How can Fish Live in Such Salty Water?

Most recent answer: 03/12/2015

How can fish live in such salty water?
- Natalie (age 18)

It may be helpful to answer another question first: "Why do not we switch irrigation with seawater?". Aqueous solutions contain many different small solutes inside. If two solutions are in contact with each other, the more concentrated one will draw water towards itself from the diluted one (and the identity of the solutes does not matter much). The source of this effect i scalled osmotic pressure.  Salty water would suck water from the roots, and - if the salt is small enough - it will also penetrate to the roots desrupting the intracellular ion balance, which affects metabolic reactions and their catalysis.

Marine organisms have specialy evolved organs that can avoid ion influx or even pump out excess ions at the expense of energy. First of all, their body is not a permeable membrane as the cell membrane is, avoiding water-ion exchange. However, some exposed surfaces are needed to do gas-nutrient-excrement exchanges. Their gills have ion pumps, that expell the absorbed ions back. They need to drink seawater, but their kidneys are also quite special in that they excrete salts quite efficiently as part of concentrated urine. All these are thermodynamically possible, because they are energy-driven. The enrgy comes from food.

The acceptable salt ranges depend on organism and there are extremes such as salmon which can live both in salty and fresh water. Now it is my turn to ask: "how can fish live in such fresh water?"


(published on 03/12/2015)

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