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In Koi their mouths are not just for feeding

article by Dr Paula Reynolds Aquatic Patho-biologist

Koi keepers have been known to feel a little heartless in passing the pond and not feeding their Koi open-mouthed in anticipation. We would accomplish nothing all day if we gave in to demands for food and it would require a bottomless tub of pellets .None the less Koi are not always requesting food they are sometimes seeking attention. There is no one to blame but the Koi-keeper for this behaviour, as it has been well learned. Koi cannot wink, or smile let alone talk to their owners but their mouths actually speak volumes.


Too wide


Koi are often observed to open their mouths extremely wide in order to be able to take in several pellets or when a tasty and nutritious insect lands on the pond surface and they want to be sure a leg or wing does not escape. In most cases, Koi are aware if they have over extended the capacity of their mouths and will open and close their lips a few times to relieve the condition. Very occasionally, the effort proves too much and a fish is unable to close its mouth at all. The hobbyist should intervene immediately and attempt to reposition the jaw as soon as the problem is spotted, as delay will make the condition worse. It is more satisfactory to sedate the fish and manipulate the jaw on both sides gently attempting to create the normal scissor action.


In large Koi, placing the thumbs inside the mouth against the cheeks on both sides and making the mouth work in the normal way can free the jaw again although it may be necessary repeat the procedure. Once the inflammation and distension subsides, the mouth will eventually be able to function normally as long as the jaw is not dislocated or broken. A small gap between the lips making the Koi appear to smile for a few days is normal as the tissues recover. In most Koi, the distress of being unable to feed is obvious but fish vary so much in their response to adversity that even in cases of severe mouth inflammation or dislocation some Koi still attempt to eat and a few even succeed.


Well Bred Koi  


Ossification is the process that forms the cartilaginous material in Koi that we term bone during the formative phase of life. How strong this material is in later life depends on the parentage of the fish and the diet of the Koi when fry and juvenile fish. A diet too rich in fats and lacking in certain vitamins and minerals can create weaknesses. As the Koi grows, the body may fail to develop normally, the anatomy may be unable to cope with periods of rapid growth or the changes that take place at maturation. Malformations may not have been spotted during culling and some abnormalities appear later in life. Weaknesses in the jaw and mouth distortion are rare possibilities and extremely unlikely in Koi that have not been intensively reared and have a strong genetic history.




Cyprinids do not possess dentition in the mouth but instead have up to three rows of paired pharyngeal teeth, so termed because they are located in the pharynx or throat. The number, size, shape, and arrangement of pharyngeal teeth vary with species and can be used for identification when externally a fish proves hard to classify possibly due to hybridisation. These type of teeth grind against a thick masticating plate sometimes called the Carp stone, located on the roof of the pharynx. A muscular pad pushes food into the pharynx and against the teeth. Koi do not always swallow while taking in food they store the pellets or the prawns and find a quiet spot in order to process the meal via the   Carp stone before actually digesting it. This accounts for vast quantities of food Koi take in at a time.


The nostrils


The olfactory organs responsible for detecting smells are found all over the head and the front of the body, but are far denser on the barbels and in the nostrils or nares. There are two paired nostrils located on either side of what is commonly called the snout. These function as an inlet and outlet system as the water passes through them during swimming and diffusion the process equivalent to breathing. The nostrils perform a vital role, as an acute sense of smell is required for survival in the wild. Even in the sheltered environment of the Koi pond, danger lurks. For example, the feet of Heron release a substance into water that Koi find attractive and they are tempted to swim towards the source of the smell. Unfortunately, many Koi become an easy meal for this well-known pond predator.


New aromas


Koi can refuse to eat a new food because of a failure to recognise its smell. The  receptor cells work defensively protect the fish from  an alien food that they perceive may harm them , when in reality it may be the best Koi food on the market . These cells are located in a special type of skin that lines the nasal cavities behind the nostrils. The receptor cells are seated in many folds and fingers that form a network of sensory tissue. Olfactory tracts are then created from the base of both receptor and supporter cells and ultimately this links to the telecephlon. This is located in the brain for the purpose of processing sensory messages. It is amazing with all that is going on in the nostrils that it only takes seconds for Koi to assess the safety of a food or anything else. In comparative experiments Koi and Goldfish, both appear to have at least a 50 times better sense of smell than humans.


Bacterial disease


There are several species of bacteria frequently isolated from lesions that can affect the mouth and lips. Some infections are localised to the mouth area alone and will respond to topical applications of antiseptic products or bactericidal products in the water and most clear up without becoming serious. None the less, disfigurement of the mouth and jaw can happen if the infection is not spotted soon enough, as the lower jaw is not always visible and an infection can penetrate beyond the skin to the jaw itself. In other cases, the mouth can be the site of an infection that later spreads internally, and the fish then requires a course of antibiotics. The quality of life for any Koi is more important than how long it lives. A Koi with a serious mouth distortion or damage that is unable to eat is worth seeking help for in case treatment is possible. If the Koi is suffering and nothing can be, done euthanasia is the kindest option. The humane way to carry this out is to sedate the fish and once unconscious leave it in the sedative solution for an hour.                                                                                  


The perils of Paula


In my own defence, I have many thousands of Koi to study and care for in my facilities and my colleagues and I dare not watch them all day or research and laboratory work would suffer. None the less, one of our Kumonryu named Glo had a narrow escape. Being miffed that her breakfast was long digested and her lunch seemed to have been delayed Glo had to assuage her terrible hunger pangs somehow. Spotting what she took to be a tasty morsel on the pond floor, she bit off far more than she could chew. When it was eventually feeding time, it was noticed that Glo was in trouble and unable to close her mouth. We netted her immediately and could not believe that at the back of her mouth and highly inaccessible was a very large stone.


