Around 1,300 species of freshwater crabs are distributed throughout the

tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referr ...

and subtropics, divided among eight

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 ...

. They show direct development and maternal care of a small number of offspring, in contrast to marine crabs, which release thousands of planktonic larvae. This limits the dispersal abilities of freshwater crabs, so they tend to be

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...

to small areas. As a result, a large proportion are threatened with extinction.

Systematics

More than 1,300 described species of freshwater crabs are known, out of a total of 6,700 species of crabs across all environments. The total number of species of freshwater crabs, including

undescribed species In taxonomy, an undescribed taxon is a taxon (for example, a species) that has been discovered, but not yet formally described and named. The various Nomenclature Codes specify the requirements for a new taxon to be validly described and named. U ...

, is thought to be up to 65% higher, potentially up to 2,155 species, although most of the additional species are currently unknown to science. They belong to eight families, each with a limited distribution, although various crabs from other families are also able to tolerate freshwater conditions ( euryhaline) or are secondarily adapted to fresh water. The phylogenetic relationships between these families is still a matter of debate, so how many times the freshwater lifestyle has evolved among the true crabs is unknown. The eight families are: ;Superfamily

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 ...

Trichodactylidae 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 known f ...

(

Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...

and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...

) ;Superfamily

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, ...

: *

Potamidae Potamidae is a family of freshwater crabs. It includes more than 650 species and nearly 100 genera, which are placed into two subfamilies: Potaminae and Potamiscinae. Subfamily Potaminae The Potaminae Ortmann, 1896 are distributed around the Me ...

( Mediterranean Basin and

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...

) * Potamonautidae (

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

, including

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...

) * Deckeniidae ( East Africa and

Seychelles Seychelles (, ; ), officially the Republic of Seychelles (french: link=no, République des Seychelles; Creole: ''La Repiblik Sesel''), is an archipelagic state consisting of 115 islands in the Indian Ocean. Its capital and largest city, ...

) – also treated as part of Potamonautidae * Platythelphusidae (East Africa) – also treated as part of Potamonautidae ;Superfamily Gecarcinucoidea * Gecarcinucidae (Asia) *

Parathelphusidae ''Arachnothelphusa rhadamanthysi''Cave crab, Borneo Parathelphusinae is a subfamily of freshwater crabs, which was formerly placed in the family Parathelphusidae; they are mainly found in South and Southeast Asia, but also found elsewhere in As ...

(Asia and

Australasia Australasia is a region that comprises Australia, New Zealand and some neighbouring islands in the Pacific Ocean. The term is used in a number of different contexts, including geopolitically, physiogeographically, philologically, and ecologi ...

) – nowadays treated as a junior synonym of Gecarcinucidae ;Superfamily Pseudothelphusoidea: * Pseudothelphusidae (Central America and South America) The

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

of freshwater organisms is typically poor, so few fossils of freshwater crabs have been found. The oldest is '' Tanzanonautes tuerkayi'', from the Oligocene of East Africa, and the evolution of freshwater crabs is likely to postdate the break-up of the supercontinent Gondwana. Members of the family Aeglidae and '' Clibanarius fonticola'' are also restricted to fresh water, but these "crab-like" crustaceans are members of the infraorder

Anomura 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 grou ...

(true crabs are Brachyura).

Description and lifecycle

The external morphology of freshwater crabs varies very little, so the form of the

gonopod Gonopods are specialized appendages of various arthropods used in reproduction or egg-laying. In males, they facilitate the transfer of sperm from male to female during mating, and thus are a type of intromittent organ. In crustaceans and millipe ...

(first abdominal appendage, modified for insemination) is of critical importance for classification.

Development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...

of freshwater crabs is characteristically direct, where the

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

hatch as juveniles, with the larval stages passing within the egg. The broods comprise only a few hundred eggs (compared to hundreds of thousands for marine crabs), each of which is quite large, at a diameter around . The colonisation of fresh water has required crabs to alter their water balance; freshwater crabs can reabsorb salt from their

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellular ...

, and have various adaptations to reduce the loss of water. In addition to their

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s, freshwater crabs have a "pseudolung" in their gill chamber that allows them to breathe in air. These developments have preadapted freshwater crabs for terrestrial living, although freshwater crabs need to return to water periodically to

excrete Excretion is a process in which metabolic waste is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after lea ...

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

Ecology and conservation

Freshwater crabs are found throughout the tropical and subtropical regions of the world. They live in a wide range of water bodies, from fast-flowing rivers to swamps, as well as in tree boles or

cave A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

s. They are primarily nocturnal, emerging to feed at night; most are omnivores, although a small number are specialist predators, such as '' Platythelphusa armata'' from Lake Tanganyika, which feeds almost entirely on

snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class G ...

s. Some species provide important food sources for various

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

s. A number of freshwater crabs (for example species from the genus ''

Nanhaipotamon ''Nanhaipotamon'' is a genus of freshwater crabs, in the subfamily Potamiscinae, found in southern China and Taiwan. As of 2018, 18 species have been described. The genus is named after the South China Sea (; literally: 'South Sea'), for it oc ...

'') are

secondary host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...

s of flukes in the genus ''

Paragonimus ''Paragonimus'' is a genus of flukes (trematodes) and is the only genus in the monotypic family Paragonimidae. Some tens of species have been described, but they are difficult to distinguish, so it is not clear how many of the named species may b ...

'', which causes

paragonimiasis Paragonimiasis is a food-borne parasitic disease caused by several species of lung flukes belonging to genus '' Paragonimus''. Infection is acquired by eating crustaceans such as crabs and crayfishes which host the infective forms called metacerc ...

in humans. The majority of species are narrow endemics, occurring in only a small geographical area. This is at least partly attributable to their poor dispersal abilities and low

fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...

, and to habitat fragmentation caused by the world's human population. In

West Africa West Africa or Western Africa is the westernmost region of Africa. The United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, M ...

, species that live in

savanna A savanna or savannah is a mixed woodland- grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to ...

s have wider ranges than species from the rainforest; in East Africa, species from the mountains have restricted distributions, while lowland species are more widespread. Every species of freshwater crab described so far has been assessed by the

International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of nat ...

; of the species for which data are available, 32% are threatened with extinction. For instance, all but one of Sri Lanka's 50 freshwater crab species are endemic to that country, and more than half are critically endangered.

References

External links

*{{cite web , url=http://faculty.nmu.edu/ncumberl/Neil/MainFWC-website/FWCBiology.html , title=Freshwater Crab Biology , author1=Neil Cumberlidge , author2=Sadie K. Reed , date=April 4, 2009 , publisher=

Northern Michigan University Northern Michigan University (Northern Michigan, Northern or NMU) is a public university in Marquette, Michigan. It was established in 1899 by the Michigan Legislature as Northern State Normal School. In 1963, the state designated Northern a uni ...

, url-status=dead , archive-url=https://web.archive.org/web/20110720014222/http://faculty.nmu.edu/ncumberl/Neil/MainFWC-website/FWCBiology.html , archive-date=July 20, 2011 Crabs Freshwater crustaceans