Freshwater crab

There are around 1,300 species of freshwater crabs, 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.

There are more than 1,300 described species of freshwater crabs, out of a total of 6,700 species of crabs across all environments. 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. 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, and it is therefore unclear how many times the freshwater lifestyle has evolved among the true crabs. The eight families are:

The fossil record of freshwater organisms is typically poor, and 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 post-date the break-up of the supercontinent Gondwana.

Members of the family Aeglidae and Clibanarius fonticola are also restricted to freshwater, but these "crab-like" crustaceans are members of the infraorder Anomura (true crabs are Brachyura).

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