Koi Anatomy

For Koi owners, it is helpful to understand the ins and outs of your living jewels. These fish have complex and interconnected systems adapted for life underwater. Here, we will briefly examine Koi’s external and internal anatomy.

External Anatomy

Though shape differs subtly between varieties, a Koi’s body tapers at both ends like a torpedo. It is broader in the front, which reduces turbulence at the rear and helps to streamline it, enabling the Koi to glide efficiently. However, a modern Koi’s shape is not identical to its ancestors. Years of selective breeding and the relative safety captivity provides have caused Koi to develop a more stocky core, which has slowed it down in the modern age (although it’s still pretty fast).


Koi have three single fins and two pairs. The caudal (tail) fin attaches to the vertebral column, which extends to the fin’s upper lobe. It is responsible for the force that propels Koi forward at high speeds. With a side-to-side thrust, the caudal fin also acts like a rudder to help the fish steer. The Koi’s caudal fin is heart-shaped, which provides a relatively weak force compared to fish in the wild that must hunt prey and escape predators.

The dorsal (top) fin runs down the spine from head to tail, providing forward-motion stability. It reduces drag, allows precise maneuvering, and stabilizes the fish when it feeds upright. This fin consists of soft rays and hardened spikes called denticles. It is prone to bacterial infection and ulceration, so inspect it when you perform a wellness check. The anal fin is located on the bottom of the Koi, just behind its vent, and also is used for stabilization.

Paired pectoral fins help with steering, slow swimming, backward swimming, and braking. They are also helpful in stirring sediment when Koi are searching for a lost or forgotten snack. The fins are slender and strongly vascularized, making them good additional areas to check for problems like disease, parasites, fraying, or damage. Ventral fins, also known as pelvic fins, are lower on the body than the pectoral fins but serve a similar function. They are responsible for controlling pitch and roll, as well as ascending and descending.


Koi have three skin layers. The first, the epidermis, sits atop the scales. One of its key components is the cuticle, also known as the slime coat, a non-cellular mucous layer that protects against waterborne irritants and bacteria. It also helps reduce drag while in motion.

The dermis is the skin’s second layer, which contains nerves, blood vessels, and pigment cells that determine the quality and thickness of the Koi’s color. It contains cells that form and regenerate the scales. The scales (which are different sizes depending on their location) overlap at five points.

The hypodermis is a fatty layer that stores energy and connects the skin to the muscle or bone. It forms a protective layer that shelters the Koi’s skeletal system, muscles, and organs.


Koi scales grow concentrically and have a smooth texture and edges. Koi will sport different scale colors, sizes, and patterns depending on the variety. Most Koi are fully-scaled, but some types have scales exclusively on the dorsal and lateral lines (Doitsu), while others are entirely scaleless (leather). Additionally, some Koi develop fukurin, which occurs when the dermis creates a distinct netting pattern.

The scales have regenerative properties and, if lost, will replace themselves within a few weeks. However, keeping scales intact is best for hobbyists, as their repaired color might change slightly. You can help prevent scale damage by feeding your Koi a nutritious diet, ensuring their water is high quality, and keeping them safe from backyard predators. Scales also can change depending on the location and time of year. For example, during the fall season, when the water temperature decreases, Koi grow body armor to protect them in cold water.

Lateral Line

The lateral line consists of a series of porous scales that run the length of the body. This sensory organ transmits low-frequency vibrations to the peripheral nervous system through mucous-filled canals. It plays an essential role in helping Koi to determine threats and their proximity to objects.

Gills And Operculum

The gills are a complicated system of rigid bony and cartilaginous arches. They are the gateway to the Koi’s respiratory system, absorbing oxygen from the pond water and releasing carbon dioxide. Blood flows through the gills in the opposite direction of the water flow. Gills remove nitrogenous waste from the Koi’s body, chiefly ammonia.

The operculum is the bony flap that protects gills and regulates water pressure. It acts as a one-way valve, allowing water to leave the gill chamber while blocking oxygen-deficient water from re-entry. If you need to inspect the gills, lift the flap slightly and gently. Tearing is possible if handling is too rough.


Five sections comprise your Koi’s tail: the base, the middle section, the upper portion, the lower portion, and the tip. The tail’s base is where it begins, comprising about half of its length. The upper portion bridges an area wider than the base and narrower than the middle. The middle section extends to the end of the tail fin. This lower portion is still narrower and connects the tail to the body. At the end of the tail is the tip. It is shaped like a wedge and is often colored differently than the rest of the tail. Some varieties may even have spots or stripes.


