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Temporal range: Norian–Recent
Dardanus megistos2.jpg
The hermit crab Dardanus megistos
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
(unranked): Reptantia
Infraorder: Anomura
Macleay, 1838

Anomura (sometimes Anomala) 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 (the two groups together form the clade Meiura). [1]



The name Anomura derives from an old classification in which reptant decapods were divided into Macrura (long-tailed), Brachyura (short-tailed) and Anomura (differently-tailed). The alternative name Anomala reflects the unusual variety of forms in this group; whereas all crabs share some obvious similarities, the various groups of anomurans are quite dissimilar. [2]

The group has been moulded by several instances of carcinisation – the development of a crab-like body form. [3] Thus, the king crabs (Lithodidae), porcelain crabs (Porcellanidae) and hairy stone crab (Lomisidae) are all separate instances of carcinisation. [3]

As decapods (meaning ten-legged), anomurans have ten pereiopods, but the last pair of these is reduced in size, and often hidden inside the gill chamber (under the carapace) to be used for cleaning the gills. [4] [2] Since this arrangement is very rare in true crabs (for example, the small family Hexapodidae), [5] a "crab" with only eight visible pereiopods is generally an anomuran. [2]


The infraorder Anomura belongs to the group Reptantia, which consists of the walking/crawling decapods (lobsters and crabs). There is wide acceptance from morphological and molecular data that Anomura and Brachyura (true crabs) are sister taxa, together making up the clade Meiura. [3] The cladogram below shows Anomura's placement within the larger order Decapoda, from analysis by Wolfe et al., 2019. [6]


Dendrobranchiata (prawns) Litopenaeus setiferus.png


Stenopodidea (boxer shrimp) Spongicola venustus.png


Caridea (true shrimp) Macrobrachium sp.jpg

Reptantia  (crawling/walking decapods)

Achelata (spiny lobsters, slipper lobsters) Panulirus argus.png

Polychelida (benthic crustaceans)

Astacidea (lobsters, crayfish) Lobster NSRW rotated2.jpg

Axiidea (mud shrimp, ghost shrimp, or burrowing shrimp)

Gebiidea (mud lobsters and mud shrimp)


Anomura (hermit crabs and others) Coenobita variabilis.jpg

Brachyura (crabs) Charybdis japonica.jpg

Some of the internal relationships within Anomura can be shown in the cladogram below, which shows Hippidae as sister to Paguroidea, and resolves Parapaguridae outside of Paguroidea: [6]


Porcellanidae (porcelain crabs)

Munididae (squat lobsters)

Parapaguridae (deep water sea anemone hermit crabs)

Eumunididae (squat lobster-like)

Hippidae (mole crabs or sand crabs)


Lithodidae (king crabs)

Paguridae (hermit crabs)

Diogenidae (left-handed hermit crabs)

Coenobitidae (terrestrial hermit crabs)


The infraorder Anomura contained seven extant superfamilies: [7] [8] [9] [10]

Aegloidea Aegla Aeglidae Aegla sp. from Nahuel Huapi Lake, Bariloche, Argentina.jpg
Aegla sp.
Chirostyloidea squat lobsters Chirostylidae
Eumunida picta.jpg
Eumunida picta
Eocarcinoidea Eocarcinus
Galatheoidea squat lobsters
porcelain crabs
Munidopsis tridentata.jpg
Munidopsis serricornis
Hippoidea mole crabs
or sand crabs
Blepharipoda occidentalis
Lithodoidea king crabs Hapalogastridae
Lithodes santolla
Lomisoidea hairy stone crab Lomisidae
Paguroidea hermit crabs
coconut crab
Caribbean hermit crab.JPG
Coenobita clypeatus

The oldest fossil attributed to Anomura is Platykotta , from the NorianRhaetian (Late Triassic) Period in the United Arab Emirates. [8]

Related Research Articles

Crab Infraorder of decapod crustaceans

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen), usually hidden entirely under the thorax. They live in all the world's oceans, in fresh water, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers. They first appeared during the Jurassic Period.

Decapoda Order of crustaceans

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 Suborder of crustaceans

Reptantia is a clade of decapod crustaceans named in 1880 which includes lobsters, crabs and many other well-known crustaceans.

Pleocyemata Suborder of 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.

Achelata Infraorder of crustaceans

The Achelata is an infra-order of the decapod crustaceans, holding the spiny lobsters, slipper lobsters and their fossil relatives.

