In this post, I’m going to discuss a topic that I thought about a lot when I was younger. Have you ever wondered why, exactly, saltwater fish can’t live in fresh water and freshwater fish can’t live in salt water? When I was creating my Insta-pets kit and watching the little guys swimming around in their saltwater tank, I decided to find out why the amount of salt in their water was so critical to their well being.

As most of us know, there are two main types of fish: saltwater and freshwater. Saltwater fish can’t survive in fresh water, and freshwater fish can’t survive in salt water. In nature, they live in different environments (the ocean vs. a stream, for example) and in captivity we keep them in different types of aquariums. If a saltwater fish somehow makes its way into a freshwater aquarium, it will die after only a few hours. Most fish can survive in just one type of water. These are referred to by scientists as stenohaline. There are also euryhaline fish, like salmon, that can survive in salt water OR fresh water. Why are fish so picky about what kind of water they live in? Read on to find out.

The key to the answer is osmosis. To explain osmosis, let’s start with a cell – the building block of life. Cells are responsible for the vital functions of all living things. We’re made of them, plants are made of them, and so are fish. Before I talk about osmosis, let’s take a look at two important parts of a cell: the cell membrane and the cytosol.

Think of the cell membrane as the “skin” that surrounds a cell and think of the cytosol as the substance that fills the cell and surrounds its contents. The cell membrane’s job is to keep everything inside safe, and to keep a balance between the levels of chemicals (mostly salts) in the cell and in the surrounding environment. It’s made up of water and lots of different kinds of dissolved chemicals that a cell needs to work properly. Cells like to keep the concentration of chemicals on the inside the same as on the outside. For example, if a cell that was previously surrounded by a fairly diluted solution (like fresh water) is suddenly exposed to a salty solution, the cell membrane will allow most of the water inside the cell to leave. This makes the cell shrink, and increases the concentration of salts in the cytosol so that the concentration inside the cell becomes equal to the concentration outside the cell again.

Confusing? Think of it this way – the cell membrane lets water move through it from areas of low concentration (less chemicals, more water) to areas of high concentration (more chemicals, less water). This helps keep the concentration of chemicals the same on the inside and outside of a cell.

Check out the illustration below to get a better idea of how osmosis works:

Osmosis Diagram

An example of osmosis – water is moving from an area of low concentration of salt, outside the cell, (notice the cell is smaller) to an area of high concentration of salt, inside the cell. Once the process is finished, the concentration of salt is the same on the outside and inside of the cell (now the cell is larger because there is more water in it). Before is on the left and after is on the right.

Fish need a certain amount of salt in their bodies to stay healthy. Too much or too little can cause problems. Since fish’s natural habitats are always changing due to weather and other forces, they have to be able to adapt to small changes in salinity by controlling how much salt is present in their bodies. This is called osmoregulation, and it’s one of the functions of the kidneys – for both fish and people. The gills and kidneys of saltwater fish get rid of salt because they live in such a salty environment. Freshwater fish concentrate salt in their bodies because they live in an environment where salt is harder to come by.

Saltwater fish’s cells naturally contain a very high concentration of chemicals so that they don’t shrink when exposed to the salt water they live in. When a saltwater fish is exposed to fresh water, all of that water floods into their cells in order to dilute the chemicals inside and keep a balance with the surrounding environment. Two things can happen when water floods in. Some cells fill up with so much water that they burst (scientists call this cytolysis), while others just stop working because the chemicals inside them are too diluted. Both of these things will kill the fish. The opposite happens to freshwater fish in a saltwater environment – water flows out of their cells, dehydrating the fish and disrupting cell function.

Here’s how osmosis can affect the size of blood cells:

Osmotic Pressure on Blood Cells

An example of osmosis in blood cells. On the left are cells in a solution with more salt in it than there is inside the cells (a hypertonic solution); in the middle are the cells in a solution with the same amount of salt in it as there is inside the cells (an isotonic solution); on the right are cells in a solution with less salt in it than there is inside the cells (a hypotonic solution).

Most fish are stenohaline, and will die when exposed to the wrong environment (salt water vs. fresh water). But what about euryhaline fish like salmon, eels, and some trout? Salmon, for example, have evolved to survive in both fresh and salt water because of their life cycle. Salmon are born in freshwater streams, but then migrate into the ocean where they mature and spend about 1-5 years. After that, they migrate back to the same freshwater streams they were born in so they can reproduce. Their bodies have adapted to use osmoregulation to either retain or get rid of salt depending on their surroundings.

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