Do Koi have teeth?
Koi are equipped with rather large teeth at the back of their throat. They do not use them defensively or aggressively but rather to process any hard-to-chew food they come across at the pond bottom.
As the gill slits perforate the sides of the pharynx, along which the food passes, the delicate lamellac are protected by a series of stout bars called gill rakers. These lie across the internal entrance to the gill slit, and slits are supported by the gill bars. Gill rakers not only stop particles passing through the gill slits but, in trapping these particles, make them available as food. This is why fish that food on fine particles have many slender gill rakers. Koi, feeding on larger morsels by choice, have between 20-30 stout gill rakers on the first gill arch. In the archetypal fish there are 5 gill arches.
Koi do have teeth
In the Koi, which lack teeth in the jaws, the fifth is modified into the very strong, tooth-bearing pharyngeal bone. This is roughly sickle-shaped, expanded in the middle where the teeth are supported. The teeth point inward and upward, and grind food against a gristly pad in the base of the skull. Strong muscles, a development of the usual gill muscle, provide enough strength to smash shells. The shape of the pharyngeal teeth gives a clue to the diet of the fish- Koi are molariform, showing that it grinds up relatively massive food items.
Putting aesthetics and longevity aside, koi do not possess stomachs. But they do have expandable intestines that function as pseudo-stomachs. This is why you should be careful feeding your koi in winter while their metabolism slows down almost to a halt, most especially with protein based diets, because this will only rot inside their intestines and make them sick.
For more information on Koi care call
Sunland Water Gardens @ 818 353 5131
Resource: Ultimate Koi Book