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Cancer is a genus of marine crabs in the family Cancridae. It includes 8 extant species and 3 extinct species, including familiar crabs of the littoral zone, such as the European edible crab (Cancer pagurus), the Jonah crab (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 species placed in the genus Cancer are united by the presence of a single posterolateral spine (on the edge of the carapace, 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.
- Cancer bellianus Johnson, 1861 – toothed rock crab
- Cancer borealis Stimpson, 1859 – Jonah crab
- Cancer irroratus Say, 1817 – Atlantic rock crab
- Cancer johngarthi Carvacho, 1989
- Cancer pagurus Linnaeus, 1758 – edible crab or brown crab
- Cancer plebejus Poeppig, 1836
- Cancer porteri Rathbun, 1930
- Cancer productus J. W. Randall, 1840 – red rock crab
Three fossil species are also included:
- 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.
When zoological nomenclature was first standardised by Carl Linnaeus in the 1758 10th edition of Systema Naturae, 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's 1802 work Histoire naturelle, générale et particulière des Crustacés et des Insectes ("Natural history in general, and specifically that of crustaceans and insects"). Latreille designated C. pagurus to be the type species in 1817.
In 1975, J. Dale Nations divided the genus Cancer into four subgenera: 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.
The earliest fossils that can be confidently ascribed to the genus Cancer are those of C. fujinaensis from the Japanese Miocene. The genus is therefore thought to have evolved in the northern Pacific Ocean, perhaps during the Miocene, and have spread across that ocean and into the Atlantic Ocean by the Pliocene or Pleistocene, having crossed the equator and the Straits of Panama.
- P. K. L. Ng, D. Guinot & P. J. F. Davie (2008). "Systema Brachyurorum: Part I. An annotated checklist of extant Brachyuran crabs of the world". Raffles Bulletin of Zoology 17: 1–286.
- Carrie E. Schweitzer & Rodney M. Feldmann (2000). "Re-evaluation of the Cancridae Latreille, 1802 (Decapoda: Brachyura) including three new genera and three new species". Contributions to Zoology 69 (4): 223–250. Also available as PDF.
- Frederick R. Schram & Peter K. L. Ng (2012). "What is Cancer?". Journal of Crustacean Biology 32 (4): 665–672. doi:10.1163/193724012X640650.
- Geoff Boxshall (2007). Crustacean classification: on-going controversies and unresolved problems (PDF excerpt). In Z.-Q. Zhang & W. A. Shear. "Linnaeus Tercentenary: Progress in Invertebrate Taxonomy". Zootaxa 1668: 313–325.
- J. Dale Nations (1975). "The genus Cancer (Crustacea: Bachyura): systematics, biogeography, and fossil record" (PDF). Natural History Museum of Los Angeles County Science Bulletin 23: 1–104.