Part I: Koi Digestive System
Koi have maxillary barbels (most cyprinids do not) and forage in mud like catfish. They ingest water, sediment and detritus and filter out non-food particles. How do they do it?
They have gill rakers in the form of a mesh at the entrance of the pharynx. The ingested stuff gets filtered and large items
that do not pass through gill rakers, are expelled through opercular openings behind the gills. The gap between gill
rakers (filter size) is proportional to the length of Koi. So, large gaps present between gill rakers in large Koi, expel only the largest zoo-plankton from being filtered out.
Koi lack stomach, but have a long gut which winds extensively throughout the visceral cavity. Koi gut resembles those of
higher vertebrates. Gut lengths can be 0.6-8 times the body length, depending on the age of Koi. Gut begins at mouth and ends at anus.
The filtered food moves into head gut. Head gut has two components, the oral (buccal) and gill (branchial, pharyngeal)
cavities. In the pharynx, just before the esophagus, there are 2 sets of interdigitating teeth that grind food into small pieces. Koi ingest considerable amount of plants. Chewing helps in breaking the plant cells, making them more
The mid-gut is always the longest portion of the gut and may be coiled into complicated loops. Villi of mammals are absent in Koi.
Koi have a gall bladder. The bile duct opens into the intestine just before the gall bladder. Koi liver has no specific shape
, but seems to occupy all of the empty space around the intestine.
Food is ingested in small particles in a relatively steady stream instead of intermittently in large units like we do.
Because of this, storage function of a stomach is not needed. Even if they did have a stomach, stomach would not be able to expand after eating because liver occupies all of the available visceral space.
Taste buds are usually present along the esophagus with additional mucus cells. Freshwater fish have longer (stronger)
esophageal muscles than marine fish, probably because they have to squeeze out almost all of water from their food, to
maintain osmolarity. Marine fish, on the other hand, drink seawater along with food. The esophageal sphincter (valve) works as check valve by preventing regress of food from the intestine.
The mid-gut is mildly alkaline and contains enzymes from the pancreas and the intestinal wall, as well as bile from the
liver. These enzymes attack proteins, lipids, and carbohydrates.
Hind-gut has mostly mucus cells. The blood supply to the hind-gut is usually comparable to that in the posterior mid-gut.
This means hind-gut helps in absorption just like mid-gut. Formation of feces happens in the hind-gut.
Gut emptying rate declines more or less exponentially with time. When temperature decreases, appetite, digestion rate, and amount of secretions produced all decrease.
A research study showed that gastric emptying time and total food passage time in skipjack tuna at 23-26 C was about
12 hours on average. The intestine was full in about five hours after eating and empty after about 14 hours. Defecation
often occurred 2-3 hours after a meal but the undigested material was from a previous meal. After a single meal, feces were found 24, 48 and even 96 hours after the meal. Magnuson (1969) commented that the food passage rates in
skipjack tuna were at least twice as fast as known for any other fish.
So, we can conclude that Koi take more than 12 hours to excrete the food they eat. This is important because people
tend to feed their Koi in winter on warm days. Since gut emptying can take up to 96 hours, one must feed only if the
warm temperatures last 4 days or more. If food remains in the gut, it will rot in the gut because digesting gut bacteria are not available in low temperatures.
Digestibility ranges from 100 percent for glucose to as little as 5 percent for raw starch or 5-15 percent for plant material
containing mostly cellulose (plant fiber). Digestibility of most natural proteins and lipids ranges from 80 to 90 percent.
Koi feeding on a mixture of mud and plants, excrete almost all of what they eat because very little is digestible. So, they forage all of their waking hours.
Digested food, particularly proteins, is not fully available to Koi even after it has been absorbed into the blood stream.
For example, amino acids, if used for building new tissue, could be used as absorbed. But if they were to be used to produce energy, the amino group needs to be removed first. The de-amination itself requires energy!
Koi have rectal fluid that aid in osmo-regulation. Rectal fluid has magnesium and other divalent ions just like blood. In Koi
, digestion and osmo-regulation are inter-related. If food stops moving in the gut, rectal fluid will accumulate. Magnesium
ions present in the rectal fluid will be reabsorbed instead of being excreted. This leads to high levels of magnesium ions in the blood that will eventually kill Koi.
Koi have some enzymes that humans have. Lipase in the intestine, to break down fats, glucosidase, maltase, sucrose,
lactase, melibiase and cellobiase to break down various carbohydrates. However, they make different forms of the
same enzyme to work at different temperatures. Because of this ability, they are able to digest food over a wide range of
temperatures. Whereas in humans, if the body temperature falls or rises a couple of degrees above normal, a lot of enzymes don't function.
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