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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 endemism to small areas. As a result, a large proportion are threatened with extinction.
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.
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).
The colonisation of fresh water has required crabs to alter their water balance; freshwater crabs can reabsorb salt from their urine, and have various to reduce the loss of water. In addition to their , freshwater crabs have a "pseudolung" in their gill chamber that allows them to breathe in air. These developments have preadaptation freshwater crabs for terrestrial living, although freshwater crabs need to return to water periodically to excretion ammonia.
The majority of species are endemism, occurring in only a small geographical area. 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. (2025). 9782831710631, International Union for Conservation of Nature. ISBN 9782831710631 In West Africa, species that live in 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; 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.
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