length to the carapace, projects from the rear, and below the carapace, there is a mass of legs, some with small claws.]]
Mating system
The majority of crustaceans have
separate sexes, and
reproduce sexually. A small number are
hermaphrodites, including
barnacles,
remipedes, and
Cephalocarida. Some may even change sex during the course of their life. 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.). 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.
Classification
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, 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. although the number of
undescribed species may be 10–100 times higher. Although most crustaceans are small, their morphology varies greatly and they include both the largest arthropod in the world – the
Japanese spider crab with a leg span of – and the smallest – the long
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 yet entirely clear. Under the Pancrustacea hypothesis, Crustacea and Hexapoda (insects and allies) are sister groups. Studies using DNA sequences tend to show a paraphyletic Crustacea, with the insects (but not necessarily other hexapods) nested within that clade. Although the classification of crustaceans has been quite variable, the system used by Martin and Davis 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 onwards. 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 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 first true lobsters also appear in the Cretaceous.
Consumption by man
Many crustaceans are consumed by humans, and nearly 10,700,000
tons were produced in 2007; the vast majority of this output is of
decapod crustaceans:
crabs,
lobsters,
shrimp, 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.
References
General references
External links
Crustacea.net, an online resource on the biology of crustaceans
Crustacea: 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 Singapore
Crustacea(crabs, lobsters, shrimps, prawns, barnacles): Biodiversity Explorer
Category:Arthropods
