Koi have an elegant yet simple shape and healthy ones are fitted out with a physical structure and organs that are admirably suited to their way of life in water. The body is made up from three parts: 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. Koi quickly discern the frequencies produced by various activities around the pond like their master's voice, familiar footsteps etc.
Fish have an incredible sensory system called the "lateral line". The lateral line is present in all fish. It is difficult to see it on a fully scaled fish. It is a series of scales going down each side from gill to tail, just about mid-body, that each have a "hole” in them. In doitsu koi, it looks like a seam running along the body. The lateral line is a receptor of energy waves created in water by sound and motion. Any movement in the water, no matter how minute, is detected by the fish via the lateral line. This is the famous fish "sonar”. 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. Fish are very easily stressed. It is necessary for them to be in a state of constant alert. Their whole lives are based on getting food, and watching out for predators. So if your koi are someplace where they are subjected to sudden and irregular loud noises, they'll be stressed out. And stress can kill koi.
Koi move throuhg waterby alternatively contracting and relaxing muscles on either side of their bodies. 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, causing a more pointed fin in the male than is usual in the female. The muscles in the caudal peduncle provide most of the propellant power and helps provide control of direction. The dorsal and anal fins keep the fish from rolling or yawing.
The head of a koi distinguishes it from other ornamental fish. The mouth of a koi is one identifying feature. Most koi have one noticeable pair of barbels with a second, vestigial 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. The nostrils are those little holes on their head in front of the eyes. (See illustration) Water flows through the holes past olfactory (smelling) cells. They have an excellent sense of smell, and can distinguish odors quite a ways from them. Smelling is actually their most important hunting sense.
The eyes are placed relatively low on the fish's head, which facilitates bottom feeding and almost 360 degree vision. Only a small portion of the eye is visible from the outside. This is usually small and black, but with a fair amount of mobility of the ball within a non-closing circular lid. For fish, koi have very good sight. The eyesight is however by human standard shortsighted. The vision is clear for plus minus two meters, blurred for the next 10 meters and beyond that a koi cannot see at all. Koi can distinguish colours but can better identify objects that are green or yellow. The iris cannot contract and dilate like those of land animals.
Koi possess organs of hearing but there are no external “ears”. The internal ear detects sound in association with the swim bladder. Sound causes the swim bladder to vibrate, and these vibrations are transmitted in turn to the otoliths of the inner ear. Auditory nerves are then stimulated and signals sent to the brain. The inner ear and otoliths are also important in maintaining equilibrium and balance.
The brain of a koi is relatively simple. Optic and other sensory nerves radiate from it to the head. The spinal cord protects the central nervous system which extends to all parts of the body. Fibers at the nerve endings both receive and transmit messages to the brain.
The operculum is commonly referred to as the "gill plate". It is a protective cover to accommodate the function of the gills, but should adhere closely to the head when at rest position. Under the gill covers, the gills themselves look like sheets of tissue with little ribs sticking out of them. The water the fish takes in passes over these filaments (the little ribs), which extracts the oxygen the fish needs. Extra water that is drawn through the gills goes to the kidney, and passes out as dilute urine. Another two very important functions of the gills are to excrete ammonia and carbon dioxide from the blood stream.
Koi have teeth located in the throat, which are used to crush food. They are not uniform in shape and are linked to the gills.
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. Much of the beauty of koi results from the uniformity of scale growth pattern interacting with the scale's pigment content. The cells which contain colour pigment are called chromataphores. Three kinds of chromataphores are responsible for the colours of koi:
• Erythrophores: contain red or orange pigment granules;
• Melanophores: contain the black pigment melanin;
• Xyanthophores: contain yellow pigment granules.
See also stability of colours
Shiny, metallic koi have irridocytes, cells which contain crystalline guanine, a reflective substance which usually appears silver. The crystalline guanine may be distributed evenly in both the epidermal layer and the scales, or irridocytes may concentrate in a particular area.
The total exterior of koi is covered by a protective mucus 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.
