Recent advancing industrialization and urbanization have increased salt concentrations in formerly-freshwater habitats. Freshwater animals are being affected, especially those like crustaceans that are unable to emigrate to escape the problem. The effects of increased salinity, first observed at the molecular level, are found to extend to the levels of the individual, population, community and ecosystem. Crustacean morphology, behaviour and life histories may be influenced, with growth rate, and age and size at first reproduction, being disturbed, along with clutch and neonate size. Mortality rates are also elevated where the salt concentration is high, though susceptibility to salt differs both between species and between clones of the same species. The effect may be to modify the composition of crustacean assemblages in terms of the species and clones present.