One of the fundamental needs of tropical fish, like other animals, is to have an adequate supply of oxygen. In most fish it is the gills that function to extract oxygen from the water, making it available to the red blood cells for distribution throughout the body. Oxygen is exchanged with the chemical waste product carbon dioxide, which is returned to the gills and then passed into the water. This process of gas exchange is referred to as respiration. Because of their design, gills are one of the most efficient gas-exchange organs known. Under favorable conditions, tropical fish can remove about 85 to 90 percent of the dissolved oxygen from water passing over their gills. Under conditions of low oxygen or high temperatures, this efficiency may fall to 10 to 20 percent or lower.
Even first-time hobbyists are aware of the respiratory function of gills. After all, this is one of the characteristics that makes a fish what it is. Many of us spend time and money measuring dissolved oxygen levels and water temperature in our fish aquariums or backyard ponds, and we are familiar with the latest in filtration systems and aeration devices that provide our tropical fish with all the oxygen they may need.
What we often forget or may even be unaware of is that the gills perform another function necessary for the health of fish. This function is excretion, the process of eliminating the break-down products of protein digestion, mostly in the form of nitrogen compounds. The most important of these waste products takes the form of ammonia, a toxic substance to fish even at very low concentrations. In other animals, it is the kidneys that excrete nitrogenous wastes, in the form of urine.
As long as the tropical fish live in an environment free of toxic ammonia, this compound can easily diffuse across the gills. For example, carp and goldfish excrete 56 percent of nitrogenous wastes as ammonia through the gill tissue. Closely linked to the excretion of ammonia is the reciprocal absorption of ions, the most important of which is sodium. Other ions absorbed by the gill tissue include calcium, lithium, cobalt and strontium. The absorption of these ions may be necessary to replace salt losses or to supplement minerals in the fish’s diet. Thus, the gills also function to maintain salt and water balance, a process known as osmoregulation.
Because of this duel function of the gills, they are acutely sensitive to changes in water quality. Factors such as temperature, pH, salinity and dissolved oxygen and carbon dioxide affect the function of the gills. For example, gill damage caused by high ammonia levels or parasitic infections compromises the two important functions of respiration and excretion. This gill damage will obviously adversely affect the normal physiology of the tropical fish.
Gill tissue is the most permeable of the tissues, and because of this the gills are most sensitive to stresses. It is the design of the gill that makes them so efficient, but also makes them more susceptible to damage. If we examine gill tissue under the microscope, we see feather-like structures called lamellae. These lamellae provide a large surface area for the exchange of oxygen and carbon dioxide. On closer inspection, we see that the gill tissue is very thin, only one or two cells thick, compared with multi-cell layers found in other tissues. In order for there to be efficient exchange of gases, the distance traveled by the gas should not be too great. In gill tissue, the oxygen from the water need only pass across one cell before being picked up by red blood cells.
In an effort to protect this sensitive tissue from the stresses of poor water quality or parasites, the gill uses a few “tricks.” Although offering protection, these strategies are at the expense of gill efficiency. For example, in poor quality water, the gill tissue may increase the number of cells or thickness of the lamellae to put more distance between itself and the stressor. This process, known as hyperplasia, increases the number of cells and is a response to chronic irritation or infection. As a result of this response, the normal distance of one cell may be increased to five or six cells.
The oxygen in the water now has several times the distance to travel across the gills before being picked by red blood cells. The same is true for the carbon dioxide and ammonia waste products moving across the gills in the opposite direction. This situation may severely compromise the respiratory function of the gills. In addition, the gills contain cells that secrete mucus. Mucus provides a barrier and alleviates the irritation. But the mucus also provides a barrier to the efficient exchange of gases and ammonia excretion.
If the environment of the tropical fish is improved through more efficient biological filtration and water changes, the gill tissue will return to its normal structure. The damage is reversible. Chronically poor water quality, however, will result in continuous stress and finally lead to disease.