The mucus layer in koi By Dr. Paula Reynolds What is mucus? It is easy to assume that all mucus is the same when it must differ to perform many important roles in all living things.  In fish, mucus has to protect and defend them in vastly different environments, some more hostile than others.  The composition of mucus has developed over time to shield fish from the numerous threats they are exposed to in rivers, lakes, seas and oceans. In comparison, the garden koi pond suggests greater safety when, in reality, captivity creates its own unique risks. Protect and defend In koi, their mucus layer protects them by forming a barrier over the whole body including the fins and this layer is contained in what is known as the cuticle. The goblet cells that secrete the mucus are located in the epidermis or skin. There are various biochemical and antimicrobial properties contained in mucus to defend koi from pathogens such as toxins, parasites, viruses, bacterial infections, and other diseases and conditions. The mucus layer is the first stage of the immune system and when it is missing for any reason the fish are vulnerable. The mucus layer in koi is important to keep them healthy and how well it functions depends on the conditions the fish live in and that requires a high standard of pond maintenance. The role of mucus When bacteria land on the skin of koi it is the role of various components in their mucus including lysozymes to work as a team to kill off the invading organisms and prevent the skin from being penetrated. The other important function of the mucus layer is to maintain separation between the internal condition of the fish and the external situation in the pond, particularly the water quality. Koi require continuous support and when pond water is high in nitrite koi are subjected to reactions inside the body as well as outside. A safe balance has to be achieved and that can put the immune system under pressure. Without their mucus intact, stability cannot be maintained indefinitely and, as the mucus is shed in response to the nitrite, secondary health issues can develop. Shedding mucus Shedding mucus is a defensive mechanism and it can occur in response to disease, irritation by parasites, skin damage due to toxins, and pollution due to poor water quality. Mucus can be seen leaving the body if koi are exposed to high levels of nitrite and the skin often has a soapy appearance. Mucus is occasionally shed and replaced naturally as the immune response ensures the composition of the mucus is adequate for its defensive role which is an automatic process does not put the fish at risk in the same way that chemical treatment can if the mucus layer is removed. Misunderstanding why koi may be exuding mucus can trigger pond treatments to remove it and investigating why mucus is leaving the body is always advisable before using chemicals. Using chemicals wisely The capacity of mucus to maintain koi well-being can be compromised by the use of pond treatment products. Occasionally fish have to be chemically treated although removal of their mucus layer is not necessary unless there is a specific parasite problem. When a treatment product has been ineffective against a stubborn parasite the mucus is thought by some to have been too thick to allow the product to work. The mucus is then removed and the parasite is treated.  Most chemical treatment products can penetrate the mucus to eliminate parasites. Anti parasite products fail to work for numerous reasons; mucus is rarely the issue and product failures should be discussed with the manufacturers.  Gyrodactylus or skin fluke is not a true parasite; it is a free-living organism and many koi keepers struggle to eliminate it. In some cases, it is a species of skin fluke that is highly resistant to certain chemicals that are now spreading into UK ponds that is the problem and, in other situations, it is the pond conditions that make chemicals less effective. Leaving the UV on during pond treatment is a common example. It is not knowing if there is a parasite infection and, if there is, what species it is that leads to many unnecessary chemical treatments. Taking mucus smears to assess what is happening on the skin is the best method of protecting koi from chemicals. Potassium permanganate Potassium permanganate is a generic material historically used to treat fish although never fully researched. My laboratory undertook an in-depth study many years ago of how safe and effective it is for koi use. Potassium permanganate removes the mucus layer in koi and one of its side effects is that the rate at which the mucus regenerates afterwards is longer than it is with other substances that also remove mucus. We carried out analysis on the mucus as it returned to the body after potassium permanganate use and found that many vital elements in its composition are missing. This means it can take time for the koi to be fully protected. Our research also demonstrated that, without their mucus layer, koi are predisposed to health problems, particularly certain forms of bacterial disease. I do not recommend using potassium permanganate when fish have bacterial disease and, as a parasite treatment, its use should be limited. Whilst we have passed on our findings to others who are also looking into new and safer products, those currently selling potassium permanganate are unlikely to be aware of this information. Comparing koi with wild fish I began this article with a reference to fish living in the wild and, if we look at their lives compared to that of koi, the one obvious difference is that they are never chemically treated. Fish in the wild rely on natural cleaning mechanisms to keep parasites under control. If at risk from toxins or other threats, they might have the opportunity to swim away from whatever adversities they encounter. Whilst confined in a limited volume of water koi can still self-clean to a degree when the problem is minor. It is when the behaviour or appearance of koi changes that an investigation is needed so that the problem is diagnosed before pond treatment. Where do health problems come from? It is a fact that if koi are properly quarantined and after the sale they live in optimum conditions in a well-managed pond, they should never need chemical treatment. It is a fact that some of the microscopic parasites and free-living organisms that are found in koi mucus are introduced to ponds on new koi despite all previous chemical treatments. Health problems develop in ponds for numerous reasons and the introduction of new fish when the existing koi community have lived together for years can result in disease. Koi with unknown origins and no record of their medical history are a bigger risk as they can carry more serious diseases. Fish lice and fish leeches are parasites that can be introduced by birds and ducks that live in unclean water are common culprits. My aim New koi introductions and filter hygiene issues are factors in outbreaks of ulcerative forms of bacterial disease. However, some outbreaks are also preceded by mucus removal and others by the overuse of chemicals. Higher dose levels of pond treatment products than those recommended are not advisable as the manufacturer's dose levels should always be adhered to. Fish farms and koi dealers will use more chemicals in preparing koi for sale although this exposure is usually short term. Occasionally some ponds have to be treated for parasites when pond management slips below the ideal. Having studied fish at a scientific level, my advice is that treating koi as little as possible and never removing their mucus unless it is essential is the way to support koi well-being. If the mucus has to be removed, the pond conditions should be kept pristine so the koi are less at risk until their mucus returns. The purpose of mucus is a biological fact with no counter argument, after all why did nature provide koi with an amazing defence system if it is an acceptable practice to weaken or remove it?