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Koi anatomy

Fin and scale regeneration


Most koi keepers will know that scale loss can and will occur and fins will get damage during the course of koi keeping. The majority of reasons have to do with injury or parasites and the resulting bacterial infections that sometimes follow. The question is, will they grow back?


Scales can be lost because of injury caused by protruding objects in the pond, feeding frenzies, flashing while trying to get rid of parasites, spawning, netting, jumping out of the pond, parasites, bacterial infections and many more reasons depending on the specific pond.

The question about regeneration or re-growth of scales has been posed to me many times. The normal reply is that it all depends on the specific injury or lesion. To fully understand the re-growth of scales, one must unfortunately get a bit technical.

In fully scaled fish, scales make up a significant part of where up to 20% of the total body calcium may reside. Fish scales are dermal bone plates that form armor for protection. Scales also provide the fish with an important and readily exchangeable calcium and phosphate buffer. Scalar calcium may, for instance, be recruited during periods of increased calcium demand such as when environmental sources for calcium are low (e.g. in very soft water).

In the emptied scale pocket that results from removing a scale, esenchymal progenitor cells, called scale pocket lining cells, reorganize to rapidly form a same-sized, new scale. This takes one to three weeks, sometimes longer. It all depends on the ambient temperature of the fish, water parameters and quality of feeding. Please note that the above is the ideal situation when a scale is plucked from the skin, with no complications. When a bacterial infection or more severe injury destroys the whole scale pocket or the scale pocket lining cells, scale re-growth will not occur.

A scale that is replaced by this process may be undistinguishable from the rest of the scales or it may be easily noticeable to the trained eye. The reason for this is the fact that the whole scale is replaced in a relative short time, while the rest of the original scales have went through a long process of growth with the fish. During the lifetime of a fish, the growth of the original scales could have been slow during winter or fast during the growth season. Disease, egg formation etc. may also have contributed to periods of slow scale growth. The thickness, colour intensity and growth ring formation can therefore in all probability differ from the new re-generated scales. After the initial re-growth of the scale it will add rings at the same rate as the other scales.  Because of the overlapping nature of koi scales, the loss of a single scale will not have a serious effect on the aesthetics of the fish. The loss of several scales in close proximity to each other may however have a detrimental effect.

The following photo clearly demonstrates a substantial ulcer. When one inspects the lesion closely, it is clear that the infection has damaged the scales, scale pockets and the epidermis to varying degrees. It is a pitty that one could not follow this case up to determine the degree of regeneration that will take place.

12 August 2010

Damaged fins

Fins can be damaged for the same reasons as those that were mentioned for the loss of scales. Fins can also be nipped by predators, eaten away by bacteria and sometimes partly amputated by a koi owner while treating bacterial infections. When a fin is amputated for whatever reason, the stump is covered by a specialized wound epidermis within one day (it depends on temperature) to protect the fish from infections and also to protect the blastema. Beneath this wound epidermis mesenchymal progenitor cells start to accumulate to start the regeneration process of the fin. It is uncertain through what process, but the guess is that it happens through a differentiation process. These cells will make up the blastema. Connective tissue cells, the main cell population of the fin, are excellent candidates for blastema formation. It has also been shown that intra-ray cells located up to seven segments away from the wound can migrate along the ray and contribute to the blastema. Genes also play a role in the re-generation of fins and it is believed that most genes for bone-regulatory and bone matrix are well conserved among vertebrates.

So, in short fins can grow back but the extent of the damage to the fin will determine how successful the re-growth will be, because fin regeneration depended on the presence of musculature and endoskeleton at the site of amputation. Regeneration will occur where fin tissue has been damaged. But where the fin rays have been damaged to a point close to or up to the muscle tissue, regeneration will not be possible. There is also evidence that caudal fin regeneration happens more readily than in other fins like the dorsal fin.

The first photo demonstrate the original damaged fin on 22 June 2010 and the subsequent photo of the same fish taken on 29 August 2010, shows the progress with regeneration during the two months at temperatures around 20 degrees Celsius. Subsequent photos demonstrate the improvement in rising temperature three weeks and then 5 weeks later.

finrot2 progress_1
Photo: Jack Birkenbach, 22 June 2010 Photo: Jack Birkenbach, 29 Aug 2010

progress_2_17092010 progress_3_27102010
Photo: Jack Birkenbach, 17 Sept 2010                                                                         Photo: Jack Birkenbach, 27 October 2010

Regenerated fins may show some scaring. It all depends on the type of injury and the base colour of the fin. It is also very interesting to note that a genetically deformed fin cannot be rectified by partial amputation. The fin will grow back similar to the original fin.

The following is an attempt to demonstrate the safe areas to amputate a fin if necessary. The green lines on the photo represent a rough guide for safe amputation. Any damage or amputation closer to the skin and muscle represents the “danger zone” where fins will in all probability, not regenerate.



The next photo shows a Tosai that lost a substantial part of the bottom rays of the caudal fin in June 2010. After two months one can clearly see the progress of regeneration at the average winter temperature of 20 degrees Celsius. The pink dots indicate the edge of the original lesion. Attempts will be made to place more photos over time of the progress. The red veins in the caudal fin indicate the stress experienced by the fish during the catching and sedating process.


16 August 2010



Same tailfin as on 10 February 2011



28 August 2011


The caudal fin has completely regenerated.


Last Updated on Sunday, 28 August 2011 17:37