''Cancer'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of marine

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 in the family Cancridae. It includes eight extant species and three extinct species, including familiar crabs of the

littoral zone The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal areas ...

, such as the European edible crab (''

Cancer pagurus ''Cancer pagurus'', commonly known as the edible crab or brown crab, is a species of crab found in the North Sea, North Atlantic Ocean, and perhaps the Mediterranean Sea. It is a robust crab of a reddish-brown colour, having an oval carapace wi ...

''), the

Jonah crab The Jonah crab (''Cancer borealis'') is a marine brachyuran crab that inhabits waters along the east coast of North America from Newfoundland to Florida. Jonah crabs possess a rounded, rough-edged carapace with small light spots, and robust claws ...

(''Cancer borealis'') and the red rock crab ('' Cancer productus''). It is thought to have evolved from related genera in the Pacific Ocean in the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

Description

The species placed in the genus ''Cancer'' are united by the presence of a single posterolateral spine (on the edge of the

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

, towards the rear), anterolateral spines with deep fissures (on the carapace edge, towards the front), and a short extension of the carapace forward between the eyes. Their claws are typically short, with grainy or smooth, rather than spiny, keels. The carapace is typically oval, being 58%–66% as long as wide, and the eyes separated by 22%–29% of the carapace width.

Species

The genus ''Cancer'', as currently

circumscribed In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...

, contains eight extant species: Three fossil species are also included: Also available a
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*'' Cancer fissus'' Rathbun, 1908 – Pliocene, California *'' Cancer fujinaensis'' Sakumoto, Karasawa & Takayasu, 1992 – Miocene, Japan *'' Cancer parvidens'' Collins & Fraaye, 1991 – Miocene, Netherlands As their generic delimitation was based on characters of the dorsal carapace, Schweitzer and Feldmann (2000) were unable to confirm the placement of ''Cancer tomowoi'' in the genus, since it is known only from parts of the sternum and the legs. Other species until recently included in the genus ''Cancer'' have since been transferred to other genera, such as '' Glebocarcinus'', '' Metacarcinus'' and '' Romaleon''.

Taxonomic history

When

zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. It is also informally known as the ICZN Code, for its publisher, the In ...

was first standardised by

Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...

in the 1758 10th edition of ', the genus ''Cancer'' included almost all crustaceans, including all the crabs. Linnaeus' cumbersome genus was soon divided into more meaningful units, and ''Cancer'' had been restricted to one group of true crabs by the time of

Pierre André Latreille Pierre André Latreille (; 29 November 1762 – 6 February 1833) was a French zoologist, specialising in arthropods. Having trained as a Roman Catholic priest before the French Revolution, Latreille was imprisoned, and only regained his freedom ...

's 1802 work ' ("Natural history in general, and specifically that of crustaceans and insects"). Latreille designated ''C. pagurus'' to be the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

in 1817. In 1975, J. Dale Nations divided the genus ''Cancer'' into four

subgenera In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between t ...

: ''Cancer (Cancer)'', ''Cancer (Glebocarcinus)'', ''Cancer (Metacarcinus)'' and ''Cancer (Romaleon)''. Each of these is now treated as a separate genus, as is the genus '' Platepistoma'', erected by Mary J. Rathbun and resurrected in 1991. Since that time, further genera have been described to accommodate species previously included in ''Cancer'', and the genus ''Cancer'' now contains only eight extant species.

Evolutionary history

The earliest fossils that can be confidently ascribed to the genus ''Cancer'' are those of '' C. fujinaensis'' from the

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

ese

. The genus is therefore thought to have

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...

in the northern

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, perhaps during the Miocene, and have spread across that ocean and into the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

by the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

, having crossed the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

and the Straits of Panama.

References

{{Authority control Cancroidea Decapod genera Extant Miocene first appearances