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The air we breathe is mostly nitrogen (78%) and 21% oxygen. The water that fish ‘breathe’ also contains oxygen, but at a much lower concentration, less than 1%. In addition, since water is 840 times denser than air and 60 times more viscous, it takes more effort for fish to ‘breathe’ to extract oxygen, around 10% of their metabolic energy. In comparison, terrestrial animals only use about 2% of their metabolic energy to extract oxygen from air. For example, rainbow trout need to move approximately 600 ml of water past their gills per minute per kg weight while, in comparison, terrestrial reptiles such as turtles only need to move 50 ml air min-1 kg-1. As a result, even though fish gills are quite efficient, obtaining enough oxygen from the surrounding water can be difficult and sometimes life threatening.
Fish capture oxygen using their gills which are in direct contact with the surrounding water and are easy prey for parasites and bacterial infections. The total surface area of the gills is approximately 10 times the surface area of the whole body. Gills are also important in ion exchange (maintaining the acid-base balance) and waste elimination, such as ammonia. Thus, fish basically urinate via their gills as well as breathe through them. To obtain oxygen, water is drawn into the mouth cavity and then the mouth is closed to force water out through the two opercula. This pumping movement creates a unidirectional flow of water, unlike the inhaling and exhaling through the same orifice in terrestrial mammals. Some fish, such as sharks, can keep their mouth open while swimming, which apparently provides enough flow of water over the gills to breathe normally. If your tanks allow it, you can try to measure the heart frequency of your fish indirectly by counting the opercular frequency – the times that the opercula open and close during one minute. This measurement can be used as an indirect indicator of animal welfare since stressed fish have high opercular frequencies.
Most fish have four gill arches on each side of their body (Figure 2). Each arch consists of a white bony rod which runs from top to bottom (ventral-dorsal) from which stem the V-shaped primary filaments in a caudal direction. The primary filaments or primary lamellae are red since they are full of blood. Each primary lamella has secondary lamellae which cross it perpendicularly and carry individual blood cells to facilitate gas exchange (release CO2 and capture O2 using the haemoglobin in the red blood cells). The flow of the blood runs against the flow of water, which increases its efficiency. In addition, fish can open or close the set of primary filaments to expose more secondary lamellae to the water, effectively taking deeper breaths. After filling up with oxygen the blood cells continue to move through the body via arteries.
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