There have been quite a few threads here in the past concerning koi/goldfish hybrids, and since recently I found myself the new owner of a hybrid, I thought I would share my research into the topic. First of all let me introduce my own young hybrid. He was sold to me as a "black comet" by a well-known vendor. I suspect the foreign wholesaler was not entirely honest with the vendor and I place no blame upon the vendor, who is well-known and has an excellent reputation. Either way, when the fish came to me I photographed it, as I generally do with new fish, and noticed while examining the photos that the fish had tiny barbels. This was my first clue. More on that later. Here is a photo first. His name is Sygnus, by the way. First of all, a bit about the... Viability & Genetics of Hybrids Koi carp (Cyprinus carpio) and goldfish (Carassius auratus) can hybridize in the average backyard fish pond, and while it is somewhat common, the hybrids are often sterile and do not reproduce themselves. In the wild, hybrids may or may not be sterile depending on the subspecies of carp involved. There are dozens of carp subspecies that may hybridize with introduced goldfish. For example: So depending on which subspecies is involved, the hybrids can be sterile or reproduce as easily as if they were full-blooded. As documented here: Additionally, some crosses may produce fertile hybrids of one gender but not the other. See the end of the first paragraph here: http://www.wdc-jp.biz/pdf_store/isj/publication/pdf/16/163/16304.pdf A bit more about the viability of hybrids between goldfish and carp subspecies here: http://www.cababstractsplus.org/abstracts/Abstract.aspx?AcNo=20053041051 So my fish may or may not be sterile - we have yet to see. Anyway, I wanted to figure out precisely how to identify him as a hybrid... Identification Using Meristics First I noticed the barbels, and so I set upon researching the meristic (physical) differences between goldfish, koi, and hybrids. The most easily identified features include barbels, lateral line scales, and fin rays. An experienced goldfish or koi keeper will also notice body shape, mouth position, and fin shape, but these are highly variable between individuals and can't really be counted on for a solid ID. The best way is to use the shape and number of the pharyngeal teeth, those bony ridges in the jaws that help to grind up food. However, it's impossible to use those unless you kill the fish. So understandably, I wanted something a little less... uh, lethal. Lateral line scale count is one of the more reliable ways to ID a hybrid. Sygnus has 33 lateral line scales and 20 dorsal fin rays. He fits right into the hybrid range - on the low end for koi, too many for goldfish. Another marker is the presence of barbels. Koi have two pairs of barbels - one set just up and back from the mouth, another at the corners of the mouth. See here for photos: http://www.koiquest.co.uk/Anatomy.htm Goldfish have no barbels whatsoever, and hybrids either show no barbels or only one set, very small, at the corners of the mouth. Sygnus has one set of "reduced posterior" barbels, just visible against the rock here: You can see more photos of possible hybrids with similar small barbels here: http://www.koiphen.com/forums/showpost.php?p=1442380&postcount=5 You will also notice them in this fish: http://www.tnfish.org/PhotoGalleryF...bridCommonCarpGoldfishMeltonHillNegus_jpg.htm As I mentioned earlier, an experienced keeper will immediately be able to differentiate between a koi and goldfish by body shape, mouth position and fin shape. These were the first features that tipped me off to the possibility that Sygnus was a hybrid. These are, of course, only to be used as... Informal Identification Keys Notice the differences in shape between him and one of the midnight shubunkins: Right away you should see that the hybrid has a much flatter belly line than the shubunkin's, which curves upward. The hybrid's mouth is set at the bottom of his head, while the shub's is midway between the bottom and top. The hybrid has round fins, the shub has pointed fins. This fin difference is even obvious between the hybrid and my common goldfish. Here you can see more details: See the rounded fins? Also notice those sensory nubs around the eyes - koi have them, goldfish do not. They are visible on the photo of koi eyes at the anatomy page I referenced earlier. I have seen them on some hybrids, but others do not have them. It may be another difference between subspecies hybrids. Other features that stand out to me were the thick, strong caudal peduncle - the muscle that connects his caudal (tail) fin to his body. Notice how much more muscled it is than that of the goldfish. The top photo shows this well. Also the lack of a V-shaped tail. The hybrid has a nearly solid triangle for a tail fin. In this photo you can just barely make out a very subtle hump between the end of his head and his dorsal fin. This hump may become more prominent as he grows older and larger. Some goldfish will develop a hump like this, and you will see it in many large koi. It seems to be a common characteristic among domestic hybrids. This website has dozens of photos of domestic hybrids: http://www.shortypen.com/koi/hybrid/index.htm This is a very quick and dirty look at hybrids and how to identify them, but you can find much more information by exploring the links I provided. Questions and comments are welcome!