Can Saltwater Fish Be Converted To Freshwater

[William]

Of course salmon, eels and some trout hatch in freshwater rivers, then spend a large portion of their life in the sea before navigating back to their native river to restart the cycle.

 

[kribensis12]

Why cant you aclimate saltwater crabs into a freshwater enviroment?

 

[nmonks]

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?

 

[kribensis12]

What if you kept them in a brackish setup? Would they still die from cell flooding?

 

[nmonks]

Yes.

A saltwater crustacean might only tolerate salinity changes between 90% and 100% normal seawater. Anything less than that and its cells cannot compensate for the reduced salinity (= excess water) and they swell up and burst. Or else they lose salts so fast things like nervous impulses and muscular contractions can't work.

A euryhaline crustacean like a fiddler crab can compensate because, unlike the saltwater crab, it's an osmoregulator not an osmoconformer. It sets its own tissue fluid salinity. So as it moves between salty water and freshwater, it changes the rate at which it absorbs/excretes salt and takes up/pumps out water.

Some marine invertebrates in the hobby are actually euryhaline ones, and can be adapted to reduced salinity. The blue-legged hermit crab is probably the best known. But these are exceptions: everything else, such as corals, anemones, starfish, shrimps, hermit crabs, etc. should all be assumed to be stenohaline osmoconformers unless you know otherwise.

Cheers, Neale

What if you kept them in a brackish setup? Would they still die from cell flooding?

 

[Squidman]

I dont know about that. They might be able too. Dont follow with my info though. Im more of a Fish health care kinda guy. But i wouldnt do it due to the fact that i think saltwater fish shouldnt be in a freshwater since its not there natural habitat/enviorment.

 

[brooksbychris]

its all to do with osmoregulation of the body and water and salt passing between the body cells, well most of it is to do with that.