If you plopped a marine fish in fresh water, it would start absorbing lots of water across its gills. It wouldn’t take long for it to get into trouble with the balance of water and salt in its body. It might even swell up!
This is because their bodies are more salty than fresh water, so they absorb water via osmosis. Freshwater fish face this same problem, but they’re adapted to get rid of water quickly – mainly by peeing a lot. Some fish, like eels and salmon, move between sea and fresh water by altering their biology, but it takes a lot of energy and time to switch from sea and freshwater living.
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Asked by: Pad Scanlon, via email
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