Hello,
The answer has to do with osmosis. Say you have a glass of water with
a semi-permeable filter dividing the glass into two parts. You pour
some food coloring into the left half of the glass. At first, the left
half will change color quickly, but eventually, it will even out, and
both halves of the glass will have the same amount of color. "Osmosis
is the passage of water from a region of high water concentration
through a semi-permeable membrane to a region of low water
concentration." (1)
Fish cells and skin are semi-permeable membranes. Eventually, the
concentration of stuff on either side of them will even out. A fish
that lives in salt water will have somewhat salty water inside itself.
Put it in the bathtub, and the freshwater will, through osmosis, enter
the fish, causing it or its cells to swell and die. Put a freshwater
fish in the ocean, and the water inside the fish will leave for the
outside. The fish or its cells would suffer from dehydration and
associated health effects.
Here are some sources:
Fisheries and Oceans Canada, "Do Fish Drink the Water?" 22 Nov. 2002,
http://www.dfo-mpo.gc.ca/canwaters-eauxcan/bbb-lgb/school-ecole/activities/act1_e.asp.
(1) Gondar Design Biology, "Osmosis," http://www.purchon.com/biology/osmosis.htm.
Terry Brown, "Osmosis,"
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm.
D.J. Daniels, "Osmoregulation and the Evolution of the Kidney," 20 May
2003, http://home.earthlink.net/~dayvdanls/physio_kidney.html
I hope this answer meets your needs. If not, please request clarification.
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