| Achelata Temporal range: | |
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| The phyllosoma larva is characteristic of the Achelata (drawing by Haeckel) | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Crustacea |
| Class: | Malacostraca |
| Order: | Decapoda |
| Suborder: | Pleocyemata |
| (unranked): | Reptantia |
| Infraorder: | Achelata Scholtz & Richter, 1995 |
| Families [1] | |
The Achelata is an infra-order of the decapod crustaceans, holding the spiny lobsters, slipper lobsters and their fossil relatives.
The name "Achelata" derives from the fact that all the members of this group lack the chelae (claws) that are found on almost all other decapods (from the Ancient Greek ἀ- , a- = "not", χηλή , chela = "claw"). They are further united by the great enlargement of the second antennae, by the special "phyllosoma" form of the larva, and by a number of other characters. [2]
The infraorder Achelata belongs to the group Reptantia, which consists of the walking/crawling decapods (lobsters and crabs). The cladogram below shows Achelata's placement within the larger order Decapoda, from analysis by Wolfe et al., 2019. [3]
| Decapoda |
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Achelata contains the spiny lobsters (Palinuridae), the slipper lobsters (Scyllaridae) and the furry lobsters (Synaxidae, now usually included in Palinuridae), [4] as well as two extinct families, Cancrinidae and Tricarinidae. [1]
Both Palinuridae and Scyllaridae have a fossil record extending back to the Cretaceous. [5] [6] The two fossil families contain a single genus each; [1] Tricarina is known from a single Cretaceous fossil, [7] while Cancrinos is known from the Jurassic and Cretaceous. [6] One estimate of the divergence between Achelata and its closest relatives places it at about 341 million years ago. [8]
The Caridea, commonly known as caridean shrimp or true shrimp, are an infraorder of shrimp within the order Decapoda. This infraorder contains all species of true shrimp. They are found widely around the world in both fresh and salt water. Many other animals with similar names – such as the mud shrimp of Axiidea and the boxer shrimp of Stenopodidea – are not true shrimp, but many have evolved features similar to true shrimp.
Spiny lobsters, also known as langustas, langouste, or rock lobsters, are a family (Palinuridae) of about 60 species of achelate crustaceans, in the Decapoda Reptantia. Spiny lobsters are also, especially in Australia, New Zealand, Ireland, South Africa, and The Bahamas, called crayfish, sea crayfish, or crawfish, terms which elsewhere are reserved for freshwater crayfish.
The Decapoda or decapods are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is estimated to contain nearly 15,000 species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp and Anomura including hermit crabs, porcelain crabs, squat lobsters making up the bulk of the remainder. The earliest fossil decapod is the Devonian Palaeopalaemon.
Reptantia is a clade of decapod crustaceans named in 1880 which includes lobsters, crabs and many other well-known crustaceans.
Pleocyemata is a suborder of decapod crustaceans, erected by Martin Burkenroad in 1963. Burkenroad's classification replaced the earlier sub-orders of Natantia and Reptantia with the monophyletic groups Dendrobranchiata (prawns) and Pleocyemata. Pleocyemata contains all the members of the Reptantia, as well as the Stenopodidea, and Caridea, which contains the true shrimp.
Anomura is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans include the word crab, all true crabs are in the sister group to the Anomura, the Brachyura.
Slipper lobsters are a family (Scyllaridae) of about 90 species of achelate crustaceans, in the Decapoda clade Reptantia, found in all warm oceans and seas. They are not true lobsters, but are more closely related to spiny lobsters and furry lobsters. Slipper lobsters are instantly recognisable by their enlarged antennae, which project forward from the head as wide plates. All the species of slipper lobsters are edible, and some, such as the Moreton Bay bug and the Balmain bug are of commercial importance.
