It has been said that to become a successful koikeeper, one must learn to think like a koi. It might be easier to learn to think like a koi if one knew exactly how a koi is put together. This article proposes to illustrate "what goes where" (without the need for you to perform necropsies), and to standardize communication with correct scientific terminology.
Cyprinus carpio, a large, soft-finned freshwater fish has, through centuries of cross-breeding and in-breeding of mutations, become our beloved koi. Koi have an elegant simplicity of shape and healthy ones are fitted out with a physical structure and organs that are admirably suited to their water way of life.
The length of a koi, as used by a hobbyist, is the measurement from the tip of the nose to the end of the tail - the total length. Length is the most commonly used measurement: there are expected increments at certain ages; it is a factor in pricing and purchasing, and it determines size categories at shows.
Koi have been bred to have less body depth than natural carp. Depth is not as noticeable as length because of the swimming position. It is not critical as long as the fish is generally in proportion, although females usually have more depth than males, especially when laden with roe.
Slicing through the body vertically, there are three basic divisions: the head region, trunk region, and caudal region. Horizontally, the upper portion is the dorsal area, and below the lateral line, the body is called the ventral area. The lateral line is present in all fish. It is difficult to see it on a fully scaled fish, but is quite evident on a kawa (leather) koi. The lateral line is a richly endowed receptor of energy waves created in water medium by sound and motion. Koi quickly discern the frequencies produced by their master's voice (or any sound used to call them to food). The lateral line is very sensitive, even to minor variations, and loud noises institute immediate protective/evasive action. Although koi have no ears per se their "hearing" is exceptionally keen. They have a wider range (especially in the lower range) than most fish.
The pectoral and pelvic fins of the koi occur in symmetrical pairs: the dorsal, anal, and caudal fins are single. All the fins are divided by rays, the fine supporting segments that appear as pattern. The pectoral fins are multipurpose fins, used for retrograde swimming, balance and position: they revolve in a wide arc and enable the fish to turn with agility. The leading spine has a tendency to be more highly developed in the male than the female, the heavier. straighter support helping to form a more pointed fin in the male than is usual in the female. Both the pectorals and the dorsal fin are used in quick stops. The muscles ending in the caudal peduncle provide most of the propellant power: the mobility and maneuverability of the caudal fin helps provide control of direction.
The head of a koi distinguishes it from any other fish: the differences may he minimal, but they add up to a distinctive shape. The mouth of a koi is one identifying feature: the position of the pre- maxilla and maxilla is one of the features that makes a koi head easy to recognize. Most koi have one noticeable pair of barbels with a second, vestigal pair directly above. However, they may have one, two or three pairs of well- developed barbs. The barbels are extremely sensitive sensory organs, both to touch and to chemicals.
There is an appeal in koi eyes that goes far beyond the physical description of them. They are placed relatively low on the fish's head, which facilitates bottom finding. As in human eyes, only a small portion of them is visible from the outside. Usually small and black, but with a fair amount of mobility of the ball within a non-closing circular lid, a smart koi makes, and holds, eye contact with human beings (which makes willing slaves out of most of us!)
The operculum is commonly referred to as the "gill plate". It is an expansile cover to accommodate the function of the gills, but should adhere closely to the head when at rest position.
The anus is the excretory opening. In adult koi. a slight variation in shape is considered in determining sex: the female tends to be round, and the male to an elongated triangle.
Scales, whether they are the small regular ones or the generally larger Doitsu, are important in koi. They may cover the entire body, or as in Doitsu, certain portions, or be scaleless. Doitsu may have scales over the entire dorsal area (Yoroi or Armor), have parallel rows at the base of the dorsal fin and along the lateral line (Kagami or Mirror), or none (Kawa or leather).
Much of the beauty of these fish results from the uniformity of scale growth pattern interacting with the scale's pigment content. The cells which contain color pigment are called chromatophores. Three kinds of chromatophores are responsible for the colors of koi:
Shiny, metallic koi have iridocytes, cells which contain crystalline guanin, a reflective substance which usually appears silver. The crystalline guanin may be distributed evenly in both the epidennal layer and the scales, or iridocytes may concentrate in a particular area.
The total exterior of koi is covered by a protective mucal coating, slippery to the hand when healthy and intact. Although not visible to the eye when the fish is in water, it performs a germicidal function, and is a necessary feature of the outside of koi.