Most marine invertebrates are what are called
osmoconformers. This means their tissue fluids have an identical osmotic pressure (essentially salinity) to the seawater. Water and salts are neither lost to nor gained from the environment. If the ambient salinity changes, they have no ability to compensate, and their tissue fluids change along with the environment. If the change is too much, their tissue fluids become either dehydrated (if the salinity goes up) or diluted (if the salinity goes down) and the animal dies.
So, if you took a saltwater crab and put it in a freshwater tank, its cells would be flooded with water and it would lose salt. Over time, it's tissues would become too diluted to work properly and the cells would die, and then so would the crab.
Most fish are
osmoregulators. All fish have approximately the same salinity blood and tissue fluids. In practise, the salinity is about halfway between freshwater and seawater. This is why freshwater fish have to remove water while conserving salt, and seawater fish have to conserve water will removing salt. All fish have some ability to adapt to changes in salinity around them. In the case of brackish water fishes this ability is very great, and they are called
euryhaline fish. Fishes with a limited ability to compensate for ambient changes in salinity are called
stenohaline.
Some invertebrates are euryhaline. Shore crabs (Carcinus maenas) is a good example, and it will adapt to anything from 25-100% normal seawater salinity. Malayan Livebearing Snails will do well in anything from freshwater to 50% seawater. Fiddler crabs (Uca spp.) are another example, and in the short term at least will tolerate freshwater to 100% seawater. When kept in freshwater for very long periods though they do tend to die.
This same sort of the observation has been observed in laboratory.* Some species of salt water Takifugu that will adapt to freshwater in the short term steadily lose condition in the long term, specifically showing changes in the composition of the blood and increased mortality. Other species will adapt fully to freshwater conditions. A
fully euryhaline fish or invertebrate is one that does equally well in freshwater or salt water. Mollies are perhaps the best-known example to aquarists.
Cheers, Neale
*
Takifugu obscurus is a euryhaline fugu species very close to Takifugu rubripes and suitable for studying osmoregulation; BMC Physiology, 2005.
Why cant you aclimate saltwater crabs into a freshwater enviroment?