The digestive tract of the goldfish is interesting because it completely lacks a stomach. This means that food moves through the goldfish system faster than it does in animals with a stomach, because there is no space for large amounts of food to be stored. It also means the intestines are broken up into discrete areas which perform the different functions that a stomach normally would. The entire digestive tract of a goldfish is quite long, typically twice the body length (McVay 1940), and is broken up into parts which form various specific functions.
The Mouth, Pharynx, and Esophagus
Food first enters the goldfish system by the mouth. The goldfish opens its mouth to create suction, drawing food items along with water, into its mouth. The water is expelled through the gills, and the food items are retained in the mouth. The food items then enter the pharynx, located at the very back of the mouth. There are taste buds in the pharynx, along with teeth (called pharyngeal teeth), which grind against a hard chewing surface to break the food items down into small pieces that can pass through the narrow intestines. There are typically 4 teeth on each side of the pharynx; each tooth having a small immature tooth at the base, in preparation to replace the mature tooth when it falls out (McVay 1940). After the food items have been chewed by the pharyngeal teeth, they go through a very short esophagus and enter the intestinal bulb.
Click for Photos of Goldfish Teeth
The Intestinal Bulb
Goldfish intestines can be divided into two basic parts; the intestinal bulb/midgut, and the caudal intestine/hindgut. The function of the intestinal bulb portion is to provide temporary storage of food items and to absorb lipids from the food items as they pass through the system (Caceci 1984). The intestinal bulb is capable of expansion for temporary storage of food items, and has been observed expanded up to 3 times its normal size in goldfish examined immediately after feeding (McVay 1940).
The Caudal Intestine
The caudal intestine is the last portion of the goldfish digestive system. It is much narrower in diameter than the intestinal bulb portion, and does not have the expanding capabilities of the intestinal bulb (McVay 1940). The function of the caudal intestine is to absorb protein from the food items as they pass through (Caceci 1984).
Knowing more about the goldfish digestive system can help hobbyists choose foods that are better-suited to the needs of their fish. It has been suggested that because of their relatively fast-moving digestive system, goldfish should be fed foods which can be quickly and easily digested. Because goldfish lack a stomach, and thus are only capable of very short-term food storage and only in small food quantities, it has also been suggested that they should be fed multiple small meals throughout the day rather than one large meal. One large meal per day could potentially overload their digestive system and cause the food items to become impacted inside the intestines.
Caceci, T. 1984. SCANNING ELECTRON MICROSCOPY OF GOLDFISH, CARASSIUS AURATUS, INTESTINAL MUSCOSA. Journal of Fish Biology. 25:1-12.
McVay, Jean A. and Helen W. Kaan. 1940. THE DIGESTIVE TRACT OF CARASSIUS AURATUS. The Biological Bulletin. 78:1:53-67.