Mission impossible


The level of inflammation suggested the stone had been there for some time. When and how she came across a stone in the particular pond she lived in was a mystery for later investigation. We sedated Glo and struggled, as it was impossible to get tweezers or anything more robust on either side of the stone which was stuck fast. Next, we applied gentle pressure trying desperately not to damage her jawbones or any other part of her anatomy so tricky was the procedure. There was a sudden collective gasp as the stone moved barely perceptibly, unfortunately, this minute shift was in the wrong direction towards her pharynx. Once there it would be beyond the point of retrieval without surgical intervention and its risks, which we knew Glo would not survive.


If at first you do not succeed -----


It was time to forget her delicate anatomy and try brute force. Although time was against us, we were able to prevent dehydration and keep the gills supplied with oxygen throughout most of the procedure. Glo was subjected to the indignity of having her mouth well lubricated with glycerine and being held upside down although firmly supported so that gravity was on our side even if we had to work at a very acute angle. We made slow progress in easing the stone towards her lips, as it was neither smooth nor round in shape and its projections required tiny manipulations in all directions to make any headway at all.


Try again


Once the stone was actually at her lips, we thought we were home and dry. However, we soon realised that Glo being both a koi and a female meant that the old adage “What goes in must come out” might not be applicable. It was obvious the stone had to be turned so the narrowest aspect was visible at the lips so we could use a clamp to remove it. This meant the widest part would still be inside her mouth. I had the choice to make an incision either side of the mouth and release the stone quickly knowing suturing would be needed or take a chance with the clamp. Glo, being anaesthetised, did not “perceive the vibration “of the cheer that went up in the laboratory as that stone emerged. Whilst her lips were very inflamed, they were intact. It had taken the lab staff and I most of the afternoon, we were behind with our work but Glo would survive


Glo recovers


We treated her mouth and lips with antiseptic for the abrasions and put her in a large hospital tank so we could monitor progress for a few days. There was a mild bactericidal product in the water and a high level of dissolved oxygen. Having been sedated through at least the latter aspects of her trauma she was not overly stressed which was fortunate. Her mouth was  much distended but we kept her amused by hand feeding her on worms and natural foods, which we placed at the back of her mouth so that she did get some  nourishment , after all hunger  had seemingly  played a major role in her condition . It was over two weeks before she condescended to forgo the personal nursing and attention and go back to her own pond. 


The mystery


Glo was so named because she lit up the pond she lived in for 15 years. It is an indoor facility used for Koi food trials. We have to keep the Koi warm and they are used to the luxury of a very varied diet. There has never been any use for stones in this pond as may have been the case had it contained pond plants. We assume the stone found its way in via the vent in the roof of the building. We wondered if the reason Glo took such an interest in the stone and subjected her lips and mouth to even processing it at all could be due to the fact she had not come across such an object since she was a humble fry in a Japanese Koi farm. If this is the case, it is likely that Koi in ponds in which there are stones are less likely to experiment with them. We have many ponds and some are planted, yet none of the Koi that live with stones has been daft enough to get so intimately involved with one but then Glo is female.


And finally


It is ironic that one of the trickiest removals of any foreign body   had to involve one of my own fish. Having removed countless objects from the gills, nostrils, and mouths of many fish species over the years, stones, and large frogs have proved the most troublesome. It is so nice to tell a hobbyist that the gill disease they suspected is no more than a few frogs’ legs clinging to the gill filaments as the throat of their Koi contained a large frog. It is rare for Koi to be severely damaged by frog encounters as long as the problem is spotted in time. Frogs that ride on the head of a Koi can prevent the gills functioning but this a seasonal activity and not a common problem. Koi can get their mouths in some tight situations and their owners may need to be resourceful on occasions and act fast. The mouth is a means of communication between  Koi and  Koi-keeper rarely it says help me ,  most of the  time it says feed me,  and occasionally  it says watch me and marvel at my beauty and my antics – and we  all do .

The function of the mouth


In Koi, the function of the mouth is not merely for feeding. They use their mouths to complete many tasks that in humans require all the senses and we have the benefit of a pair of hands to hold and assess objects. Pressure receptors are located over the head, lips, and barbels so that Koi know when they are in contact with large or dangerous objects or even a predator. A network of highly sensitive cells is located on the lips and barbels and inside the mouth is suffused with special sensory cells. The sense of touch   being focused on the lips and in the mouth allows Koi to feel as well as taste the objects they pick up. Everything small and interesting is tested on the lips and passed into the mouth for full scrutiny in a process that takes a split second and anything inedible is quickly rejected. Although greed appears to be the motive for this behaviour, it is the instinctive feeding pattern of Carp to hunt continuously for their food. The only slight pause is when there is a delivery of pellets to the pond.


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