Fish have a very, very thin layer of skin that covers the scales. It’s so thin that you can't even feel it. What this skin does is secrete mucus, and it is this mucus that give fish that slimy feel. This mucus is not living tissue. It is constantly being washed off. It acts as the first line of defense against viruses, bacteria and parasites, because it contains antibodies that the fish creates. Behind this mucus barrier is the first layer of living tissue - the skin over the scales.
The scales come next and overlap like tiles on the roof of a house. This scaling provides the fish with a flexible, bony, layer of protection against their environment. Scales start to form when the fish is about a week old. If the fish loses one, it'll grow back. In a regularly scaled fish, only the front end of the scale is attached to the skin. In a doitsu, the whole scale is attached to the skin. The next layer of skin, as we continue in to the koi, is the dermis. This skin actually contains nerves, blood vessels, and the pigments which give the koi its color. The fish's scales are attached to this layer of skin
Koi are a part of a group of fish called "teleosts." It means "bony skeleton." These bones provide a protective framework for all of the fish's internal organs, and attachment points for its muscles.
A Koi’s circulatory system is not that different from ours. They have a heart that pumps blood (also with red and white blood cells and plasma) through arteries, then back, via the veins, to the heart.
The liver helps remove waste from the blood and controls the use of digested food. It also produces bile which is used in the digestion / absorption of fat. Located just below the liver, the Gall Bladder stores bile and releases it to help with digestion.
The spleen produces lymph cells (a yellow fluid consisting mostly of blood plasma and white blood cells) and stores red blood cells.
The urinary bladder plays an important part for the koi. As the salt content of the koi is higher then that of the water in which it lives the Koi’s body is continually taking in water which tries to equalize the salt concentration, this is known as osmosis. As a result of this process the koi must release the excess water, otherwise it would blow up like a balloon.
The anus is the excretory opening. In young koi a slight variation in shape is considered in determining sex. The female tends to be round, and the male normally has an elongated triangle. In mature koi the difference is more pronounced. The anal pore is located just forward of the anal fin. The waste products of the Koi’s digestive system are expelled via the anal pore. Water in the form of urine is also expelled via the anal pore.
The internal sex organs of the male are the testes and the ovaries of the female. In both the male and female they are located below the swim bladder. Eggs and sperm exit the body via the gonophores which are located just in front of the urinary opening.
The "swim bladder." is a two chambered structure located just beneath the spine and is filled with gas. The components of this gas are quite similar to air. The swim bladder consists of a cranial and a caudal lobe separated by a sphincter. The swim bladder is filled with air through a fine network of blood vessels that constantly deposits small aliquots of air into the caudal lobe of the swim bladder. What this organ does is provide buoyancy in the water, and allows the fish to stay at whatever depth it wants. In the absence of the gas bladder, fish are denser than water, causing them to sink. The other function of the swim bladder is to transmit sound to the inner ear - helping the koi hear. Like all fish, Koi have internal ears that contain bony otoliths. These rest on a bed of sensory hair cells, and vibrate when sound (travelling as particle displacement waves) passes through them. Because the gas bladder is filled with gas, it changes shape when sound (travelling as pressure waves) passes through it. Otophysan fish have exploited this by connecting the gas bladder to the inner ear via a series of bones called the Weberian apparatus. Because pressure waves travel further, this improves their ability to detect sound.
The digestive system of koi is more or less like that of any higher animals but differs from many as the koi doesn't have a stomach as such. Food enters via the mouth and is crudely crushed by the pharyngeal teeth (bony projections from the gill supports). From there it passed into esophagus and then into the intestines. The anterior part of the intestines is swollen and looks a lot like a stomach. The intestines are long and coiled, usually 4-5 times the length of the koi. This is due to the fact the vegetable matter eaten by the koi require more time within the body to be broken down so that the goodness is released. The intestines exit the body at the anal pore. That is one reason they should be frequently fed smaller amounts of food at a time. The koi takes in food, grinds it up (or not), and then it passes directly on to the gut. What is digested is absorbed into the bloodstream. What is not absorbed is simply passed out as feces.
Updated 03 Jan 2010