Q. What are those “whiskers” on the sides of a koi’s mouth for? What do they do? The reason I ask is that one of my koi contracted mouth fungus and the whiskers on the left side of his mouth rotted off. Will this hurt him?
A. Those “whiskers” are called barbels. Carp have one pair of barbels on each side of their mouths, with the top one being noticeably shorter than the bottom one.
Barbels are like whiskers in mammals in that they are indeed sensing organs. But they are functionally much more important for tasting than for tactile sensing. The barbels are actually covered with tastebuds. Because carp are omnivorous scavengers, having tastebuds on the barbels outside the fish’s mouth allows it to literally taste things without ingesting them. Given the carp’s habit of foraging in along murky pond and rivers bottoms, this seems like a useful anatomical innovation.
Experimental studies show that the barbels are not very sensitive to sweet tastes, but they react strongly to bitter and sour tastes. Your koi will not suffer any long-term consequences from losing a pair of barbels. Because most hobbyists feed their koi floating foods, koi rarely need their barbels to locate food. It is also possible that the barbel organs will regenerate if enough live tissue survived infection.
Article source: http://www.fishchannel.com/setups/ponds/koi-whiskers.aspx