Anomura

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
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Clade:	Reptantia
Infraorder:	Anomura Macleay, 1838
Superfamilies
Aegloidea Chirostyloidea † Eocarcinoidea Galatheoidea Hippoidea Lomisoidea Paguroidea

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]

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 Parapylochelidae Pylochelidae Pylojacquesidae	Coenobita clypeatus (Coenobitidae)

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

References

1. Chablais, Jérôme; Feldmann, Rodney M.; Schweitzer, Carrie E. (11 August 2010). "A new Triassic decapod, Platykotta akaina, from the Arabian shelf of the northern United Arab Emirates: earliest occurrence of the Anomura". Paläontol Z . 85 (2011): 93–102. doi:10.1007/s12542-010-0080-y. S2CID   5612385.
2. Gerhard Scholtz; Stefan Richter (1995). "Phylogenetic systematics of the reptantian Decapoda (Crustacea, Malacostraca)" (PDF). Zoological Journal of the Linnean Society . 113 (3): 289–328. doi:10.1111/j.1096-3642.1995.tb00936.x.
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.
4. Shane T. Ahyong; Kareen E. Schnabel; Elizabeth W. Maas (2009). "Anomuran phylogeny: new insights from molecular data". In Joel W. Martin; Keith A. Crandall; Darryl L. Felder (eds.). Decapod Crustacean Phylogenetics. Crustacean issues. Vol. 18. CRC Press. pp. 399–414. doi:10.1201/9781420092592. ISBN   978-1-4200-9258-5.
5. Jonas Keiler; Stefan Richter (2011). "Morphological diversity of setae on the grooming legs in Anomala (Decapoda: Reptantia) revealed by scanning electron microscopy". Zoologischer Anzeiger . 250 (4): 343–366. Bibcode:2011ZooAn.250..343K. doi:10.1016/j.jcz.2011.04.004.
6. Carrie E. Schweitzer; Rodney M. Feldmann (2001). "Differentiation of the fossil Hexapodidae Miers, 1886 (Decapoda: Brachyura) from similar forms" (PDF). Journal of Paleontology . 75 (2): 330–345. Bibcode:2001JPal...75..330S. doi:10.1666/0022-3360(2001)075<0330:DOTFHM>2.0.CO;2. S2CID   85997166.
7. Wolfe, Joanna M.; Breinholt, Jesse W.; Crandall, Keith A.; Lemmon, Alan R.; Lemmon, Emily Moriarty; Timm, Laura E.; et al. (24 April 2019). "A phylogenomic framework, evolutionary timeline and genomic resources for comparative studies of decapod crustaceans". Proceedings of the Royal Society B. 286 (1901). doi: 10.1098/rspb.2019.0079 . PMC   6501934 . PMID   31014217.
8. Sammy De Grave; N. Dean Pentcheff; Shane T. Ahyong; et al. (2009). "A classification of living and fossil genera of decapod crustaceans" (PDF). Raffles Bulletin of Zoology . Suppl. 21: 1–109.
9. Jérôme Chablais; Rodney M. Feldmann; Carrie E. Schweitzer (2011). "A new Triassic decapod, Platykotta akaina, from the Arabian shelf of the northern United Arab Emirates: earliest occurrence of the Anomura" (PDF). Paläontologische Zeitschrift . 85 (1): 93–102. Bibcode:2011PalZ...85...93C. doi:10.1007/s12542-010-0080-y. S2CID   5612385.
10. K. E. Schnabel; S. T. Ahyong; E. W. Maas (2011). "Galatheoidea are not monophyletic – molecular and morphological phylogeny of the squat lobsters (Decapoda: Anomura) with recognition of a new superfamily". Molecular Phylogenetics and Evolution . 58 (2): 157–168. Bibcode:2011MolPE..58..157S. doi:10.1016/j.ympev.2010.11.011. PMID   21095236.
11. WoRMS (2018). "Anomura". WoRMS. World Register of Marine Species . Retrieved 2018-09-28.

External links

• Crustaceans portal
• Arthropods portal

• Media related to Anomura at Wikimedia Commons
• Data related to Anomura at Wikispecies