Southwest Koi and Pond Association




Koi Anatomy

By Phil Ishizu & Spike Cover

Last revised 3-23-02

Anatomy is defined as the morphologic structure of an organism. Morphology is defined

as the science concerned with the configuration or the structure of animals and plants.

This section is supplemental to the "Anatomy of Koi" section of Chapter One of

Advanced Koi Care, by Nicholas Saint-Erne, DVM. This book is scheduled to be

published in 2002 and is intended to be made available through the AKCA Bookstore.

KHA students should refer to the prepublication manuscript p. 36.

External Anatomy:

Form – Koi have what is known as a fusiform shaped body (tapering toward each end) as

seen in the diagram below from Living Jewels (reproduced with the kind permission of

the authors, Ronnie Watt and Servaas de Kock).

Fins – Koi have 3 single fins and two sets of paired fins.

The caudal or tail fin is primarily used for forward swimming especially fast swimming.

The dorsal (top) fin is used for stabilization during forward motion. The largest leading

ray becomes very stiff, sharp and thorn-like as the koi grows older.


The anal fin, like the dorsal, is used for stabilization. Also, the largest leading ray

becomes very stiff, sharp and thorn-like as the koi grows older.

The pectoral fins are paired and are used for numerous functions including: steering

during forward motion, slow swimming both forward and backward, breaking and to

counteract the jet effect of the water being forced out of the opercular openings.

The pelvic (or ventral) fins are also paired and serve to control pitch and roll and to

counter lift.

The fins are thin and well vascularized, which makes them vulnerable to damage. It is

also easy to see or detect changes and damage to the fins. Therefore, diseases are often

first detected in the fins, which can appear to be damaged, torn, or hemorrhaging.


The skin and scales are covered by the cuticle, a non-cellular mucus coating. More

commonly known as the slime coat, the cuticle of the koi is a thin layer of mucus that

contains many protective substances including antibodies, lysozyme (an enzyme that is

destructive to cell walls of certain bacteria), and C-reactive protein (a protein that may

have some antibacterial properties). The cuticle is the koi’s first line of defense against

water-borne irritants and parasites and it assists the skin with drag reduction for better



The cellular layers of the skin consist of an epidermis, dermis, and hypodermis. The

epidermis (outermost cellular layer) is very thin, usually only 6 to 8 cells thick and

contains unicellular mucous glands with a network of very small capillaries. The dermis

(middle layer) contains the scales, the scale-forming cells, pigment, blood vessels, and

nerves. The hypodermis is a vascularized fatty layer between the epidermis and the

muscle or bone beneath. It is the interface between the skin and the rest of the body.

The following illustration is of the skin of a swordtail (tropical livebearer) but with the

exception of the outline shape of the scales (which are shown later in this section), is very

similar to carp (koi) and is reproduced with the kind permission of Tetra Press from the

book, Aquariology, The Science of Fish Health Management – Master Volume, edited by

John Gratzek and Janice Matthews.


Scales – All koi scales are of the cycloid type and most koi have scales over most of their

bodies but lack scales on the head. These scaled koi are referred to as wagoi in Japanese.

Some koi have scales only along the dorsal line and the lateral line; these are called

German scaled koi (Doitsu, in Japanese). Some koi/carp are scale-less and are referred to

as leather koi/carp. Other koi/carp have a heavy scales appearing almost randomly and

are referred to as armored scaled koi/carp.

Scales are thin flexible plates with a layered structure that grow from the dermis. Note

that since the scales are produced in the dermal layer, removing a scale creates an ulcer,

or hole in the skin, which can be a potential area for pathogens to enter the body. The

scales grow essentially from the center outward. The actual origin of growth is in the

center of the scale and is seen in the scale below (courtesy of Mark Whalen): Note

Scales are normally flat, however the picture of the scale below had been off the fish and

dried for one to two years prior to scanning.


A detailed look at the growth rings as seen through a microscope appears like the

following at 100x (courtesy of Brett Rowley)


Note: the age of a koi cannot be determined by simply counting the rings on a scale (as

can be done with a tree from a cross sections of the trunk).

There is a tremendous about of overlapping in scales and there can be up to six or seven

layers in some spots (small dark areas). About 20% of most scales are exposed to the

exterior, i.e., the portion of the scale without overlapping scales, see large dark area of

illustration above (provided by Masaki Okamoto).

Note: On some koi, the dermis grows from beneath the scale

and is seen as ‘fukurin.’ Varieties such as ogon and asagi

typically display fukurin especially on the shoulder area. It

appears around the external edges of the scales.

