Koi are blessed with a multi-layered external barrier consisting of both skin and scales that protects them from the perils of pond life as well as gracing them with a magnificent livery. We can be guilty of regarding scales a the koi’s primary means of protection, yet it is their skin that provides koi with their first line of protection as well as their colour. In fact, even though koi possess scales, their beauty quite literally only remains skin deep.
Koi Skin – Protection and Pigmentation
Believe it or not, a koi’s skin actually overlays its scales. This clearly shows how thin and delicate the skin is as when we view koi, it is their scales that actually appear to form the final outer barrier of their body. The very delicate skin actually envelopes each scale in a pocket of skin, giving each scale protection.
One of the most important protective functions of the skin is to continually secrete mucus onto the external surface of the skin. Mucus is a very simple protein and is secreted to provide koi with a ‘buffer zone’ and a layer of primary protection against external attack. Any micro-organism that wants to break through a koi’s protective barrier, must first penetrate the sticky mucus layer, that is being continually sloughed off.
If koi are irritated by their environment through extremes in water quality or as a result of chronic parasitic infestations, koi then increase mucus secretion as a means of increasing their level of defence. In fact a dulling of the colours, where reds appear orange and blacks grey is the likely result of excess mucus secretion and should be a warning sign that the koi is under stress.
The skin is also responsible for colouration, being the site where the colour cells (chromatophores) are located. Again, perhaps contrary to common belief, the scales themselves are not coloured, but adopt the colour of skin that covers them. As we can all testify (some of us to our dismay), the skin can change over time, causing patterns and depth of colour to change. This occurs largely as a result of inbuilt or genetic factors.
The skin consists of an outer epidermis that is made up of an outer layer of protective cells, interspersed with mucus-secreting cells. Below the outer epidermis is the deeper dermis, an area of the skin where the colour-producing chromatophores are located, as well as stronger connective tissues and blood capillaries. It is these capillaries that dilate and become visible against white skin (making a white koi look brown or pinkish) during periods of stress.
Skin also has a role to play in producing different colours from similarly coloured chromatophores. Those melanophores (black colour cells) that are located closer to the epidermis produce a black colouration while those embedded deeper yield a blue skin colour as seen in Shusui.
Care of the Skin: It is imperative for the health of our koi that the skin and mucus layers are kept intact. Netting and handling of Koi should be kept to a minimum as such episodes increase the risk of damage to their front line of protection. If it is necessary to handle koi, they should be netted with soft hand nets and lifted with wet hands.
The protective mucus layer is the first line of attack for pathogenic organisms, and the extent of pathogenic organisms can be determined by taking a skin scrape (or more accurately, a mucus scrape). Taking a glass microscope slide, it should be pulled gently down either dorsal flank, and the mucus viewed under a relatively powerful light microscope. When taking a mucus sample, it is of course impossible not to remove any protective mucus, so sampling should be kept to a minimum, bearing in mind the additional risks that koi can face as a result.
We should also be aware that should we have to remove a scale (damaged or partially dislodged) we also remove the overlying protective layer of skin. Interestingly, even though the scale will never be replaced, the repaired tissue will take on the original pigmentation of the affected area of skin.
Scales provide koi with a flexible armour, and a third line of defence against physical attack. The number of scales on a koi are fixed at the start of life, but grow in size to keep koi protected as they increase in volume. They overlap like tiles on a roof, offering a compromise between a complete armour and flexibility.
The seasonality experienced in our climate means that there are periods of rapid growth, followed by periods of negligible growth. This stepped pattern of growth is recorded in each scale, actually making it possible for koi to be aged by reading the number of bands on a scale.
Anyone who has seen a scale that a koi has lost will soon appreciate that once the skin has been removed, they are transparent and offer no colour to the koi. Scale types do vary between Doitsu and fully scaled koi, with the former German-descendants exhibiting relatively heavy mirror-type scales, with the majority of the koi’s body being free from scales.
As with the majority of external physical features exhibited by koi, the Doitsu scale pattern occurs through selective breeding and is due to the fact that their German ancestors were bred to be scale-less, making them easier to be prepared for the table.
If you ever need to remove a loose scale, do so with a clean pair of tweezers, and a sharp movement, treating the localised are with a topical anti-bacterial treatment.
General care of skin and scales
1. Skin and scales are made up of a combination of protein and minerals which must be made available to koi through their diet and pond environment. Koi can absorb minerals from the water as well as their diet so ensure that their pond water is high in calcium and magnesium salts (ie. it is hard).
2. The condition of the skin can be greatly improved by the regular addition of clay, acting as a mineral supplement. This replenishes ‘tired’ pond water with an abundance of minerals, while ‘polishing’ the water, improving its overall clarity.
3. Chromatophones in the skin that give a koi its colour can only exhibit carotenoids if they are made available in the diet. If koi are offered a carotenoid-deficient diet, then their colours are likely to fade. Top their diet up with carotenoids to keep their colour topped up. (Look out for ingredients such as spirulina, marigolds, astaxanthin and canthaxanthin).
In summary, the koi’s skin and scales perform both functional and glamorous roles of protection and colouration. They must be protected to keep koi from succumbing to disease, and must be given the correct dietary and environmental conditions for them to grow and thrive. The skin and colour quality of a koi can be used as a health indicator, and can even be enhanced, possibly increasing the value of a fish.g the value of a fish.