Around 1,300 species of freshwater crabs are distributed throughout the tropics and subtropics, divided among eight families. 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 to small areas. As a result, a large proportion are threatened with extinction.
More than 1,300 described species of freshwater crabs are known, out of a total of 6,700 species of crabs across all environments. [1] The total number of species of freshwater crabs, including undescribed species, 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. [1] 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. [1] 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. [1] The eight families are:
The fossil record 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. [2]
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 (true crabs are Brachyura). [3]
The external morphology of freshwater crabs varies very little, so the form of the gonopod (first abdominal appendage, modified for insemination) is of critical importance for classification. [1] Development of freshwater crabs is characteristically direct, where the eggs hatch as juveniles, with the larval stages passing within the egg. [1] 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 1 mm (0.04 in). [4]
The colonisation of fresh water has required crabs to alter their water balance; freshwater crabs can reabsorb salt from their urine, and have various adaptations to reduce the loss of water. [4] In addition to their gills, freshwater crabs have a "pseudolung" in their gill chamber that allows them to breathe in air. [4] These developments have preadapted freshwater crabs for terrestrial living, although freshwater crabs need to return to water periodically to excrete ammonia. [4]
Freshwater crabs are found throughout the tropical and subtropical regions of the world. [1] They live in a wide range of water bodies, from fast-flowing rivers to swamps, as well as in tree boles or caves. [1] They are primarily nocturnal, emerging to feed at night; [1] most are omnivores, although a small number are specialist predators, such as Platythelphusa armata from Lake Tanganyika, which feeds almost entirely on snails. [4] Some species provide important food sources for various vertebrates. [1] A number of freshwater crabs (for example species from the genus Nanhaipotamon ) are secondary hosts of flukes in the genus Paragonimus , which causes paragonimiasis in humans. [4]
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, [1] and to habitat fragmentation caused by the world's human population. [5] In West Africa, species that live in savannas have wider ranges than species from the rainforest; in East Africa, species from the mountains have restricted distributions, while lowland species are more widespread. [4]
Every species of freshwater crab described so far has been assessed by the International Union for Conservation of Nature; [6] of the species for which data are available, 32% are threatened with extinction. [5] 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. [5]
Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. They live in all the world's oceans, in freshwater, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers on each arm. They first appeared during the Jurassic period, around 200 million years ago.
Clinothelphusa kakoota is a species of crab in the family Gecarcinucidae, which is endemic to Sri Lanka. Its natural habitats are subtropical or tropical moist lowland forests, subtropical or tropical swamps, and rivers. It is threatened by habitat loss, and is listed as a critically endangered species on the IUCN Red List. It is known from a single site with an area of less than 100 km2 (39 sq mi). Its nearest relatives are another Sri Lankan endemic genus of crabs, Ceylonthelphusa.
Oziotelphusa is a genus of freshwater crabs in the family Gecarcinucidae. Its members are found in Sri Lanka and southern India. The genus was formerly placed within family Parathelphusidae, but now it is accepted that Parathelphusidae is the junior synonym of Gecarcinucidae.
Platythelphusa is a genus of freshwater crabs endemic to Lake Tanganyika. It has been placed in a number of families, including a monotypic family, Platythelphusidae, as well as Potamidae and its current position in the Potamonautidae, and has also been treated as a subgenus of Potamonautes. It forms a monophyletic group, possibly nested within the genus Potamonautes, which would therefore be paraphyletic. The genus is the only evolutionary radiation of crabs to have occurred in a freshwater lake, and it occurred recently, probably since the Pliocene. This parallels the better known radiation of cichlid fishes in Lake Tanganyika. Only one other species of freshwater crab is found in Lake Tanganyika, Potamonautes platynotus.
Potamonautes is a genus of African freshwater crabs in the family Potamonautidae. It is both the most widespread and most diverse genus of African freshwater crabs, including more than half the species of this continent. They are found in most freshwater habitats of the African mainland and some species are semi-terrestrial.
Potamonautes mutandensis is an endangered species of freshwater crab in the family Potamonautidae. It is found in Lake Mutanda and Lake Kivu in Uganda, Rwanda and DR Congo.
Potamonautes raybouldi is a species of freshwater crab. It lives in water-filled tree holes in forests in the eastern Usambara Mountains of Tanzania and the Shimba Hills in Kenya. It is threatened by deforestation resulting from the expansion of the human population, and is listed as a vulnerable species on the IUCN Red List. The species was described in 2004, and named after Professor John N. Raybould of the University of Bristol, who collected the first specimens of the species.
