Freshwater crab

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Around 1,300 species of freshwater crabs are distributed throughout the tropics and subtropics, divided among eight families. Freshwater crab_sentence_0

They show direct development and maternal care of a small number of offspring, in contrast to marine crabs, which release thousands of planktonic larvae. Freshwater crab_sentence_1

This limits the dispersal abilities of freshwater crabs, so they tend to be endemic to small areas. Freshwater crab_sentence_2

As a result, a large proportion are threatened with extinction. Freshwater crab_sentence_3

Systematics Freshwater crab_section_0

More than 1,300 described species of freshwater crabs are known, out of a total of 6,700 species of crabs across all environments. Freshwater crab_sentence_4

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. Freshwater crab_sentence_5

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. Freshwater crab_sentence_6

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. Freshwater crab_sentence_7

The eight families are: Freshwater crab_sentence_8

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The fossil record of freshwater organisms is typically poor, so few fossils of freshwater crabs have been found. Freshwater crab_sentence_9

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. Freshwater crab_sentence_10

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). Freshwater crab_sentence_11

Description and lifecycle Freshwater crab_section_1

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. Freshwater crab_sentence_12

Development of freshwater crabs is characteristically direct, where the eggs hatch as juveniles, with the larval stages passing within the egg. Freshwater crab_sentence_13

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). Freshwater crab_sentence_14

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. Freshwater crab_sentence_15

In addition to their gills, freshwater crabs have a "pseudolung" in their gill chamber that allows them to breathe in air. Freshwater crab_sentence_16

These developments have preadapted freshwater crabs for terrestrial living, although freshwater crabs need to return to water periodically to excrete ammonia. Freshwater crab_sentence_17

Ecology and conservation Freshwater crab_section_2

Freshwater crabs are found throughout the tropical and subtropical regions of the world. Freshwater crab_sentence_18

They live in a wide range of water bodies, from fast-flowing rivers to swamps, as well as in tree boles or caves. Freshwater crab_sentence_19

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 snails. Freshwater crab_sentence_20

Some species provide important food sources for various vertebrates. Freshwater crab_sentence_21

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. Freshwater crab_sentence_22

The majority of species are narrow endemics, occurring in only a small geographical area. Freshwater crab_sentence_23

This is at least partly attributable to their poor dispersal abilities and low fecundity, and to habitat fragmentation caused by the world's human population. Freshwater crab_sentence_24

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. Freshwater crab_sentence_25

Every species of freshwater crab described so far has been assessed by the International Union for Conservation of Nature; of the species for which data are available, 32% are threatened with extinction. Freshwater crab_sentence_26

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. Freshwater crab_sentence_27

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: crab.