Without osmoregulation fish will die
Osmoregulation is the control of the concentration of body fluids. If
a fish is unable to regulate the effects of osmosis it will die.
Clearly, osmoregulation is a vital function affecting all aspects of
Osmosis, diffusion and fish health
If we look at the structure of the gill or gut under a powerful
microscope we would see the tissue that lines these structures, the
epithelium, is very thin, usually only one cell thick. Such areas are
rich in blood vessels with very thin walls. The combined thickness of
epithelium and blood vessels is less than the size of a full stop. The
structure and thinness of these areas means that the minute particles
whizzing around in the water (and blood) can pass through the epithelium
and blood vessels with relative ease; in fact in many cases it is as if
there wasn't any barrier at all!
A substance will diffuse from an area where it
is more concentrated to an area where it is less concentrated.
Osmosis is a special
case of diffusion. Water will move from an area where it is more
concentrated - i.e. more pure - to an area of less concentration
- i.e. containing more dissolved substances, for example more
it is this difference
in water 'saltiness' between the fish's blood and the
surrounding water that drives the constant osmosis
If the gill and gut surfaces were unfolded and laid out flat we would
have a massive area, much greater than the combined surface area of the
skin and scales. At least half of the exposed surface of a fish is
permeable to small particles. It is this permeability that leads us to
describe a fish as an open system, constantly exchanging particles
between its inside and the surrounding water. Now, perhaps, it is
possible to understand why changes in the surrounding water have such an
important effect on fish.
Diff and 'Oz'
The permeability of the gills and gut and, to a lesser extent, the
skin, which are all in close contact with the water, leads to a further
challenge for aquatic animals. Diffusion or osmosis alone would
eventually result in the fluid of a fish's body being identical to that
of the surrounding water, just as a sugar cube, left in a hot cup of
tea, would eventually fully dissolve and be spread evenly throughout the
tea (by diffusion). If the fluid within the fish were to be dissolved
thoroughly with the water surrounding it we would, I'm afraid, have a
dead fish! But the composition of the body fluids is obviously different
to the pond water and has to be maintained that way if the fish is to
remain alive and healthy. So how does a fish manage to do this? What is
it that keeps a fish's body fluids stable and different to the
The body fluids of a freshwater contain more dissolved salts and ions
than the surrounding water. As a result of this imbalance there is a
constant influx of water into its body and a loss of salts and ions from
the blood outwards. A similar effect occurs if you put a dried raisin or
apricot into distilled water. There is a net influx of water molecules
through the skin of the dried fruit and it swells up to look like a
grape again. (Try it!)
The mechanism may seem complex but essentially a fish has to rid its
body of excess inflowing water by constantly excreting a weak solution
of urine. Fresh-water fish can urinate approximately 30 per cent of
their body mass each day. Salts are removed from the urine before it is
excreted (fish are not wasters, remember) and they are also actively
taken up from the water by way of the gills in order to maintain
internal salt levels. This constant active absorption of salts requires
energy but is essential to the fish's survival, and anything that
affects this vital function will have serious effects on the health or
even survival of the fish.
The reverse situation exists in marine fish; the environment contains
more dissolved salts and ions than the fish's body so there is a net
movement of water out of the fish's body into the stronger
sea-water. To replace this constant loss of water, marine fish drink
sea-water and excrete the excess salts. Special cells in the gills
called chloride excretory cells are involved in this process.
Clearly, any interference to the fish's osmoregulatory systems,
either fresh water or marine, would quickly prove fatal! Fresh water
fish would rapidly accumulate water (a typical sign seen in dropsy),
while marine fish would dehydrate. Clearly, our health considerations
must take aboard the importance of osmoregulation which can be affected
by instances such as water hardness, ulcer disease and stress.
To summarize, we have seen that a fish, because it lives in water,
has to adapt to two major challenges, namely:
low levels of available oxygen,
the need to constantly regulate and
maintain the composition of its body fluids.
It is the interaction between the chemical composition of the pond
water and the need to maintain constant internal conditions that results
in water quality being so crucial to fish health.