Squat lobster Decapod crustaceans in the infraorder Anomura

Squat lobsters are dorsoventrally flattened crustaceans with long tails held curled beneath the cephalothorax. They are found in the two superfamilies Galatheoidea and Chirostyloidea, which form part of the decapod infraorder Anomura, alongside groups including the hermit crabs and mole crabs. They are distributed worldwide in the oceans, and occur from near the surface to deep sea hydrothermal vents, with one species occupying caves above sea level. More than 900 species have been described, in around 60 genera. Some species form dense aggregations, either on the sea floor or in the water column, and a small number are commercially fished.

Carcinisation Evolution of non-crab-like crustaceans into crab-like forms

Carcinisation is an example of convergent evolution in which a crustacean evolves into a crab-like form from a non-crab-like form. The term was introduced into evolutionary biology by L. A. Borradaile, who described it as "one of the many attempts of Nature to evolve a crab". Most carcinised crustaceans belong to the infraorder Anomura.

Porcelain crab Family of crustaceans

Porcelain crabs are decapod crustaceans in the widespread family Porcellanidae, which superficially resemble true crabs. They have flattened bodies as an adaptation for living in rock crevices. They are delicate, readily losing limbs when attacked, and use their large claws for maintaining territories. They first appeared in the Tithonian age of the Late Jurassic epoch, 145-152 million years ago.

Astacidea Infraorder of crustaceans

Astacidea is an infraorder of decapod crustaceans including lobsters, crayfish, and their close relatives.

Hairy stone crab Species of crustacean

The hairy stone crab is a crab-like crustacean that lives in the littoral zone of southern Australia from Bunbury, Western Australia, to the Bass Strait. It is the only species in its family. It is 1.5–2.5 cm (0.6–1.0 in) wide, slow-moving, and covered in brown hair which camouflages it against the rocks upon which it lives.

Cyclodorippoida is a group of crabs, ranked as a section. It contains the single superfamily Cyclodorippoidea, which holds three families, Cyclodorippidae, Cymonomidae and Phyllotymolinidae.

Hippoidea Superfamily of crustaceans

Hippoidea is a superfamily of decapod crustaceans known as sand crabs, mole crabs, sand bugs, or sand fleas.

Dromioidea Superfamily of 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.

Podotremata Group of crabs

Podotremata is a traditionally used section of crabs (Brachyura), now considered to be invalid. It was the smaller grouping of crabs, separate from the larger Eubrachyura section. Morphological and molecular analyses do not reveal a monophyletic Podotremata, but rather that it is paraphyletic, and so the most recent classifications divide "Podotremata" into three sections: Dromiacea, Cyclodorippoidea and Raninoida.

Dromiacea Group of crabs

Dromiacea is a group of crabs, ranked as a section. It contains 240 extant and nearly 300 extinct species. Dromiacea is the most basal grouping of Brachyura crabs, diverging the earliest in the evolutionary history, around the Late Triassic or Early Jurassic. Below is a cladogram showing Dromiacea's placement within Brachyura:

Polychelida Infraorder of crustaceans

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.

Hexapodidae is a family of crabs, the only family in the superfamily Hexapodoidea. It has traditionally been treated as a subfamily of the family Goneplacidae, and was originally described as a subfamily of Pinnotheridae. Its members can be distinguished from all other true crabs by the reduction of the thorax, such that only seven sternites are exposed, and only four pairs of pereiopods are present. Not counting the enlarged pair of claws, this leaves only six walking legs, from which the type genus Hexapus, and therefore the whole family, takes its name. Some anomuran "crabs", such as porcelain crabs and king crabs also have only four visible pairs of legs. With the exception of Stevea williamsi, from Mexico, all the extant members are found either in the Indo-Pacific oceans, or around the coast of Africa.

Gebiidea Infraorder of crustaceans

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.

<i>Eocarcinus</i> Extinct genus of crustaceans

Eocarcinus praecursor is a Jurassic species of decapod crustacean, sufficiently distinct from its relatives to be placed in its own family (Eocarcinidae). Often considered the oldest true crab, it was considered by a 2010 study to be an early member of the Anomura. However, a reanalysis in 2020 again found it to be the earliest known stem-group crab.

Platykotta akaina is a species of decapod crustacean from the Triassic of the United Arab Emirates. It is the oldest known fossil from the infraorder Anomura, and is most closely related to Eocarcinus praecursor.


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  2. 1 2 3 Gary Poore (2004). "Anomura – hermit crabs, porcelain crabs and squat lobsters". Marine Decapod Crustacea of Southern Australia: a Guide to Identification. CSIRO Publishing. pp. 215–287. ISBN   978-0-643-09925-8.
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