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Saltwater fish have a different body chemistry than their freshwater counterparts. Their bodies have hypotonic cells, which keep more salt inside the cells (than outside of them). Therefore, putting them in freshwater will make the water rush into their cells and and eventually kill them.
You might tend to believe that when it comes to surviving in water, all fish are absolutely identical, i.e., all fish can survive in all types of water, whether it’s in a river, lake, pond or ocean. After all, the only thing fish need to stay alive is water. So, regardless of whether the water has come from the sea or a lake, at the end of the day, it’s just water. Going by that reasoning, any fish should be able to survive in any type of water, right?
But alas! That isn’t the case. As it turns out, most fish can survive in only one kind of water – either freshwater (no salt content) or saltwater.
Before we delve further into this, it would be very helpful if we first understood the biological process of osmoregulation.
What is osmoregulation?
As the name implies, it is the process through which living organisms regulate the effects of osmosis to protect the health of their cells. This obviously leads to another question – what is osmosis?
You have almost certainly read about osmosis and its ubiquitous presence in the living world. To put it simply, it is the process that makes a liquid (typically water) pass through the wall (specifically called a membrane) of a living cell. More specifically, it is the net movement of water from an area of low solute concentration to one of high solute concentration across a semi-permeable membrane.
As mentioned earlier, we are constantly surrounded by osmosis in one form or the other. Some common examples include dehydrated foods and vegetables, the function of plants’ roots, bloating of animal carcasses and so on.
The reason freshwater fish cannot survive in saltwater and vice versa has a lot to do with a property of a liquid called tonicity. In simple words, it is the ability of a solution to exert osmotic pressure upon a membrane.
Tonicity comes in three types: hypertonic, hypotonic and isotonic. For the scope of this article, we are only interested in the first two.
Hypotonic and hypertonic solutions
A hypotonic solution has a lower concentration of solutes (solutes are the substances that are dissolved in a solution, e.g., sugar is the solute in a sugar solution) inside the cell than outside of it. On the other hand, a hypertonic solution has a higher concentration of solutes outside of the cell than inside it.
How freshwater and saltwater fish survive in their corresponding waters
Seawater is hypertonic to the fish living in it, which means that the salt content of the surrounding water is higher than the content inside the fish. As a result, they lose the water inside their body to the surrounding seawater due to osmosis. To compensate for this water loss, saltwater fish drink huge amounts of water and are therefore able to survive in highly saline waters.
On the other hand, fresh water is hypotonic to the fish that live in it, i.e., the salt content in their body is higher than the salt content of the water surrounding them. Due to osmosis, therefore, water continuously flows into their body (i.e., the area of high solute concentration, salt being the solute, in this case).
To survive in the face of this continuous supply of water, freshwater fish have to urinate very frequently.
What would happen if you put a freshwater fish into saltwater and vice versa?
If you put a freshwater fish into saltwater, water from inside their body would flow out into the highly saline water surrounding them until they die of dehydration.
If you put a saltwater fish into freshwater, the surrounding water would flow into their body until their cells swelled up with water, eventually killing them.
There are certain fish, however, that can survive in both saltwater and freshwater (provided there’s a long enough acclimation period – a chunk of time for their bodies to adjust to a different salinity than they are used to. Such species of fish are known as euryhaline fish. Salmon, bull shark, trout, shad and striped bass are some of the examples of this specialized kind of fish.
Fish are very sensitive to even the slightest fluctuations in the salinity of the water in which they live. That’s why it’s recommended to fully understand the biological requirements of a fish before putting it into an aquarium at your house!
- Osmoregulation – Wikipedia
- Tonicity – Wikipedia
- Osmosis – Wikipedia
- Euryhaline Fish – Wikipedia
- Acclimation of Osmoregulatory Function in Salmon – The University of New Mexico
- Osmoregulation, Red Drum, And Euryhaline Fish: Environmental Physiology – University of Kentucky College of Agriculture, Food and Environment