When looking at 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.
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 in the resting 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, where as the male tends to be 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 even be scaleless. Doitsu may have scales over the entire dorsal area (Yoroi or Armor), or have ta parallel row either side of the body from the base of the dorsal fin and along the lateral line (Kagami or Mirror), or even no scales at all (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:
Melanophores: contain the black pigment melanin;
Xyanthophores: contain yellow pigment granules.
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. To notible identify a metalic koi the name will be shown with Gin. For example Gin Matsuba.
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. The mucus is the first barrier of defence for the fish.
