by Dr David Huchzermeyer
“ … Like A Fish To The Water”, And Other Miracles!
Water is fascinating. Not only for water sports and swimming pool enthusiasts but especially for anyone with an interest in fish. All too easily the fisherman takes life forms in the water for granted, until one day he finds no catch. Then the questions start. Why does a sick goldfish sometimes lose his balance in the water? Why does the belly swell and the eyes bulge? What problems face a Koi with a large ulcer on its body? And what becomes of a goldfish gasping for air when the water has turned bad?
Consider the fact that life on our planet first evolved in water. People can generally relate to those features of fish that are also represented in animals that left their aquatic habitat and evolved into the land living animals of today. For instance, the bilateral symmetries of the appendages, the eyes, nostrils, mouth and vent. Many other aspects of fish are, however, hidden from the observer and only closer examination of the fish will open our eyes to the wonders and difficulties of an existence under water.
Water carries only approximately 5 % or less of the oxygen in the air. In place of lungs, oxygen is efficiently taken from water and passed to the blood via the gills of fish. The blood-rich nature of gills gives them their normal dark red colour. Gills are extremely sensitive and are exposed to any harmful substances in the water. Consider how our so-called civilised lifestyles abuse the aquatic environment through pollution, canalisation, dam building and draining of wetlands.
A Weighty Business…
Water does not simply flow over the gills. A double pump action, exerted between the mouth cavity and the operculum or gill cover, forces water through the gills. As water is 1300 times heavier than air, this process requires considerable exertion. Breathing is hard work in the aquatic environment, even without pollutants complicating the process.
Pressure To Survive
The weight of water exerts enormous pressure on any organism living within water. You may have felt the pressure on your ears when diving even in shallow water. The entire pressure exerted by the atmosphere at sea level is equivalent to 760 mm of Hg. As one moves deeper into the water, every additional 10 meters of water depth increases the pressure by another atmosphere or a further 760 mm Hg. Any structure containing air is at risk of being obliterated at such pressure. Our lungs cannot function under such pressure, unless of course the air is supplied from a pressurised source such as the scuba tank of a diver.
Built-in Buoyancy Tanks
Like other animals, fish are also composed mainly of water and as water is virtually incompressible they are able to function normally, that is without being squashed, even at great depths. Many fish have gas- filled swim bladders that act as buoyancy tanks allowing them to swim unhampered at their optimum depth range. Some fish fill their swim bladder through an open duct by gasping air at the surface. Other fish have a sealed swim bladder and rely on a unique gas gland by which they are able to secrete gas into the swim bladder even against very high pressures exerted by the weight of the surrounding water. Similarly fish are able to reabsorb excess gas from the swim bladder as and when needed, in order to maintain buoyancy at different depths.
The Battle Of The Bulge
Fish living in fresh water have a tendency to swell up with water that potentially enters their bodies through osmosis as a result of the relative saltiness of their tissues compared to the water. To counter this water influx, fish have an almost impermeable skin. They also have special cells in their gills that actively absorb salts from the water. Excess water that enters the body via the gills of freshwater fish is lost through the production of large amounts of very dilute urine by the kidneys. Damage to the skin or gills of fish will invariably lead to over-hydration which is evident in a bulging of the soft parts of the fish, particularly the eyes. Fish with large skin wounds or ulcers will sometimes be seen to have both eyes bulging out of their sockets.
The ‘Glide’ Factor
The majority of fish rely on a streamlined body shape to allow them to glide through the water. A thin layer of red muscle situated under the skin and along the lateral side of most fish performs the movements required for normal restful swimming. This muscle functions aerobically, requiring a ready supply of oxygen just like our own muscles.
The muscles of fish that make up the white flesh of fish that you may recognise from fish you have eaten are in fact emergency muscles. These are anaerobic muscles. They can give an extremely powerful burst of activity, but only for a very short period. Unlike the 100 meter sprinter who catches his breath for a few minutes after a sprint, the fish caught on a hook or escaping from some other predator takes a much longer time to break down the lactic acid that has been produced in the muscles after such an exertion. This is partly due to the fact that the white muscles of most fish have a very poor blood supply. In place of blood, these muscles must rely on lymphatic circulation and lymph carries neither red blood cells nor haemoglobin, both of which make it possible for blood to carry oxygen so efficiently in our own bodies.
Now that you’re ‘in the swim’ about fish, you might just appreciate how well things are really going when someone tells you, “Things are going swimmingly!”