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 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.