Koi have panoramic vision up to six feet and in color. Their eyes can also see in two directions simultaneously and independently of one another. However, eyes are a luxury for Koi, which don’t require them for survival. Blind fish can find food using their barbels, lips, and mouths. The lateral line also helps them detect the whereabouts of other fish and pond structures.


Koi have multifaceted mouths that they use for a variety of sensory tasks in addition to feeding. The protruding mouth toward the bottom of the head provides a Koi’s primary sense of touch and is ideal for foraging on the pond’s floor. The Koi uses pressure receptors to feel and assess items it finds. Two sets of muscle-controlled barbels flank the mouth and contain sensitive taste buds. It will reject inedible debris within a few seconds.


Koi have U-shaped nostrils (nares) on either side of the snout between the eyes and mouth. The base of these nostrils contains olfactory organs. They are fashioned like miniature tubeswater enters through the front and escapes through the back. Hair-like cilia within the nares help detect substances. Water does not pass from the nares to any other body part.

Internal Anatomy

Koi are vertebrates of the teleosts group, among the most skeletally advanced freshwater fish. They have two types of muscle: white and dark. White muscle enables fast swimming, while dark muscle activates during slower movement or stabilization. All muscles are oriented in thick rings called somites that band the sides of Koi in orderly rows.

Skeleton & Teeth

As a teleost fish, Koi skeletons contain a large number of bones. Each bone is thin, light, and absent of marrow, which helps the fish attain an appropriate level of buoyancy. The skeletal system is diverse from head to tail, with a skull that protects the brain and bones guarding delicate gills. The head joins with the backbone, which connects to the tail and is composed of tiny bones that enable swift maneuvering. Koi rely on their skeletons for overall body support, though not as much as most other animals.

Koi’s teeth are lodged in the throat, attached to the pharyngeal bones behind the gill chambers. (This makes it impossible for them to bite you.) Once Koi ingest food with gulp and suction, these serrated teeth grind hard and crunchy parts of foods such as insects and crustaceans. And because Koi regenerate teeth throughout their lives, no aquatic dentistry is needed.


Located in the Koi’s core, the swimbladder controls buoyancy. Without it, Koi would need to exert significant energy to keep from rising to the water’s surface and to remain there. Several blood vessels within this organ control gas content, and fish adjust the amount stored to float upward or downward at will. Difficulty floating, sinking, or maintaining a normal position indicates a common problem called swimbladder disease.

Cardiovascular System

Koi have a four-chambered heart with a ventricle and atrium used to pump blood through a system of arteries and veins and two backup chambers to counter pressure changes that could strain it. However, even with added support, a Koi’s cardiovascular system is relatively weak compared to other animals, and their blood pressure is considerably lower. This is one reason it is crucial to keep your pond well-aerated.

The respiratory system of Koi consists of a mouth and gills. Koi take in water that flows into two opercular cavities, which house a system of capillaries and blood vessels. In contrast to mammals, fish have just one circulation pattern. In addition to oxygen, the system transports nutrients to the tissues and pumps out nitrogen waste products.


Koi have two kidneys: caudal and cranial. The caudal kidney is long and narrow, running nearly the length of the body cavity and located just below the spine. The cranial kidney sits above the heart. Though most nitrogen waste exits the gills in the form of ammonia, the kidneys receive and excrete some of it and excess water as urine.


The stomach is an organ essential to mammals and is absent in Koi, leaving these fish incapable of digestion. Food proceeds directly from the esophagus (where the pharyngeal teeth break it down) to the intestine, which absorbs nutrients and expels the rest. This digestive inability makes feeding Koi too close to the cold season dangerous. If Koi enter torpor with a full gut, harmful bacteria can pass through the intestinal wall and into the bloodstream.

Once the food is digested in the intestines, it is passed to the liver, distributed, and stored in the tissues. The liver converts excess protein, processes old red blood cells to help form bile, and breaks down other toxins. The right lobe of the liver covers the gallbladder, and the left lobe encases the spleen.


The gonads are located between the swim bladder and intestines. However, Koi lack external sex characteristics, so distinguishing males from females can be tricky. Females tend to have slightly rounder body structures and often grow larger than males. However, these characteristics can have a variety of causes and alone cannot be used to sex Koi.

Want to add a few new fish to your backyard pond? Browse our full selection of Koi and goldfish today.

1 response

  1. I’ve just ordered some koi from you, I’m soooo excited to get them! I’m wondering what the best plants are to plant in their pond/aquarium for them to graze on

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