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DO FISH DRINK WATER
A fish walks into a bar...
Everyone knows that fish live in water. But do they actually ever drink the stuff? "Do fish drink water" might sound like a trick question (or even the start of a joke) but the answer actually tells us a great deal about osmosis and how it works to help animals survive in their environments. Lets investigate the answer to this surprisingly scientific question.
Some conform, some dont.
Invertebrate animals (those without backbones) living in the salty seawater of oceans are generally osmoconformers. This means that the concentration of solutes (including salts, sugars, and other dissolved molecules) in their cells is the same as the concentration of solutes in seawater. Since seawater is isotonic to their cells, invertebrates do not need to gain or lose water.
Most aquatic vertebrate animals are osmoregultors, meaning that their cells contain a concentration of solutes that is different than the water around them. The internal systems of osmoregulators work to regulate their internal water levels.
Fish have a concentration of solutes in their cells that is less than that of seawater, but greater than that of freshwater. (See the measured quantities of these concentrations in the chart below:
| Solute Concentrations in Units of mOsm/L | |
| Substance | mOsm/L |
| Freshwater Lakes and Ponds | 20 to 40 |
| Cells and Blood of Most Vertebrates | 300 |
| Seawater Oceans | 1000 or more |
Osmosis occurs in the gills
Water doesn’t move easily through a fish’s skin (like it can through a cell membrane), but it can move through the linings of the fish’s gills with great ease. A fish’s gills must be extremely permeable so that oxygen from the water can enter the capillaries in the gills and provide the fish with oxygen.
When saltwater (from the ocean) flows over the gills of a fish, the water is hypertonic to the blood in the capillaries in the gills, causing water to move out of the gill tissue where it is lost. This means that saltwater fish are constantly losing water out of their bloodstream through osmosis.
When fresh water (from lakes and streams) flows over the gills of a fish, the water is hypotonic to the blood in the capillaries in the gills, causing water to move into the gill tissue. This means that freshwater fish are constantly gaining water into their bloodstream through osmosis.
The Loss and Gain of Water by Osmosis in Fish Is Dealt with by Osmoregulation
To counter the effects of osmosis in their gills, both freshwater and seawater have to undergo osmoregulation, but since the results of osmosis are opposite on them, so too must be the type of osmoregulation in which freshwater and seawater fish engage.
The figure below shows all of these key differences occurring between freshwater and seawater fish. We have already discussed the different effects of osmosis at the gills of fish. Now we need to look at what is coming in through their mouths and at what each is excreting.
To see the whole thing, download the PDF:
http://www.mediafire.com/view/wlea7f0wonf3pve/01_fish.pdf
