Fish live their lives completely surrounded by water on all sides. It surrounds them externally in their habitat and also comprises much of their body mass. Fish must therefore strike some sort of balance between these two separate, and very different, water environments which determine so much about their health. This balance is met through the processes of osmosis and osmoregulation. To explain osmosis, we'll first discuss the concepts of diffusion and equilibrium. All molecules have energy associated with them known as kinetic energy. This energy of motion allows for the random movement of molecules throughout the space which they are in.
Diffusion is simply the tendency of molecules to passively move from an area where they are more highly concentrated to an area where they are at a lower concentration - as a result of random movement. Note here, that it is the molecules as a whole which move from higher to lower areas of concentration. The movement of individual molecules is random and while some may move down the concentration gradient (from higher to lower concentration), others will move up the concentration gradient. There will be a net movement of molecules down the concentration gradient until a point of equilibrium is reached. At this point, the molecules are moving at equal rates up and down the concentration gradient and there is no further net movement of the particles. Diffusion and equilibrium can be illustrated by placing a drop of methylene blue in a glass of water. One will observe that after some time has passed, this dye has dispersed throughout the water until an even color can be observed in all areas. The dye molecules diffused through the water randomly and reached a point of equilibrium such that there is no further net color change in the glass. Keep in mind that although there are equal numbers of dye molecules in all areas of the glass, individual molecules aren't static and don't remain in the same place. It is merely their concentration which is maintained. In a different scenario, one can imagine being in a large room where someone lights a cigarette. At first only the people nearest to the cigarette will smell the smoke. However, after some time, everyone in the room will smell it. When the cigarette is no longer burning, the scent will be evenly detectable throughout the room as the smoke molecules diffuse and reach equilibrium.
Osmosis is a concept similar to diffusion which involves the passive movement of water through a membrane which is permeable to the water, but not to the solutes dissolved in the water. With osmosis, the water moves from an area of lower solute concentration to an area of higher solute concentration. While this may initially seem to be the opposite of diffusion, one must consider that the water is still moving from where the water is most abundant to where it is least abundant. This is illustrated in the figure below.
The figure illustrates two U-shaped tubes, each of which contain two different salt solutions separated by a semi-permeable membrane which allows only water to pass through it. In Tube A, the left side contains a hypertonic solution - having a higher concentration of solutes, and the right side contains a hypotonic solution, having a lower concentration of solutes. Isotonic is a term for solutions of equal solute concentration. Water moves from the hypotonic side to the hypertonic side and in doing so, increases the salt concentration on the right side of the tube and decreases the salt concentration on the left side of the tube until the two concentrations are equal. Tube B illustrates equilibrium. At equilibrium water is moving though the membrane in both directions at an equal rate. The pressure needed to stop the water level from rising on the left side of the tube is called the "osmotic pressure". Water always has a net movement toward the solution of higher osmotic pressure. At equilibrium, both sides of the tube have equal osmotic pressures.
The biological importance of hypertonic, hypotonic, and isotonic solutions and their effects on living organisms can be observed by subjecting a single cell to each of them, when the cell has no means of adapting to each new environment. A cell placed into a hypotonic solution will have water rush inside of it (where solutes are more concentrated) and cause the cell to burst. A cell placed into a hypertonic solution will experience dehydration as water leaves the cell for the surrounding environment. A cell placed into an isotonic solution will experience water entering the cell at the same rate at which it leaves, and so will be at equilibrium and appear normal.
Fish do not always find themselves in isotonic environments. Thus, their body cells must have a means by which to adapt to changing salt concentrations in their bodies and environments. Osmoregulation controls this balance of water/salt concentrations. Freshwater fish are hypertonic to their water environment and therefore, water is continually diffusing into the fish through the gill membranes into the blood. The gills are also permeable to respiratory gases, ammonia waste products, and ions. Therefore, while water moves in towards the higher osmotic pressure of the blood, sodium and chloride ions also diffuse out of the fish, moving down their concentration gradients to the external environment. Freshwater fish must expend energy to regulate this ion loss and fluid uptake. Marine fish experience the opposite situation as their bodies are hypotonic to their saltwater environment.
The continual uptake of water in freshwater species is regulated by the kidneys which continually produce large amounts of dilute urine. Despite the importance of healthy kidneys to help counteract the problem of taking on water, some salts are also lost in the large amounts of urine as well as through the membrane of the gills. Fortunately, the gills are also a site of ion uptake. Special cells in gill lamellae contain sodium and chloride "pumps". These pumps are special enzymes that use energy to move the ions up their concentration gradient (remember that moving down a concentration gradient is spontaneous, as in diffusion, and requires no input of energy) to maintain their higher concentration in the body. Thus, osmoregulation is a process that requires the expenditure of much energy on the part of the fish--even when they appear to be inactive. This constant expenditure of energy to maintain an osmotic balance is a reason why proper nutrition and low stress levels are important for healthy fish. Damage to the kidneys through bacterial infection or other means is often deadly as these organs extract the large amounts of water which continually diffuse into the fish's body. Fish with ascites or bloat are often suffering from kidney damage which is, unfortunately, irreversible. Saving a fish which appears bloated is highly unlikely, if the swelling is due to kidney dysfunction as they have lost the ability of osmoregulation.
Angelfish and discus are examples of fish that have very strong and efficient osmoregulatory systems. They do well naturally in their native environment where the osmotic pressure is great due to the extremely soft water of the Amazon river basin. However, when under stress, these systems can be impaired. This is why it is advised to add some salt to the water of fish under stress. The salt reduces the osmotic pressure. The stress could come from transportation, diseases, trauma or a number of other problems. Keep in mind that salt is only good for short term success. The stress must be removed to prevent further problems.
A sudden change in osmotic pressure can put great stress on the osmoregulatory system of a fish. This is of great concern when shipping fish to locations with water different from what they're adapted to. The fish arrives under great stress and is not able to regulate any osmotic pressure differences easily. This is one reason why acclimation should be slow. It also explains why treating diseased fish must be done carefully. When putting them in a salt bath, the concentration of salt should be increased gradually.
It is important to understand osmosis and how it affects our fish. It is a vital component for their well-being. Your ability to control problems, safely ship and receive fish and treat diseases will be enhanced if you pay attention to this aspect of their lives.
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