The twin tails of some prized goldfish varieties didn't arise naturally: Ancient Chinese fish aficionados deliberately bred for this trait. Chinese started keeping goldfish for their ornamental value around the year 1000, during the Song Dynasty. About 600 years later, during the Ming Dynasty, domesticated goldfish with one right tail and another on the left appear in historical records. Such fish are rarely seen in nature. Researchers have now traced this distinctive feature to a genetic mutation in a gene that controls development of the back end of the spine. In normal goldfish, there is a single set of bones in the tail. In goldfish with the mutation, the tail skeleton splits into mirror image left and right sides. The genetic mutation arose naturally, but then twin-tail goldfish varieties were "established and maintained by breeders in Song to Ming dynasty China," the researchers report online today in Nature Communications.