Koi fish can have their teeth replaced in an alternating manner. Unlike human teeth, the pharyngeal teeth of cyprinid ornamental fish, including koi, can be continuously replaced in an alternating cycle. This means that the teeth of koi fish are constantly renewed, rather than having just one set like humans do.
The tooth structure of koi fish is quite unique. Koi fish don't have upper teeth. Their "upper teeth" are actually evolved from a special part of the skull - the pharyngeal bone, which turns into a "biting plate" that corresponds to the lower teeth. This setup allows the koi to crush food. This special tooth structure enables koi fish to chew and digest food effectively.