Freshwater crab

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

[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.

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.

[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.

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.