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Fish are truly remarkable animals. First off, the question of what a fish is is a pretty tricky one, because fish are defined by what they’re not, namely mammals, reptiles, amphibians, or birds. Secondly, they breathe by doing a direct gas exchange, but only in the region of their gills, unlike water-dwelling amphibians. Anyone who’s had a pet goldfish has watched it swim round, opening its mouth to pass water through, looking like it’s taking big drinks. In all that water, how do fish maintain the proper levels of hydration? After all, everything needs water to survive. How do fish drink?

The answer is: it depends. 

Image by catchesthelight

Let’s talk about our goldfish first (I had one once; his name was Sir Owain Barnabas Ichabod the First. He lived for 6 years and greeted us everyday at feeding time). The first thing we have to understand is the process of osmosis. Most of us heard this term in high school science classes, and since then we’ve only heard it used for non-scientific things. Diffusion is the tendency of molecules to move from an area of high concentration to an area of low concentration, with the “aim” of making everything evenly distributed. Osmosis refers to diffusion in the case of water—because water is so important for life and in so many life processes, it gets its own special name. Basically, water gets into any cell by the process of osmosis: from your stomach into the rest of your body, and so on. A quantity of pure water is always going to have a higher concentration of water molecules than water mixed with other molecules.

This is how freshwater fish get their water. Fish will either absorb or lose water depending on their needs and their surroundings. Being so close to a much higher concentration of water molecules, they do this directly through their skin. Freshwater fish also need salt to survive, and the concentration of salt in their bodies is always going to be higher than the concentration of salt in the surrounding fresh water. So a freshwater fish’s kidneys are always working to expel excess water, so that their salt doesn’t dilute.

But if you live in the ocean, you have no problem with losing salt; in fact, you have a problem with losing water, since the saltwater has way more salt in it than your body. Osmosis here is actually driven by the chemical balancing of salt quantities. Saltwater fish do actually drink water when they open their mouths to let water pass over their gills. Water is processed in much the same way we do, moving from the stomach to the rest of the body. The excess salt is then excreted on their gills, where it goes back into the ocean (or the tank, as the case may be).

So from this, it’s abundantly obvious why saltwater fish can’t live in freshwater, and why freshwater fish can’t live in saltwater. They have very different methods of dealing with water and salt in their bodies, and those methods don’t really adapt well to a different environment. Of course, there are a few fish that can live in both environments: salmon is a great example. Salmon can actually drink freshwater and process it the way a saltwater fish would, which is why they can live in both.

The differences in fish even from just saltwater to freshwater is only the beginning of the huge range of differences found in this group of organisms. No wonder we have trouble defining a fish! But now you don’t have to worry as much about your goldfish getting thirsty; if you’ve cleaned its tank and it has lots of lovely fresh water, it’s probably fine.

Sources:

http://en.wikipedia.org/wiki/Amphibian

http://en.wikipedia.org/wiki/Fish

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

http://indianapublicmedia.org/amomentofscience/do-fish-drink/

http://www.aquarium-pond-answers.com/2006/12/how-do-fish-drink.html