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Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of Arthropod that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic animal including decapoda (, , , and crayfish), ostracoda, branchiopoda, argulidae, krill, , , , , Mysida, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the Hexapoda ( and ) emerged deep in the Crustacean group, with the completed pan-group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans ( and ).
The 67,000 described species range in size from Stygotantulus at , to the Japanese spider crab with a leg span of up to and a mass of . Like other , crustaceans have an exoskeleton, which they ecdysis to grow. They are distinguished from other groups of arthropods, such as , Myriapoda and Chelicerata, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of and .
Most crustaceans are free-living , but some are terrestrial (e.g. woodlouse, ), some are Parasitism (e.g. Rhizocephala, Argulidae, Pentastomida) and some are sessile (e.g. ). 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 prawns. Krill and 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.
Each somite, or body segment can bear a pair of : on the segments of the head, these include two pairs of antennae, the mandibles and maxillae; the thoracic segments bear arthropod leg, which may be specialised as (walking legs) and (feeding legs). Malacostraca and Remipedia (and the hexapods) have abdominal appendages. All other classes of crustaceans have a limbless abdomen, except from a telson and Caudal ramus which is present in many groups. Morphology of the brain in Hutchinsoniella macracantha (Cephalocarida, Crustacea) – page 290 The abdomen in malacostracans bears , and ends in a telson, which bears the anus, and is often flanked by uropods to form a Decapod anatomy. The number and variety of in different crustaceans may be partly responsible for the group's success.
Crustacean 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. , for instance, also possessed biramous appendages.
The main body cavity is an open circulatory system, where blood is pumped into the Hemocoel by a heart located near the dorsum. Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods have . 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 Decapoda, 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 woodlouse, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs until they hatch into free-swimming larvae.
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.Shavit, Keshet, et al, A tunable reflector enabling crustaceans to see but not be seen, Science, February 16, 2023, and published in volume 379, issue 6633, February 17, 2023 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.
The subphylum Crustacea comprises almost 67,000 described species, which is thought to be just to of the total number as most species remain as yet undiscovered. 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-long (0.004 in) Stygotantulus. Despite their diversity of form, crustaceans are united by the special 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 ( and allies) are . More recent studies using suggest that Crustacea is paraphyletic, with the hexapods nested within a larger Pancrustacea clade.
The traditional classification of Crustacea based on morphology recognised four to six classes. Bowman and Abele (1982) recognised 652 extant families and 38 orders, organised into six classes: Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Ostracoda, and Malacostraca. 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. Since then phylogenetic studies have confirmed the polyphyly of Maxillopoda and the paraphyletic nature of Crustacea with respect to Hexapoda. 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).
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, 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, Dendrobranchiata and Polychelida appear in the Triassic, and Caridea and appear in the Jurassic. 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, particularly in crabs, and may have been driven by the adaptive radiation of their main predators, bony fish. The first true also appear in the Cretaceous.
(2025). 9780691170251, Princeton University Press. ISBN 9780691170251
The following cladogram shows the updated relationships between the different extant groups of the paraphyletic Crustacea in relation to the class Hexapoda.
Cylindroleberididae
(Myodocopida)
Ctenocheilocaris galvarini
Armillifer armillatus
(Porocephalida)
Perforatus
(Cirripedia)
Cylindroleberididae
(Calanoida)
Microdajus sp.
Ocypode ceratophthalma
(Decapoda)
Hutchinsoniella macracantha
Lepidurus arcticus
(Notostraca)
Speleonectes tanumekes
Mantispa styriaca
(Neuroptera)
Fossil record
Crustaceans have a rich and extensive fossil record, most of the major groups of crustaceans appear in the fossil record before the end of the Cambrian, namely the Branchiopoda, Maxillopoda (including and Pentastomida) and Malacostraca; there is some debate as to whether or not Cambrian animals assigned to Ostracoda are truly , 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, but do not appear until the Carboniferous. Most of the early crustaceans are rare, but fossil crustaceans become abundant from the Carboniferous period onwards.
Consumption by humans
Many crustaceans are consumed by humans, and nearly 10,700,000 were harvested in 2007; the vast majority of this output is of Decapoda: , , shrimp, crawfish, and Dendrobranchiata. 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.
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