This article appeared in the 1987 issue Vol 1. No.3 of ATOLL
Fish Biology
Osmoregulation: Please give me a drink!
By J. Charles Delbeek M.Sc.
Adapted from http://www2.hawaii.edu/~delbeek/delb11.html
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 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 (active transport). 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 help 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 behavior 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 move 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.
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.