The Gecarcinucidae are a family of true freshwater crabs. They are found throughout South Asia, Southeast Asia and New Guinea, with a single genus found in Australia.
Potamonautidae is a family of freshwater crabs endemic to Africa, including the islands of Madagascar, the Seychelles, Zanzibar, Mafia, Pemba, Bioko, São Tomé, Príncipe and Sherbro Island. It comprises 18 extant genera and 138 extant species. Fossil remains dating from the Late Miocene period have been attributed to the family Potamonautidae.
Potamon fluviatile is a freshwater crab found in or near wooded streams, rivers and lakes in Southern Europe. It is an omnivore with broad ecological tolerances, and adults typically reach 50 mm (2 in) in size during their 10–12 year lifespan. They inhabit burrows and are aggressive, apparently outcompeting native crayfish.
Deckenia is a genus of freshwater crabs from East Africa, in the family Potamonautidae, or sometimes in a family of its own, Deckeniidae. The genus was named by Hilgendorf after Karl Klaus von der Decken who collected the first examples during his expeditions to Africa. Both species live in swamps from Eyl in Somalia to Dar es Salaam, Tanzania, both in coastal areas and further inland. A third species, Deckenia alluaudi, lives in the Seychelles, and has been transferred to a separate genus, Seychellum.
Johora singaporensis, the Singapore stream crab or Singapore freshwater crab, is a critically endangered species of freshwater crab endemic to Singapore. It grows to a size of 30 millimetres (1.2 in) wide.
Clibanarius fonticola is a species of hermit crab from Vanuatu. It lives exclusively in fresh water, the only hermit crab in the world to do so. While a number of other hermit crabs are terrestrial or live in estuarine habitats, and certain brackish water species can tolerate low salinity levels for a time, no other hermit crab spends its entire life in fresh water; the only other fully freshwater anomurans are the South American aeglids.
The Aeglidae are a family of freshwater crustaceans currently restricted to South America. They are the only anomurans to be found in fresh water except for a single hermit crab species, Clibanarius fonticola, on Espiritu Santo, Vanuatu. They live between 20° S and 50° S, at altitudes between 320 and 3,500 m (1,050–11,480 ft).
Potamonautes sidneyi is a species of freshwater crab in the family Potamonautidae. The common name is the Natal river crab or Sidney's river crab, although they may also be referred to as "river crabs", "fresh water crabs" or "land crabs".
Tanzanonautes tuerkayi is a species of fossil freshwater crab from Tanzania, the only species in the genus Tanzanonautes. It is the oldest known freshwater crab, and probably dates from the Oligocene; the next oldest specimens are from the Miocene. A number of fragmentary remains were discovered in the Songwe Valley part of the East African Rift in Mbeya Region, Tanzania, in fragile sandstone sediments. The animal had a carapace around 50 millimetres (2.0 in) across the widest part, which is around 15 mm (0.6 in) from the front of the carapace; the carapace is 44 mm (1.7 in) from front to back, and the slightly inward-curving rear edge of the carapace is 28 mm (1.1 in) across. The genus name refers to Tanzania, the country where the fossil was found, while the specific epithet commemorates Michael Türkay, an authority on freshwater crabs. The genus Tanzanonautes is assigned to the family Potamonautidae, although the characters mentioned in the family's diagnosis are missing from the fragmentary fossils available.
Seychellum alluaudi is a species of freshwater crab endemic to the Seychelles, and the only true freshwater crab in that country. It lives in rainforest streams on the archipelago's granitic high islands. Although it may be abundant, little is known about its biology. If its habitat were to decline in quality, S. alluaudi might become endangered, but it is currently listed as Vulnerable on the IUCN Red List.
Perbrinckia fenestra is a species of freshwater crabs of the family Gecarcinucidae that is endemic to Sri Lanka. the species is categorized as vulnerable by founders due to their single locality where tourist destinations are abundant. The site is Batatotalena Cave in Kuruwita. It is rarely found, and known to live under moist rocks, near water sources and under wet litter.
Perbrinckia gabadagei is a species of freshwater crabs of the family Gecarcinucidae that is endemic to Sri Lanka. The species was once categorized as vulnerable by founders, but now considered to be critically endangered and probably extinct due to lack of recent evidences since 1996. The species first found from Adam's Peak area. It is very rarely found, and known to live under moist soil, and near water sources.
Clithon corona is a species of brackish water and freshwater snail with an operculum, a nerite. It is an aquatic gastropod mollusk in the family Neritidae, the nerites.
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