Marine fish (teleosts) have the exact opposite problem to that
encountered by freshwater teleosts. Their body fluids are, again, 1/3
of that of sea water but this time they are in sea water so their
body fluids are hypoosmotic to their environment. As a result they
will tend to lose water by osmosis to the environment through their
skin but mostly through their gills. Consequently, they have
developed mechanisms and behaviour to compensate for this water loss.
Firstly, the kidneys of marine teleosts are modified in such a way
that very little water is extracted from the blood, some species even
lack certain kidney structures and can't eliminate water (Gordon,
1977; Moyle and Cech, 1982). This results in a reduction in the loss
of water by the production of urine. However, water is still being
lost by the gills and this cannot be stopped, so the only method left
is to somehow replace the water as quickly as it is lost. Marine
teleosts accomplish this by actually drinking water, the most
reliable drinking rates reported in the literature range from 3-10
ml/(kg hr) (Gordon, 1977). However, drinking water by itself cannot
solve the problem, a complex series of events must first occur in the
digestive tract. These events are not yet well understood but it is
known that most of the water is absorbed as are the monovalent ions
Na+ and Cl- (they are, after all, drinking salt water!), while the
divalent ions (such as magnesium and sulfates) are excreted by the
kidneys (Gordon, 1977). Sodium (Na+) and chloride (Cl-) also move by
diffusion into the body through the gills. Therefore, Na+ and Cl-
ions will accumulate in the body of the fish and must be eliminated,
this is accomplished by special cells in the gills called chloride
cells, which me these ions out of the body by active transport (Moyle
and Cech, 1982; Gordon, 1977).
From the above information some practical tips for the hobbyist can
be gained. Since marine fish must constantly expel various solutes,
such as sodium and chloride ions, against an osmotic gradient, a
great deal of energy is required. Therefore, anything that you can do
to lower the osmotic gradient will benefit the fish in terms of
energy expenditure. The simplest way of doing this is to lower the
salinity of the water as much as possible, particularly for a fish in
distress (i.e. diseased). This alone can sometimes be enough to ease
their burden. Of course any such change must be extremely gradual and
must not get to the point where the fish is in obvious stress.
Another problem comes when invertebrates are added, especially the
soft-bodied ones such as anemones and corals; a drop in salinity can
be disastrous for them. Since marine fish produce very concentrated
urine, their waste products can pollute a tank far quicker than a
freshwater fish which produces much more dilute wastes. That is why
you can usually put in many more freshwater fish than marine fish in
the same volume of water. That is why paying attention to the water
quality of a marine tank is so much more critical than in a
freshwater tank. With the advent of dry/wet filter systems from
Europe, the load in marine aquaria can now be greatly increased due
to the superior ability of the filter to handle waste products. That
is why the so called "mini-reef" systems are becoming so popular with
hobbyists, many more animals can be kept in a smaller volume of water
with little risk of pollution.
Fish Biology
Osmoregulation: Please give me a drink!
by J. Charles Delbeek M.Sc.
here are two paragraphs that break it down rather well. seems it is more work for the fish the higher the salinity. so it seems I was slightly incorrect in thinking its not hard on the fish at higher salinities. I wont swear this as gospel or anything but it seems relevant and accurate, the auther reads as a knowledgable person....
