Why can’t saltwater fish survive in freshwater?
In: Biology
Their bodies are adapted to the osmotic pressure of saltwater. In freshwater, they wouldn’t be able to maintain electrolyte balance within their cells.
Saltwater fish will explode if put in freshwater because freshwater goes into their gills too easily. It’s like a baby bottle nipple being too big but instead of drooling out the milk the baby explodes
Most animals, including fish and humans, do something called osmoregulation, which is regulating the concentration of water and salts in our tissues.
If we humans drink seawater, for instance, it will make our tissues too salty. To regulate it, water will move out of our cells and into the solution to reduce the concentration. This is bad enough but your kidneys will need even more water to filter and excrete the excess salt, and the net effect is dehydration and ultimately death. The same sort of thing happens when you put a freshwater fish in salt water.
Saltwater fish have the opposite problem. They live in water that is much saltier than their own tissues and they can’t really avoid consuming it. So to compensate, their organs are constantly pumping the excess salt out of their bodies. If put in fresh water, their body doesn’t know to stop; they keep excreting salt and holding onto water until they either bloat up and die, or their electrolytes get so low that their nervous system fails. Humans can also die in a similar way: it’s called water intoxication, although it is rare.
There are fish that can live in both salt and fresh water, e.g. salmon. They’re called “euryhaline” (as opposed to stenohaline organisms, who can only live in a narrow range of salinities). However since most fish have no reason to move between waters of different salinity, its biologically “cheaper” to adapt to just one salinity.
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