Transitioning between freshwater and saltwater
In the real world, most aquatic organisms are specialized for life in freshwater or saltwater exclusively, with a few exceptions that are able to survive in either (such as some species of eels). If you're attempting to evolve a saltwater species to live in a freshwater environment or vice versa, you'll need to make some considerations.
Overview[edit]
Whether it's a plant (polyph) or an animal (muscal), your primary consideration should be how your organism will be affected by the process of osmosis.
A freshwater organism placed into saltwater will find that it loses moisture to its environment and gains salts, which will be fatal as it ends up dehydrated. A saltwater organism, on the other hand, naturally has a higher concentration of salt in its body, which will cause it to gain moisture in a freshwater environment, bloating the organism to the point of death.
In either case, each species has adaptations that allow them to survive in their respective environments. Freshwater organisms take in all the salt they can to balance against the lower concentration of salt in the surrounding water, whereas saltwater organisms take in lots of water to counteract the high concentration of salt in their environment. To place either in an environment they're not adapted to handle will causes these processes to stop being helpful and start being harmful.
In general, a species transitioning from freshwater to saltwater needs a means of expelling excess salt from its body. A species transitioning from saltwater to freshwater needs a means of expelling excess water from its body.
Plants (Polyphs)[edit]
Plants, generally speaking, need very little salt to survive, whether they live in freshwater or saltwater. This creates a unique situation for marine plants that need to have some means of drawing water from their environment without becoming dehydrated from the salt content.
Freshwater to saltwater[edit]
In the real world, this is dealt with in different ways, and mangrove trees make a great example as a species that is able to tolerate very high salinity compared to other plants. This is achieved by filtering salt from the water as it enters through the roots. Some species take this a step further by secreting salt from glands in their leaves, or by concentrating it in old leaves or bark and discarding them.[1]
Salt glands come in many forms of varying complexity. The simplest form of salt gland is a salt bladder, which is a specialized epidermal cell containing a vacuole into which salt is pumped.[2] Salt can be pumped into the bladder through various means, such as using specialized proteins in the membranes of adjacent cells.[3]
Animals (Muscals)[edit]
Freshwater to saltwater[edit]
Using fish as a real-world example, freshwater species have no trouble at all getting water from their environment for obvious reasons. However, the bodies of most fish are saltier than freshwater, which means that freshwater species are prone to losing salt to their environment. As a result, freshwater fish are adapted to taking in and holding onto as much as they can, disposing of the excess though urine.
Saltwater to freshwater[edit]
Saltwater fish, on the other hand, have the opposite problem, and are prone to losing water to their environment. As a result, they take in lots of water and expel the excess salt. Some species do this by expelling salt through their gills.