Re: Fantailed Goldfish have two tails. Why? Does a gene account for that?

Anonymous asked:

I have four goldfish(Carassius auratus). They all have 2 tails.Fantailed and twintailed are two words for this. "Fancy" goldfish are specially bred goldfish, and most have two tails. Since the "Fancy" part is genetic, I thought the fantailed part was genetic. I don't think it is twinning,because as far as I know,twinning is not genetic. I have never seen a baby Fantail,but I am sure fish breeders don't divide embryos.Please tell me why some goldfish have two tails and if other animals could spawn a germ line similar to this. Thank you.

Interesting question. In my researches I came across the fact that all varieties of goldfish are the same species that you mentioned, Carassius auratus, and thus can be bred with each other. Fantail goldfish represent a lineage that has resulted from many generations of inbreeding goldfishes, analogous to the way that selective breeding of Canis familiaris has produced poodles, which are easily recognizable and distinguished from, say, Basset hounds. So yes, the fantail trait is genetic, although I doubt that the specific gene or genes responsible have yet been identified.

According to the Adelaide Aquariums fish store in Australia, fantail goldfish should have doubled caudal (tail) and anal fins and a short, deep body. Here's a photo from their website:

I was unable to find any information about how the doubled tail develops. However, I'd be willing to bet that, rather than being two separate fins, it begins as a single fin that divides (either partially or entirely) as the animal progresses through its early ontogeny. Information from a web site on goldfish types seems to confirm this hunch, stating that in the ideal fantail goldfish the two caudal fins are joined at the top. I'm not a developmental biologist, but it seems more likely to me for an anatomical structure such as a fish's caudal fin to be partially divided than for two of the structures to have grown separately and then subsequently fused together.

Nothing that I read indicated that the fantail trait results from anything other than selective breeding, so there's no reason to think that embryo manipulation is involved.

I hope this answers your question!

Allison J. Gong
Mad Scientist