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How Do Fish Hydrate


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Recently a friend asked me how do fish hydrate.

I felt pretty stupid when i couldnt give him a definative answer.

I thought it was probably via osmosis or absorbing H2O from their food or again it had something to do with their gills. I know they pull oxygen from water flushed through their gills.

Don't be too mean to me regarding my ignorance.

Surely i cant be the only one who doesn't know - can i ? :wacko:

PS I am bored sh####s with this weather. Cant work and cant fish :mad3:

Edited by zenman
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Many pelagic birds possess nasal salt glands near the base of the bill. They are quite active in

removing excess salt from the blood. This gland is very important as it is more effective at secreting

salt than the kidneys. This enables sea birds to drink salt water.

Small droplets of water can be visible dripping from the end of the bill within thirty minutes of


By the way, as my name suggests I ain't no scientist. Rather I'm not bad at searching the internet for all manner of crap!

PS - Sorry, still searching for the fish one.

Edited by Boofhead
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Haha. Not a dumb question at all - is there such a thing anyway?. To my understanding:

* Fresh water fish hydrate by osmosis in their gills.

* Marine fish hydration is from the water content in the food the eat and from the sea water they take in when they engulf their food. Fish have better systems than humans for filtering salt so the sea water is effectively filtered by their kidneys.

Please correct me if I'm wrong but I hope this is correct and hope it helps.

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this was actually a topic in hsc biology and has to do with the natural concentration gradients in environments.

More simply, in freshwater environments the cell fluids in fish are more concentrated (with electolytes/salts) then the surrounding water therefore they tend to gain water by osmosis, as water will move from an area of high conc (i.e low in solutes eg salt) to an area of low conc (one which is high in solutes). therefore freshwater fish dont "drink" and they excrete large amounts of dilute urine and actively absorb salts through their gills to maintain this concentration gradient.

in marine environments organisms have a lower internal salt conc then the area around them and they must have adaptations to prevent excessive water loss. they constantly "drink" water and the gills excrete salt. they also produce a very conc urine. therefore the water lost by excretion is less then the water taken in by drinking and they remain hydrated.

they are known as osmoregulators:

in brief: freshwater fish excrete water and absorb salts

saltwater fish: conserve water and excrete salt.

sharks and rays: osmoconformers (their internal concentration changes with external environment)

sorry for the long response but hope it answers the question.

cheers pk.

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