Contrary to the thinking of many people (from a biological standpoint)...goldfish, grass carp, big head carp, buffalo etc. are not related to carp. Common carp, mirror carp, leather carp and koi are however the same fish....and all started as common carp. Common carp is the one we're most familiar with. The monks when spreading Christianity took carp with them from place to place to raise in their monistery (sp)? ponds for the purposes of a food source. If you've ever caught a carp with "missing" or "no" scales (some people seeing this think...sick carp in bad water)....they were bred this way. Monks didn't like scaling them so bred scales off of them. The mirror carp will have irregular sized scales or be missing a lot of scales...a leather will have no scales. Koi were bred to have the "colors" they have and were more used for show/ponds. If left alone long enough in the same waters and all these fish were to breed with common carp...not with another mirror or leather...eventually the dominant gene of the common carp will work it's way back out and the others will disappear. Point of trivia...carp have been recorded to live in excess of 50 years...a very commonplace thing in the UK where carp are caught repeatedly....but, the practice of very safe "on the bank" treatment is practiced. This will sometimes include medication.....carping is big business there and in most parts of Europe. Many carp (the bigger one's) have even been given names and kids grow up with the goal of catching the same fish dad or granddad caught.