Fish Biology
Osmoregulation: Please give me a drink!
by J. Charles Delbeek M.Sc.

Diffusion is defined as the movement of a substance from an area of
high concentration to an area of low concentration. Osmosis is
defined as the diffusion of water from an area of high concentration
to an area of low concentration across a semi-permeable membrane.
These two mechanisms are of prime importance in the lives of not only
fishes, but aquatic invertebrates as well. Living cells need to be
surrounded by an environment characterized by exact concentrations of
certain substances dissolved in water (Moyle and Cech, 1982).
Problems occur, however, when this environment is altered, therefore,
organisms spend a great deal of time insuring that this doesn't
happen. In fish, the external environment often varies considerably
from that which exists within the body of the fish. This results in
the movement of various substances, such as water and salts, in and
out of the fish by osmosis and diffusion. This article will explore
the various ways in which fish overcome these imbalances and what we,
as aquarists, can do to help them.

There are basically four different strategies of regulation of
internal water and total solute concentrations used by fish. This
depends, in part, on the environment in which they live. The first
osmoregulatory strategy is that used by the hagfishes (Agnatha,
Myxiniformes) which are slimy eel-like animals that are found only in
deep- water marine habitats. Actually they have a very simple method,
their body fluids have basically the same total salt concentration as
sea water and they are the only vertebrates with this characteristic
(Moyle and Cech, 1982). In other words they are isotonic (equal
concentrations) with their environment and there is no osmotic
gradient by which fluids or salts can be lost.

The second strategy is that employed by marine elasmobranchs (sharks,
skates and rays). Although their body fluid has a lower concentration
(hypoosmotic) of salt than sea water (about 1/3 of sea water) they
have developed a strategy to overcome this. Instead of passing urea
(which is mostly composed of organic salts) out of their bodies, it
is put into their blood stream, effectively raising the concentration
to that of sea water! Even with this method, they must still
eliminate excess sodium (Na+) and chloride (Cl-) ions (Moyle and
Cech, 1982). This is performed by a special gland known as the rectal
gland, which concentrates Na+ and Cl- ions into a solution which is
passed out of the body (Gordon, 1977). The coelacanth also uses this
mechanism.

Freshwater fish (teleosts) have the exact opposite problem, their
body fluids (1/3 the concentration of sea water) have a greater
concentration than their surrounding environment (hyperosmotic). As a
result they are constantly taking on water by diffusion through their
skin and, to a much larger extent, through the thin membranes of
their gills. Therefore, to maintain the high concentration of their
body fluids, they must continuously excrete the excess water they
have absorbed. This is accomplished by highly efficient kidneys which
produce a very dilute urine (Moyle and Cech, 1982). The only problem
which such a high rate of urine production is that a loss of salts
and other solutes is unavoidable. Salts, mostly Na+ and Cl-, are
also lost by diffusion through gill membranes. Some of these can be
replaced by ions contained in food but by far the most common method
is through the movement of a substance against an osmotic gradient
through the use of energy. This usually involves the exchange of one
substance for another. In the case of freshwater fish, Na+ ions are
taken from the water and ammonia ions are taken from the fish and
they are exchanged. This effectively rids the fish of ammonia.
Chloride ions are exchanged for carbonate ions which helps in
maintaining the pH of the body fluids.

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.

References
1. Gordon, M.S. 1977. Animal Physiology: Principles and
Adaptations. MacMillan Publ. Co., Inc., New York.

2. Moyle, P.B. and J.J. Cech 1982. Fishes: An Introduction to
Ichthyology. Prentice Hall, New Jersey.