Koi eat to live (except Chagoi that obviously live to eat!) and a koi’s physiology is adapted to be able to consume and digest a vast array of living and dead food materials. The processed and manufactured artificial diets that are routinely offered to koi bear little resemblance to the nature and diversity of the food that this opportunistic omnivore is actually adapted to digest.
Put briefly, when considering digestion, captive-reared koi are the spoilt relatives of their ancestral carp. However, even though koi have improved on the drab carp livery, they have retained the digestive capabilities of a carp and as we will see, because of their rough-n-ready natural diet, they have an ability to digest a vast array of natural food items.
What is digestion?
When koi feed, their objective is to digest the food they consume efficiently, absorbing what they require, excreting what is indigestible or surplus to their requirements. Digestion is the breakdown of insoluble food items into less complex, soluble molecules that koi can then absorb into their blood. This involves both mechanical (physical) and chemical stages, and the feeding process starts before food is offered to them as koi are likely to detect you approaching the pond with food.
Koi are alerted to the presence of food through a combination of sight, sound and smell, as they either see or hear food (or other feeding koi), or smell the scent of food that has dissolved in the water.
On discovering food, its mouth and buccal cavity (at the back of the mouth) are well equipped for the seizure, control, selection and preparation of food. The buccal cavity is lined with tough ridges of a folded membrane called the mucosa. The mucosa is covered with microscopic projections called papillae and is richly provided with mucus-secreting goblet cells and taste buds. Chewing it over Digestion in humans starts in the mouth with chewing to physically break up the food. This is also true for Koi where preparation of the food commences with the grinding action of several pairs of pharyngeal teeth. They are situated in the posterior part of the buccal cavity on the ventral surface. This “grinding mill” ensures that all food entering the gut is suitably fragmented for further digestion. The “chomping” action seen in a feeding koi occurs as a result of this grinding, often expelling any rejected fragmented material through its opercula.
The duct which carries the ground-up food from the pharyngeal region of the buccal cavity to the anterior intestine and receptive sac is called the oesophagus. A dense bed of cilia and goblet cells (secreting yet more mucus) aids the passage of the selected food material through this first constriction in the gut. Koi consume many insoluble organic nutrients in their diet, such as proteins, carbohydrates and oils with complex molecular structures and high molecular weights.
To utilise them, they have to be broken down into smaller component parts which are soluble in water and easily transported around the body to where it is needed in the fish’s blood system. Breaking It Down A koi’s intestine is a relatively featureless structure and unlike other vertebrates, has no multi-cellular secreting glands, valves or caecae. In fact, apart from the dilating receptive sac (which is not a stomach), the intestine is similar in structure along its entire length.
It is usually twice the length of the koi’s body and is lined with epithelial cells which are constantly being replenished as abrasive food material erodes them away. The passage of food is lubricated by secretions from goblet cells while other cells in the gut epithelium are dedicated to absorbing the broken down soluble nutrients from the gut.
Koi – The stomachless fish.
The grinding action of the pharyngeal teeth may remove the necessity for the action of acid on food material. The complete absence of a stomach means that unlike other fish, no acid digestion takes place and no pepsin (a protein-digesting enzyme) is produced in conjunction with hydrochloric acid to digest proteins. In those animals that do have a stomach, pepsin acts in the stomach optimally at pH2, so if no hydrochloric acid is secreted in stomachless koi then pepsin secretion would be pointless anyway.
It has also been suggested that due to the omnivorous diet of koi compared with other fish, the need for pepsin is reduced as lower levels of protein are ingested in their diet. This is probably why protein assimilation efficiency tails off when koi feed on very high protein artificial diets.
When comparing the intestines of fish it is interesting to note that those of omnivorous fish such as koi are always longer than those of carnivorous fish. Koi diets are lower in protein and are harder to digest, requiring a longer time in the gut to be broken down and absorbed – hence their lengthier guts.
Detergent-style action Ducts from the gall bladder and pancreas enter the koi’s intestine almost immediately after the oesophagus, carrying bile and enzymes respectively into the intestine. The food and enzyme mixture remains in the distended receptive sac while it digests, prior to entering the smaller intestine.
Bile which is produced in the liver and stored in the gallbladder is an emulsifier of lipids and oils. Working in a similar way to a detergent it breaks up larger bodies of lipid into smaller droplets producing a larger surface area for lipase (lipid-digesting enzymes) attack. Lipids and oils are broken down into soluble fatty acids and glycerol so that they can then be absorbed into the blood.
Proteases such as trypsin and chymotrypsin, secreted by the pancreas, digest the proteins in the food. This breaks down proteins into amino acids or small groups of amino acids which are soluble and easily absorbed into the bloodstream.
In human digestion, the action of chewing, smelling and seeing food stimulates the mouth to “water” or produce saliva. The saliva contains a starch digesting enzyme (salivary amylase) which breaks down insoluble starches and carbohydrates into soluble sugars. (Try sucking on a piece of bread (starch) and after a while you’ll find it tastes quite sweet). In its aquatic environment, it is impractical for koi to secrete salivary amylase, so it is secreted directly into the gut from the intestinal mucosa to break down starch. How long does food spend in the gut?
Gut transit time (the time taken for food to pass through the koi’s intestine) can range in koi from 16 hours at 25C up to 60 hours at 12C, showing that rates of digestion and enzyme action increase along with the temperature. Prior to excretion, the final absorption takes place in the hind area of the intestine.
Further towards the anus, the intestine is similar in structure to the oesophagus, showing thicker walls of connective tissue and being densely supplied with mucus secreting goblet cells. This aids the formation, passage and ejection of waste. In the natural carp diet, a significant percentage of faecal material is inert mineral matter of no nutritative value, inadvertently ingested with food items while grubbing around on the pond bottom.
Unlike carnivorous fish, the carp does not secrete collagenase, an enzyme required for the complete breakdown of the connective protein found in animal skin and tissue and this will also be a significant part of faecal matter when feeding on natural diets.
When comparing the diets that we offer our koi with the natural diet that they are designed to digest, we could be guilty of offering them a diet that is too easy for them to digest. It is accepted that low-waste diets lead to better water quality, but are adapted from high performance diets that are used to grow food fish to a harvestable size as quickly as possible.
Would it not make sense to offer our koi the benefits of a more challenging, roughage-rich diet, acknowledging that we too are keen on growing our koi quickly, but that their long term health is also of paramount importance to us. Having seen how their digestive system is adapted for a tough and varied diet, can this really be achieved by feeding koi a diet that is far removed from what nature intended?