External Anatomy of Koi Fish

diagram of external anatomy of koi fish

Overview of the External Anatomy of Koi Carp Fish

The koi fish is not different from any other living thing. It is a complex, interconnected system that has evolved naturally and selectively through breeding.

Hobbyists and enthusiasts have many resources that provide detailed and comprehensive information about koi anatomy and physiology. This is only a brief overview of the features you can see since you interact with the “Living Jewel’s / nishikigoi” external components.


A range of characteristics can be used to define koi, such as fins, scales, and, most important, color.  But one leading standard is koi’s shape. 

A hundred years of breeding koi have provided the perfect koi body.

Chub is good. The Koi conformation is a little bit of a compromise between the two. Practicality and aesthetics. These are practical matters. Koi fish were once bred to be food.

The better, the more complete and fuller the koi are- the better. Because raising chubby koi needed just as much space as smaller koi and as much food.

The slim ones were overshadowed; the chubby ones prevailed every time. Moderation is essential, however. A fat koi may appear to lack grace and beauty. A well-formed body is a sign of beauty and elegance.

Koi is most desired in a shape that is not too fat nor too thin.

Both bodies and fins are proportional. A koi who is too fat has an issue.  Having a large body but regular-sized fins (which do not change with weight) may appear too small. The fish looks cartoonish as a result.

The upside-down pear is the sexiest. Viewed from above, the body-perfect stretches from the pectoral fins to the leading edge of the dorsal. Fin tapers to the base of the tail. The nose should be pointed gently but not sharply. Pointed or foreshortened, as if the fish had run into the wall a few times

You will find subtle differences depending on the variety, but a koi’s shape can be fusiform or taper at both ends. The koi’s front is more comprehensive than its back. This helps minimize turbulence and streamlines the fish’s ability to glide through the aqueous environment.


They are located just below the underside gills and can move forward or backward at slow speeds, brake, and change direction. They counter the propulsion force produced by the water leaving the operculum. They can also stir up pond sediment to search for food.


Two sets of paired fins are available for koi (pectoral ventral, pelvic, or pelvic), and three single fins are available for them (caudal tail, dorsal and anal). They are responsible for stability and movement but are also thin and susceptible to damage. Often, the first sign of a disease is found in the fins.


Also known as the pelvic fins, they are lower than the pectoral and have a similar function. These fins are responsible for controlling pitch, roll, ascending, and descending.


This stabilizer is located on the back of the koi and keeps the fish upright. It reduces drag to allow for faster maneuvering and stops the fish from yawing or rolling when it is erect.


Also known as the tail fin, this is responsible for forwarding motion and thrust (especially at extremely high speeds).


This fin is located just before the caudal and stabilizes to prevent yawing or rolling.


A line of porous skin runs the length of the entire body from one side to the other of the koi. It is located around the midline. Through a series of mucus-filled canals, this sensory organ transmits low-frequency vibrations it finds from greater distances to its peripheral nervous system. It plays a vital role in identifying threats and objects’ proximity.


The epidermis is the epidermis, which lies above the skin’s scales. It is home to the mucous glands responsible for creating the cuticle or slime coat. This is the first line of defense against potentially harmful bacteria and disease. It helps reduce drag when in motion.

The dermis is the second layer. It contains scale-forming cells that produce scales. It houses the chromatophores, pigment-producing cells, sense organs, blood vessels, and nerves.

The hypodermis, located under the koi’s skin, is a fatty membrane that acts as a barrier between its skin and the rest.

Koi scales, which are teleost (bony fish) sized, are cycloid. They grow concentrically with a smoother texture and smooth outer edges.

Scales are light, calcified, and flexible. They are embedded uniformly in the dermis. The scales, which are of different sizes depending upon where they are located, overlap in the same manner as roof tiles and at five additional points.

You can leach calcium from the scales back into the koi to replenish fish’s nutritional stores when they are scarce.

Even if the disease does not set in, lost scales can be replaced in a matter of weeks. However, the replacement scale is often less sturdy. The replacement scale may not be as complete as previous sales rankings. Some uniformity might also be lost when compared to the earlier sales rankings.

If the scales cover only the dorsal or lateral lines, the koi can be considered a Doitsu (or German Koi). Some koi are not covered by scales and are called leather koi.


The bony plate protects the gills below and acts as a valve to allow the oxygenated blood out while preventing it from coming back in. It regulates the water pressure that flows along the gills. You can move it to inspect the gills as it is not attached at the back of the lower part. However, it can be damaged if handled too rough or lifted too high.


These gills are responsible for gaseous exchange. They bring oxygen into fish and then transport carbon dioxide out. Gills are also essential for removing waste from the body, such as ammonia.


Koi possess excellent vision. They can see in both directions simultaneously and independently of each other. Their eyes can see from either side of their bodies, and they can also catch up and down in a 360o panorama. This is useful for keeping an eye on predators as they feed.

They will be fine without an eye if they do lose one. Koi have a very well-tuned sensory system that uses their mouths, lips, and barbels to “sense” food. The lateral line also helps the “fight or flight” aspect of survival.


Koi do not have the “ears” we know. They are instead internal. The vibrations produced in water are transmitted from bones to the swim bladder, amplifying them.

If they are exposed to loud noises repeatedly, stress can cause stress-induced illnesses.


The protrusive jaw is placed towards the bottom of the head to best suit the koi’s bottom-feeding habits. This position makes benthic koi feeders. They can also feed on the surface and mid-water.

Two sets of muscle-controlled barbels flanking the mouth have sensitive taste buds that help them determine whether what they are about to eat is good or bad. They are responsible for finding food in the substrate through which koi hunt.

Koi have teeth in the rear of their throat for chewing food when needed.  But don’t worry, they will not bite you.


It is not a nose, at least in the sense we know. Instead, it’s two pairs of U-shaped nostrils or nares located on either side of the mouth and eyes. The forwarding motion of water onto the olfactory receptors that detect smell (located at the base of the nostrils) diffuses the water.

Click here to view the internal anatomy of koi fish.

Last Updated on March 5, 2022 by Davin