SF Bay Area Koi Club

Organization of Pond and Koi Keepers




Anatomy of Koi


Internal Anatomy


Gills: The gills have three functions. When water is passed over the gills, oxygen is absorbed and carbon dioxide and ammonium is exhaled. 75% of the ammonia excreted by the fish is through the gills. The gills also help the fish osmoregulate, (equalize body pressures). The two gills are located on the head of the fish, just behind the eyes. They are covered by a boney flap known as the operculum. Each gill consists of a bone arch with filaments extending at right angles to the arch. Lamellae are the tiny branches on the filaments. Healthy gills are bright red and do not stick together. Signs of trouble with the gills are listlessness, flashing, and rubbing. Examination of a damaged gill will show excess mucous along with hyperplasia and a pale color.


Swim Bladder or Air Sac: This glistening oval, dumbbell shaped sac is located in the dorsal area of the abdomen of the fish, just below the kidney. Its elasticity allows the fish to have buoyancy and balance. The sphineter muscle opens to let gases out to the blood stream and the rete mirabile muscle allows gases in. Futile attempts to swim near the surface or up side down swimming are signs that the swim bladder is not functioning normally.


Gonads: The sex organs of the fish are termed gonads. In females you will find the egg sac and in males, the testes. The egg sac may fill a major portion of the abdominal cavity.


Gastrointestinal Tract:  A long, tubular organ; only slightly smaller than the stomach. The stomach is at the anterior end of the intestine, behind the head and above the liver.


Spleen: The function of the spleen is to produce and store red blood cells. It is found embedded in the intestine as a small red mass.


Liver: The liver is responsible for the digestion, filtration and storage of glucose. It is found in the anterior part of the body cavity as a brownish red mass. The liver also produces many enzymes that are stored in the gall bladder. These enzymes assist in the breakdown of food. The liver functions to store food energy.


Gall Bladder: Digestive enzymes and bile are held here. The gall bladder is a small sac found embedded in the liver tissue.


Heart: The heart is a two chambered organ. The function of the heart is to circulate the blood. It is located adjacent to the gills, just behind the head.


Kidneys: There are two kidneys in a fish. The posterior kidney is located below the vertebral column and above the air sac. The anterior kidney is found in front of the air sac. The kidneys produce white blood cells and filter wastes. Diseased kidneys result in a loss of immunity.


Brain: The brain is located in the head, between but slightly behind the eyes. It is protected by the skull. The brain controls the voluntary behavior and involuntary functions of the fish. Erratic swimming or an abscess on the top of the head could be indications of a problem.

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