Heterotremata 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 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 ...

in females, but 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 ...

in males. It comprises 68 families in 28 superfamilies.

Evolution

Heterotremata is the sister group to

Thoracotremata Thoracotremata is a clade of crabs, comprising those crabs in which the genital openings in both sexes are on the sternum, rather than on the legs. It comprises 17 families in four superfamilies . Evolution Thoracotremata is the sister gro ...

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 Heterotremata are less certain, with many of the superfamilies found to be invalid. The proposed

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

Superfamilies

* Aethroidea * Bellioidea * Bythograeoidea *

Calappoidea Calappoidea is a superfamily of crabs comprising the two families Calappidae and Matutidae. The earliest fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geo ...

Cancroidea Cancroidea is a superfamily of crabs, comprising the families Atelecyclidae and Cancridae. http://boletinsgm.igeolcu.unam.mx/bsgm/vols/epoca04/6502/%2810%29Osso.pdf Four other families have been separated into new superfamilies: Cheiragonidae ...

* Carpilioidea * Cheiragonoidea * Corystoidea * Dairoidea * Dorippoidea * Eriphioidea * Gecarcinucoidea * Goneplacoidea * Hexapodoidea * Leucosioidea *

Majoidea The Majoidea are a Superfamily (taxonomy), superfamily of crabs which includes the various spider crabs. Taxonomy In "''A classification of living and fossil genera of decapod crustaceans''" De Grave and colleagues divided Majoidea into six fa ...

* Orithyioidea * Palicoidea *

Parthenopoidea Parthenopidae is a family of crabs, placed in its own Superfamily (taxonomy), superfamily, Parthenopoidea. It comprises nearly 40 genera, divided into two subfamilies, with three genera ''incertae sedis'': ;Daldorfiinae Ng & Rodríguez, 1986 * ...

* Pilumnoidea *

Portunoidea __NOTOC__ Portunoidea is a superfamily of crabs that includes the family Portunidae, the swimming crabs. Which other crab families are also placed here is a matter of some contention, and may be revised following molecular phylogenetic analyse ...

Potamoidea Potamoidea is a superfamily of freshwater crabs, comprising the two families Potamidae and Potamonautidae Potamonautidae is a family of freshwater crabs endemic to Africa, including the islands of Madagascar, the Seychelles, Zanzibar, Mafia, ...

* Pseudothelphusoidea * Pseudozioidea * Retroplumoidea *

Trapezioidea Trapezioidea is a superfamily (zoology), superfamily of crabs. Its members live symbiosis, symbiotically with corals, and have a fossil record stretching back to the Eocene. Families The World Register of Marine Species lists the following famil ...

Trichodactyloidea Trichodactylidae is a family of crabs, in its own superfamily, Trichodactyloidea. They are all freshwater animals from Central and South America, including some offshore islands, such as Ilhabela, São Paulo. Only one of the 50 species is kno ...

Xanthoidea Xanthoidea is a superfamily of crabs, comprising the three families Xanthidae, Panopeidae and Pseudorhombilidae. Formerly, a number of other families were included in Xanthoidea, but many of these have since been removed to other superfamilies. ...

However, recent studies have found the following superfamilies and

families 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 its members and of society. Ideal ...

to not be

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

, but rather

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

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 ...

: *The superfamilies

, Eriphioidea, and Goneplacoidea are polyphyletic *The superfamily

is paraphyletic with respect to Gecarcinucoidea, which is resolved by placing Gecarcinucidae within

*The

families

Epialtidae Epialtidae is a family of crabs, containing the subfamilies: * Epialtinae MacLeay, 1838 * Pisinae Dana, 1851 * Pliosomatinae Števčić, 1994 * Tychinae Dana, 1851 * Actinotocarcininae References Majoidea Decapod families {{c ...

Mithracidae Mithracidae is a family of crustaceans belonging to the order Decapoda. Genera Genera: *''Ala'' *'' Amphithrax'' *''Hemus'' *'' Maguimithrax'' *'' Microphrys'' *'' Mithraculus'' *'' Mithrax'' *'' Nemausa'' *'' Nonala'' *'' Omalacantha ...

and

Majidae Majidae is a family of crabs, comprising around 200 marine species inside 52 genera, with a carapace that is longer than it is broad, and which forms a point at the front. The legs can be very long in some species, leading to the name "spider c ...

are polyphyletic with respect to each other *The

family

Xanthidae Xanthidae is a family of crabs known as gorilla crabs, mud crabs, pebble crabs or rubble crabs. Xanthid crabs are often brightly coloured and are highly poisonous, containing toxins which are not destroyed by cooking and for which no antidote is ...

is paraphyletic with respect to

Panopeidae The Panopeidae are a family containing 26 genera of morphologically similar crabs, often known as "mud crabs". Their centers of diversity are the Atlantic Ocean and eastern Pacific Ocean. Distribution Most members of the family Panopeidae liv ...

References

External links

* Crabs {{crab-stub