On the main body of the fish shown on the right, the

platinum or white areas around the scales is the fukurin

while the darker or yellowish areas are the scales.


Koi have a protrusive mouth that is ideally suited for bottom

feeding. Koi are referred to as benthic feeders, since the

position of the mouth on the underside of the head allows

them to literally suck foods from the pond bottom.

However, they are able to feed at any level, bottom, midwater

and at the surface.


Koi have no teeth in their jaws and this is consistent with how koi in nature feed; i.e.,

suck everything up, taste it, and then decide if it is consumable or not. The Koi do

however, have teeth like structures (pharyngeal teeth) on the pharyngeal bones located

just behind the gill chambers. Food is ground by these teeth against a bony pad (carp

stone) located on the top of the pharynx, itself.

Sensory Organs


Koi do not have external ears but hear by an internal "ear" that is connected via a group

of bones, known as the Weberian ossicles to the cranial (forward) swim bladder. It is

believed that sound is amplified by the swim bladder. Koi, like other fish, are very

sensitive to sound, and can be stressed to the point of becoming ill when exposed to loud

noises on a constantly recurring basis.


Koi eyes are similar in structure to our own. However, unlike ours, they have bilaterally

placed eyes that are independently movable, which increases the range of area in their

visual field. They have cones and rods, the structures of the eye that see color and black

and white, respectively. They probably have good enough sight to see the shape of words

on a printed page. Since koi live in a water environment, they do not need protective

eyelids. This, however, makes their eyes more vulnerable to abrasion or other

mechanical trauma during netting and handling.


The lenses are spherical shaped and protrude through the pupil which allows considerable

peripheral vision, including forward and rear. In healthy fish, the lens is completely

clear. Bacterial and nutritional problems will sometimes cause the lens to become

cloudy. The pupils are under neurological control like mammals, but the response time is

too slow to be of clinical use.

Lateral Line

The lateral line runs along the side of the koi about midway down the side of the fish.

Holes in the scales lead to a canal beneath the surface that contains neuromast cells.

Water movement in any direction striking the sides of the fish will cause the mucous in

the canal to vibrate. These vibrations stimulate the neuromast cells that are linked to the

periphery nerve system and provide one the most effective perceptions for survival (flight


The lateral line is an important landmark. It is at approximately the same level as the

spine, which has a blood vessel that runs along the length of the spine just ventral

(underneath) to the spine and will be important for locating the blood vessel.


Olfaction (smell)

The olfactory organs (used to smell) are located at the base of the nostrils called nares.

Water does not flow to any other part of the body from the nares. They are exclusively

used for olfaction (smelling), are paired and located between the eyes and the mouth.

They are shaped like and can be thought of as small U-tubes into which water enters

through the leading or forward hole and exits through the rear port or opening. Just

behind the forward opening there is a flap of skin that directs water into that opening of

the nare as the koi moves forward in the water. The movement of substances through the

nares is aided by diffusion and by the motion of small hairs-like structures (cilia) within

the nares. The following illustration was slightly modified and reprinted with the kind

permission of Koi Carp magazine.


Taste buds are numerous in and around the lips, mouth and tips of the barbels. Koi have

two pairs of barbels. Three hundred years ago they had three pairs.


Internal Anatomy

The internal organs are placed approximately as shown in the illustration below

(reproduced with the kind permission of the authors of Living Jewels). The first portion

of the gut is elastic and can be used to store food although it is generally accepted that koi

do not have a stomach (wide hollow organ that functions to mix food with mucus, acid

and digestive enzymes).


Carp are Teleost fish which means literally "bony skeleton" and carp are one of the

boniest fresh water fishes. Fish bones are thin and light weight with no bone marrow in

the center. A light weight skeleton is advantageous to an animals that needs to be buoyant

to live in a water environment.

The drawing below is taken from the Tetra Encyclopedia of Koi and reproduced with the

kind permission of Interpet Publishing Ltd. It was redrawn from the book by Duncan





Based on anatomy and function, two types of muscle exits in Teleost fish, white and dark

muscle. The white muscle is thought to be for fast quick swimming and the dark muscle,

located directly under the skin is for sustained swimming. The clinical significance of

these different types of muscle is that they process medicine in very different ways,

which will affect how the fish responds to intramuscular (IM) injections. It is generally

accepted that red muscle is well vascularized (has a good blood supply), which would

facilitate absorption of medicines into the blood faster than white muscle, and sustain the

release since the blood flow is likewise sustained in comparison to white muscle which

can create an oxygen debt and requires a recovery period.


