Thoracotremata is a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

s, comprising those crabs in which the genital openings in both sexes are on the

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sh ...

, rather than on the

legs A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ...

. It comprises 17 families in four superfamilies .

Evolution

Thoracotremata is the sister group to

Heterotremata Heterotremata is a clade of crabs, comprising those crabs in which the genital openings are on the sternum in females, but on the legs in males. It comprises 68 families in 28 superfamilies. Evolution Heterotremata is the sister group to Thorac ...

within the clade

Eubrachyura Eubrachyura is a group of decapod crustaceans ( ranked as a "section") 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 ...

, having diverged during the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

period. Eubrachyura itself is a subset of the larger clade Brachyura, which consists of all "true crabs". A summary of the high-level internal relationships within Brachyura can be shown in the

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

below: The internal relationships within Thoracotremata are less certain, with many of the superfamilies found to be invalid. The proposed

below is from analysis by Tsang ''et al'', 2014:

Superfamilies and families

Ocypodoidea The Ocypodoidea, or ocypoid crabs, are a superfamily of crabs, named after the genus '' Ocypode''. It contains over 300 extant species in these eight families: * Camptandriidae Stimpson, 1858 * Dotillidae Stimpson, 1858 * Heloeciidae H. Miln ...

** Camptandriidae **

Dotillidae The Dotillidae are a family of crabs with 59 species, nearly half of which are in the genus '' Ilyoplax''. The two genera ''Scopimera'' and ''Dotilla'' are collectively the sand bubbler crab Sand bubbler crabs (or sand-bubblers) are crabs of ...

** Heloeciidae ** Macrophthalmidae **

Mictyridae ''Mictyris'' is a genus of brightly coloured crabs, placed in its own taxonomical family, the Mictyridae. It inhabits the central Indo-West Pacific region. These crabs congregate on mud flats or beaches in groups of a few thousand, and filter sa ...

Ocypodidae The Ocypodidae are a family of semiterrestrial crabs that includes the ghost crabs and fiddler crabs. They are found on tropical and temperate shorelines around the world. Some genera previously included in the family are now treated as members o ...

** Ucididae ** Xenophthalmidae * Pinnotheroidea **

Pinnotheridae The Pinnotheridae are a family of tiny soft-bodied crabs that live commensally in the mantles of certain bivalve molluscs (and the occasional large gastropod mollusc species in genera such as '' Strombus'' and ''Haliotis''). '' Tunicotheres moser ...

* Cryptochiroidea ** Cryptochiridae *

Grapsoidea The Grapsoidea are a superfamily of crabs; they are well known and contain many taxa which are terrestrial (land-living), semiterrestrial (taking to the sea only for reproduction), or limnic (living in fresh water). Another well-known member w ...

Gecarcinidae The Gecarcinidae, the land crabs, are a family of true crabs that are adapted for terrestrial existence. Similar to all other crabs, land crabs possess a series of gills. In addition, the part of the carapace covering the gills is inflated and e ...

Glyptograpsidae Glyptograpsidae is a family of crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods ...

Grapsidae The Grapsidae are a family of crabs known variously as marsh crabs, shore crabs, or talon crabs. The family has not been confirmed to form a monophyletic group and some taxa may belong in other families. They are found along the shore among rocks ...

Plagusiidae The Plagusiidae are a family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of ...

Sesarmidae The Sesarmidae are a family of crabs, previously included in the Grapsidae by many authors. Several species, namely in '' Geosesarma'', '' Metopaulias'', and '' Sesarma'', are true terrestrial crabs. They do not need to return to the sea even f ...

Varunidae The Varunidae are a family of thoracotrematan crabs. The delimitation of this family, part of the taxonomically confusing Grapsoidea, is undergoing revision. For a long time, they were placed at the rank of subfamily in the Grapsidae, but they a ...

** Xenograpsidae However, recent studies have found the superfamilies

and

to be

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...

and invalid.

References

Crabs {{crab-stub