The function and interaction of pond fish gills with the pond keeper

This is the first article in a series of 6 that looks at various aspects of a koi’s physiology and how our interaction with koi (at almost any level) can have an impact (either negative or positive) on the overall health of our koi. Each article will look at an aspect of a koi’s physiology, how it works, its structure and function so that we can gain a fuller understanding of how the way we interact with our koi can affect their health.

The series starts with a study of the gills, arguably one of the most multi-functional organs in a koi’s body and with a fascinating structure that we must fully appreciate to provide their preferred working and environmental requirements.

The Gills

The gills are a koi’s respiratory equivalent to lungs, although they have many other non-respiratory functions such as the uptake and release of dissolved compounds. In serving their primary role of gas exchange (taking in oxygen and releasing carbon dioxide) they are perfectly designed to perform their life-saving role with great efficiency.

If you have ever had the opportunity to examine gills, you will have noticed a number of features that make them ideally suited to their role.

1. A highly delicate structure that is divided into hundreds of very fine filaments providing koi with an exceptionally large surface area.
2. A blood-red colour indicating that there is a rich blood supply and an ultra-thin membrane between the blood and the pond water.

The large surface area and thin membrane makes gills very efficient at absorbing the meagre amount of oxygen that is dissolved in the water. For comparison, air has 200ml of oxygen per litre where as water will have only 10ml per litre of well oxygenated pond water. Koi are able to absorb up to 80% of the oxygen that passes over their gills dissolved in the water, compared to an almost derisory 14% absorption in our lungs. Furthermore, the rapid flow of water across the gills means that fish have to be very efficient at absorbing oxygen from the water, particularly as it only passes over them a single time.

Gills absorb oxygen and release carbon dioxide (as well as other waste products) through the process of diffusion. Diffusion is a passive process (not requiring any energy input from koi) and occurs as a result of the random movement of molecules. Over time, these molecules will move from an area of high concentration to a low concentration courtesy of such random movement. The greater the difference in concentration gradient, the faster the rate of diffusion.

What does this mean for the koi keeper? Put simply, the more oxygen we can dissolve in the pond water, the easier it is for koi to absorb it into their blood. It works in a similar way with carbon dioxide leaving the blood. Because there is a higher concentration of carbon dioxide in the blood than the pond water, carbon dioxide passes easily from the blood back into the water by diffusion.

What about other products dissolved in the blood?

Greater than 90% of the ammonia that fish excrete is released through the gills. As blood in the gills is acidic, it will be in the form of ammonium ions, leaving the blood and travelling across the ultra-thin gill membrane from the area of high ammonia concentration in the blood to the low concentration in the water.

Koi excrete ammonia because it is toxic and rely on diffusion for its removal. However, if, say a pond is experiencing a high ammonia peak through new pond syndrome or overfeeding then the diffusion gradient between the blood and the pond water will no longer exist. This will prevent the ammonia in the blood from travelling from the bloodstream into the water causing a build up of toxic ammonia in the blood. In very extreme incidences of ammonia build up in pond water it is actually conceivable that the ammonia concentration in the pond may be greater than the blood, leading to a net movement of ammonia back into the koi – Something that is completely undesirable and likely to threaten the life of a koi.

Koi, being slow moving fish, irrigate their gills by using a combination of mouth and opercula movements (called a branchial pump) which forces water into the mouth and out of the vent behind the operculum (rather like a pair of bellow operating in reverse). Sometimes it may be possible to catch a glimpse of the colour of the gills. They should be blood-red with a very clean, virtually transparent gill membrane. Gill colour can be used to determine the overall condition of a koi, causing concern if they are not a vibrant cherry-red.

Brown Gills.

Brown gills (which really reflects the fact that the blood is brown) are an indication of a blood problem can be caused by a number of factors with the most likely cause being nitrite intoxication.

Besides exchanging solutes (such as oxygen, carbon dioxide and ammonia) passively across the gills by diffusion, the gill membrane is also equipped with specialised cells that actively select particular ions dissolved in the water, transporting them against the concentration gradient back into the blood (ie from an area of low concentration to an area of high concentration). Gills are equipped with specialised columnar cells that select specific ions, particularly chloride ions and ‘grab’ them from the water, depositing them into the blood. This process is carried out to maintain a stable concentration of tissue fluid within the koi that would otherwise become diluted. Unfortunately, this process can be a little unselective in the ions that they select from the water to be deposited into the blood. For example, if a pond has a significant level of nitrite (NO2-) then the specialised columnar cells on the gill membrane can inadvertently transport NO2- into the blood instead of the desired chloride ions (Cl-). This results in the NO2- combining with the haemoglobin in the blood to form methaemoglobin, a stable compound that cannot carry oxygen. Methaemoglobin is a brown compound, giving the blood its brown colouration.

What does this mean for us?
It has long been acknowledged that dosing a pond with salt (NaCl) can reduce the impact of nitrite on koi. This procedure works by increasing the concentration of chloride ions in the water, increasing the probability that the columnar cells will select chloride ions over nitrite ions, thereby reducing the concentration of nitrite in the blood. (See Figure 1)

Gills and disease.

If you put yourself in the position of a parasite, where better to take up residence than in gill tissue. It offers a nutrient-rich blood supply that is easily accessible once the ultra-thin gill membrane has been pierced and the constant flow of oxygenated water through the gill chamber (which is protected by the bony opercula) provides further enhancement to this choice of location.

The gill fluke, Gyrodactylus, and the gill maggot, Ergasilus, are both keen residents in the gill chambers, attaching tightly to the gill tissue. They cause koi sever irritation, often evidenced by the typical flicking and flashing behaviour and their site of attachment can also be the focus of subsequent bacterial and fungal attack.

Gill irritation leading to gill disease.

The long term exposure to even a low background concentration of ammonia or the acute exposure to the gill irritant chlorine during a water change can start a chain reaction that leads to a deterioration in gill tissue and function.

Irritation of the gills can cause the gill tissue to secrete excess levels of protective mucus, or in prolonged instances, cause gill tissue itself to become inflamed. This ‘thickening’ of the ultra-thin and highly porous membrane can soon lead to the dysfunctioning of the gills. Consequently, fish will exhibit behaviour that is typical of low DO, such as gasping and rapid gill movement. Furthermore, the inflamed gill tissue or excess mucus will lead to an increase in the likelihood of bacterial infection, leading to additional respiration problems. Consequently, every effort should be made to keep ammonia and chlorine levels at zero, otherwise a chain reaction with serious koi health problems may ensue.

Gills should not simply be regarded as organs with which koi gain their oxygen from the water. We should be focused on the needs of the gills and the water in which they operate if we want to maintain the gill tissue and consequently the condition of our koi in tip-top health.