The gills are a complex system made up of both bony and cartilagnious stiffened arches.

Each side of the fish has four gill arches. On the exterior side of the gill arch are the gill

filaments, which are made up of lamallae and secondary lamellae structures, the latter

being only 2µ thick. It is these structures that are responsible for the exchange of gases

(O2/CO2) through the epithelial cells. The epithelial cells are the direct link between the

O2 in the water and the fish’s blood stream.

The above illustrations were reproduced with the kind permission of Paul Maslin and are

from his web site at:


The gills also have specialized mucus-producing cells, and chloride cells which function

in mucus production and osmoregulation, respectively.

Covering the gill arches on the outside of the fish is the operculum or gill cover. This is a

multi-purpose part of the koi’s anatomy with the primary function being to help control

the pressure of the water taken in through the mouth and passed along the gill filaments.

Also found on the gill arches, but more to the inside or anterior part of the arch, are the

gill rakers which act as food filters. Clogged rakers are often the cause of flashing or

head shaking as the fish attempts to clear the gills of excess food after eating.

Circulatory System

The heart is the pump that moves the blood through the system. It is located low in the

fish between the gills. A koi's heart is a two-chamber organ having a ventricle and an

atrium. Although it has been described as a four-chambered organ, the extra two

chambers, before and after the main pumping chambers, are called the sinus venosus and

bulbus arteriosus, respectively. They are smaller than the main pumping chambers,

functioning as accumulators (to smooth out any pressure surges and protect the

cardiovascular system from overpressure) and, as such, are not equipped for pumping

blood. They have no muscled walls but are elastic similar to balloons. The blood

pressure of fish is considerably lower than that of mammals.

Blood flows from the heart through the gills and then is distributed to the rest of the body.

Blood is collected into veins that eventually return to the sinus venosus just prior to

returning to the heart. Fish, unlike mammals, have only one circulatory pattern.

Mammals have a systemic circulation and a pulmonary circulation. One of the important

veins, the caudal vein, is located ventral (under) the spine. This vein is the most

accessible in the fish for obtaining blood samples.

Normally blood flows from an artery to an arteriole to a capillary to a venule into a vein

and then back to the heart. A portal system is one in which the blood flows from a vein

into a capillary and then back to a vein on its way back to the heart. Fish and mammals

have a portal circulation of the liver. That is, blood from the intestines flows into the

hepatic portal vein and on to the capillaries in the liver before collecting in the hepatic


Fish also have a renal portal system. Carp (koi) have a system that is somewhat modified

from the norm. In carp, the blood returning via the caudal and segmental veins is divided

into the renal portal vein and a shunt to the intestinal vein (see diagram below).

The significance of a portal system, is that the liver and kidney, which function as filters

to remove waste and toxins from the blood, might also remove medicines before they

have had a chance to circulate throughout the body, which could lead to treatment failure.



There are two kidneys in a koi. The caudal kidney is long and narrow, running nearly the

length of the body cavity and located just below the spine. The other is the anterior, head

or cranial kidney. It is located just above the heart and also contains Thyroid follicles.


The liver is smooth, dark red-brown color and is next to the cranial portion of the

intestine. The right lobe of the liver covers the gallbladder and the left lobe encases the


Swim bladder

This is a two-chambered organ located directly below the kidney that is directly below

the spine. There is a small connection between the two chambers that also connects to

the gut. The caudal-most chamber is relatively inflexible but the other chamber has some




The paired gonads are located between the swim bladder and intestines. They can be

separate or fused. Testes are white and fissured and ovaries are pink and smooth. The

gonadal pore has a separate opening from the waste pores at the anal vent. Gonads

enlarge during breeding season to almost 70% or the body weight in females and about

30% in males. Some koi can have both male and female gonadal tissues. These

hemaphrodites have external characteristics between a male and female.


This section was started by Phil Ishizu, completed by Spike Cover and was reviewed and

made better by the comments and contributions of Sandra Yosha, D.V.M, Duncan

Griffiths and Richard E. Carlson.


Fish Medicine – Stoskopf, Michael, W.B. Saunders Company, Philadelphia, PA

Aquariology, Master Volume – Gratzek, John, Tetra Press, Blacksburg, VA

The Tetra Encyclopedia of Koi – Tetra Press, Blacksburg, VA

Living Jewels – Watt, Ronnie; de Kock, Servaas – Delta Books, Johannesburg, S.A.

Koi Carp – Freestyle Publications, Poole, U.K.


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Last modified: 01/30/17.