Anomura

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).^[2]

Anomura Temporal range: Late Triassic: Norian - Recent, 227–0 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
The hermit crab Dardanus megistos
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Clade:	Reptantia
Infraorder:	Anomura Macleay, 1838
Superfamilies
Aegloidea Chirostyloidea † Eocarcinoidea Galatheoidea Hippoidea Lomisoidea Paguroidea

Description

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.^[3]

The group has been moulded by several instances of carcinisation – the development of a crab-like body form.^[4] Thus, the king crabs (Lithodidae), porcelain crabs (Porcellanidae) and hairy stone crab (Lomisidae) are all separate instances of carcinisation.^[4]

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.^[5][3] Since this arrangement is very rare in true crabs (for example, the small family Hexapodidae),^[6] a "crab" with only eight visible pereiopods is generally an anomuran.^[3]

Evolution

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.^[4] Anomura likely diverged from Brachyura in the Late Triassic period, with the earliest discovered Anomuran fossil Platykotta akaina dating from the Norian–Rhaetian aged Ghalilah Formation of the United Arab Emirates.^[1]

The cladogram below shows Anomura's placement within the larger order Decapoda, from analysis by Wolfe et al. (2019).^[7]

Decapoda

Dendrobranchiata (prawns)  Pleocyemata  Stenopodidea (boxer shrimp) Procarididea Caridea ("true" shrimp)    Reptantia  Achelata (spiny lobsters and slipper lobsters) Polychelida (benthic crustaceans) Astacidea (lobsters and crayfish) Axiidea (mud shrimp, ghost shrimp, and burrowing shrimp) Gebiidea (mud lobsters and mud shrimp) Anomura (hermit crabs and allies) Brachyura ("true" crabs) (crawling /  walking  decapods)

Some of the internal relationships within Anomura are shown in the cladogram below, which shows Hippidae as sister to Paguroidea, and resolves Parapaguridae outside of Paguroidea:^[7]

	Brachyura ("true" crabs)
Anomura	Porcellanidae (porcelain crabs) Munididae (squat lobsters) Parapaguridae (deep water sea anemone hermit crabs) Eumunididae (squat lobster-like) Hippidae (mole crabs or sand crabs)  Paguroidea  Lithodidae (king crabs) Paguridae (hermit crabs) Diogenidae (left-handed hermit crabs) Coenobitidae (terrestrial hermit crabs)

Classification

The infraorder Anomura contained seven extant superfamilies:^{[8][9][10][11]}

Superfamily	Members	Families	Photo
Aegloidea	Aegla	Aeglidae	Aegla sp.
Chirostyloidea	squat lobsters	Chirostylidae Eumunididae Kiwaidae	Eumunida picta
† Eocarcinoidea	† Eocarcinus † Platykotta	† Eocarcinidae † Platykottidae
Galatheoidea	squat lobsters porcelain crabs	Galatheidae Munididae Munidopsidae Porcellanidae † Retrorsichelidae	Munidopsis serricornis (Munidopsidae)
Hippoidea	mole crabs or sand crabs	Albuneidae Blepharipodidae Hippidae	Blepharipoda occidentalis (Blepharipodidae)
Lomisoidea	hairy stone crab	Lomisidae	Lomis hirta (Lomisidae)
Paguroidea	hermit crabs coconut crab	Coenobitidae Diogenidae Lithodidae Paguridae Parapaguridae Probeebeidae Pylochelidae Pylojacquesidae Xylopaguridae	Coenobita clypeatus (Coenobitidae)

The oldest fossil attributed to Anomura is Platykotta, from the Norian–Rhaetian (Late Triassic) Period in the United Arab Emirates.^[9]