Koi anatomy has changed very little throughout the centuries, in spite of evolution or modern breeding techniques. Understanding the basic anatomy of koi can help owners keep their fish in healthy condition and know how better and when to treat them.

Nostrils
Koi have nostrils, or olfactory organs, located just above their eyes. These are solely for scent purposes.

Dorsal Fin
The dorsal fin located on top of the koi is the fin that is most responsible for stabilization. When the koi needs to move more quickly through the water, it can lower the dorsal fin.

Lateral Line
The lateral line running along the mid-body of the koi opens into a channel of neuromast cells that are sensitive to vibrations in the water. The lateral line is located at the same level as the spine.

Caudal Peduncle
The caudal or tail fin behaves as a rudder and assists in providing speed and thrust. The caudal peduncle is narrowing of the fish from the body of the fish to the caudal fin.

Anal Fin
The anal fin of the koi is located in front of the tail and is used primarily to stabilize the fish.

Vent
The vent of the fish, also called the genital papilla, is the external sex opening of the fish and is located between the anal pore and the anal fins. The vent may vary in size and pigmentation, depending on whether the koi is male or female.

Gonads
The gonads of the koi can be found between the intestines and the swim bladder and can enlarge dramatically during breeding season. The gonopores are separate from the anal pores.

Swim Bladder
The swim bladder has 2 chambers and can be inflated and deflated, allowing the fish to swim at varying levels.

Pelvic Fins
There is a set of two pelvic fins that help control the sideways movements of the fish and counteract lift.

Intestine
The digestive system of a koi is unique in that the koi does not have a stomach. Once the food is crushed by the pharyngeal teeth, it passes into the esophagus and directly into the intestine. The intestine of the koi are roughly 4 to 5 times the length of the fish. Once the food passes through the intestine, it is expelled through the anal pore.

Spleen
The koi’s spleen is an organ that both produces lymph cells and stores red blood cells.

Liver
The liver produces bile to aid in the digestion of food, and it also helps metabolize or remove harmful byproducts from the blood.

Pectoral Fins
The pectoral fins help the fish steer, swim, and brake.

Heart
The koi has a 2-chambered heart that is located between the gills. The ventrical and atrium pump blood, while the sinus venosus and bulbus arteriosus act as spare chambers regulating the blood pressure. Blood flows from the heart, through the gills, and then through the rest of the body.

Gills
Gills are organs that absorb oxygen from the water passing over them. Carbon dioxide is then eliminated from the body through the blood vessels in the gills and back into the water.

Brain
Koi have a very simple brain that transmits and receives messages from the nerve fibers attached to the fish’s central nervous system.

Barbel
The barbell is located on the upper lip on the upper lip of the koi. It contains both taste buds and receptors to help the fish locate food.