why does a freshwater fish die In the sea and visa-versa? I’m assuming it’s due to salt but why?
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Cells are sacks of slightly salty water. In particular, they’re *permeable* sacks of slightly salty water: water can flow in and out of most cells (and needs to, for the cell to function).
If a cell is surrounded by water saltier than itself, water flows out of the cell. (This is a purely physics-driven process, it has nothing to do with whether the molecules involved are part of a living thing or not.) If enough water flows out of the cell, the cell shrivels up and can’t function, and it dies. (You can imagine the cells’ component parts as being sort of “trapped under a collapsing tent”.)
Similarly, if a cell is surrounded by water less salty than itself, water flows *into* the cell. This causes the cell to swell and, if the swelling is severe enough, burst. This kills the ~~crab~~ cell.
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Complex living things, like humans, fish, etc, maintain an internal environment for their cells. That means that our cells can rely on always being surrounded by an environment with a particular level of salinity. In humans, this is about 0.9% salt.
We maintain this by regulating the amount of salt in the water in our bodies. In both humans and fish, this process is handled by the kidneys, which respond to high salt concentrations by pumping salt out of the blood and into the urine.
But this process is limited by how efficient the machinery of the kidneys are. Humans are not adapted to living in saltwater, so we can’t drink it. That’s because our kidneys can’t concentrate our urine to the salinty of saltwater, so they have to actually use *more* water than we took in to get rid of the salt in the water we drank.
Freshwater fish are the same, but they have an extra problem: salt from the water can easily enter their body through their gills. So they don’t even have to drink it (although of course they do drink it) for the salt to become a problem. If a freshwater fish is added to salt water, salt from the water quickly dissolves into the fish’s bloodstream, and their kidneys can’t dump the salt fast enough. Salt levels rise, and the fish’s body fails as a result. (I don’t know the exact cause of death in fish; in humans this is actually a result of the salt messing with the body’s electrical systems, not direct cell death, but the fundamental problem – too much salt – is the same.)
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Saltwater fish have efficient kidneys that, unlike humans and freshwater fish, can dump salt quickly. That lets them live in seawater, although if the salt levels go too high, they can overwhelm the kidneys of saltwater fish too. That’s why no fish live in salt lakes like the Great Salt Lake or Dead Sea. (A few shrimp can survive in those conditions, but it’s called the *Dead* Sea for a reason.)
In freshwater, saltwater fish have a different problem. They don’t get *enough* salt. Freshwater fish, and freshwater-drinking land animals like humans, are efficient about picking up salt from our food and water to make sure we get enough. (This is why salt tastes good to you: it’s a behavioral motivation to get a nutrient your body, in the natural environment, would need to be sure to get enough of.) But saltwater fish, whose main problem is *dumping* salt *out* of their bodies, don’t do this. They get more than enough – too much – salt from the water around them anyway. So they’re not adapted to grabbing up salt, and they end up with too *little* salt in their bodies as a result.
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A few fish, like salmon, are *euryhaline*, and have both the salt- and freshwater adaptations to survive both. But that’s biologically expensive, so most species haven’t evolved to do so.
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