Astacoidea is superfamily of freshwater crayfish that live in the Northern Hemisphere. The other superfamily of crayfish, Parastacoidea, lives in the Southern Hemisphere. Astacoidea consists of three families: Astacidae, Cambaridae, and Cambaroididae. Crayfish are closely related to lobsters, as shown in the simplified cladogram below:
Astacidea is an infraorder of decapod crustaceans including lobsters, crayfish, and their close relatives.
Furry lobsters are small decapod crustaceans, closely related to the slipper lobsters and spiny lobsters. The antennae are not as enlarged as in spiny and slipper lobsters, and the body is covered in short hairs, hence the name furry lobster. Although previously considered a family in their own right, the furry lobsters were subsumed into the family Palinuridae in 1990, and molecular phylogenies support the inclusion of the furry lobsters in the family Palinuridae.
Glypheidea is an infraorder of lobster-like decapod crustaceans, comprising a number of fossil forms and the two extant (living) genera Neoglyphea and Laurentaeglyphea: The infraorder was thought to be extinct until a living species, Neoglyphea inopinata, was discovered in 1975. They are now considered "living fossils", with over 256 fossil species discovered, and just two extant species.
Hippoidea is a superfamily of decapod crustaceans known as sand crabs, mole crabs.
Dromioidea is a superfamily of crabs mostly found in Madagascar. Dromioidea belongs the group Dromiacea, taxonomically ranked as a section, which is the most basal grouping of Brachyura crabs. Dromiacea likely diverged from the rest of Brachyura around the Late Triassic or Early Jurassic, and the earliest fossils attributable to the Dromioidea date from the Late Jurassic.
The phyllosoma is the larval stage of spiny, slipper and coral lobsters, and represents one of the most significant characteristics that unify them into the taxon Achelata. Its body is remarkably thin, flat, and transparent, with long legs.
Polychelida is an infraorder of decapod crustaceans. Fossil representatives are known dating from as far back as the Upper Triassic. A total of 38 extant species, all in the family Polychelidae, and 55 fossil species have been described.
Eubrachyura is a group of decapod crustaceans comprising the more derived crabs. It is divided into two subsections, based on the position of the genital openings in the two sexes. In the Heterotremata, the openings are on the legs in the males, but on the sternum in females, while in the Thoracotremata, the openings are on the sternum in both sexes. This contrasts with the situation in other decapods, in which the genital openings are always on the legs. Heterotremata is the larger of the two groups, containing the species-rich superfamilies Xanthoidea and Pilumnoidea and all the freshwater crabs. The eubrachyura is well known for actively and constantly building its own burrows. The fossil record of the Eubrachyura extends back to the Cretaceous; the supposed Bathonian representative of the group, Hebertides jurassica, ultimately turned out to be Cenozoic in age.
Gebiidea is an infraorder of decapod crustaceans. Gebiidea and Axiidea are divergent infraoders of the former infraorder Thalassinidea. These infraorders have converged ecologically and morphologically as burrowing forms. Based on molecular evidence as of 2009, it is now widely believed that these two infraorders represent two distinct lineages separate from one another. Since this is a recent change, much of the literature and research surrounding these infraorders still refers to the Axiidea and Gebiidea in combination as "thalassinidean" for the sake of clarity and reference. This division based on molecular evidence is consistent with the groupings proposed by Robert Gurney in 1938 based on larval developmental stages.
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow. The larvae of crustaceans often bear little resemblance to the adult, and there are still cases where it is not known what larvae will grow into what adults. This is especially true of crustaceans which live as benthic adults, more-so than where the larvae are planktonic, and thereby easily caught.
Cancrinos is a genus of fossil crustacean closely allied with the slipper lobsters. One species is known, C. claviger from the Jurassic of southern Germany.
Tricarina is a genus of crustaceans of uncertain phylogenetic placement, known from a single incomplete fossil specimen from the Cretaceous of western Iran. It has a flattened body with three longitudinal ridges, which give it its name.
Media related to Achelata at Wikimedia CommonsSubgroups of Order Decapoda | ||
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| Dendrobranchiata |   | |
| Pleocyemata | ||
