Evolutionary Insights into Muscle Fiber Distribution in the Twin Tails of Ornamental Goldfish.

Twin-tail ornamental goldfish have a bifurcated caudal fin with a morphology that is extremely divergent from the conventional body plan of the vertebrates. Here, we investigate the musculoskeletal histology of this bifurcated caudal fin. From some of the investigated twin-tail goldfish, we found a twin-tail goldfish specific muscle (hereafter referred to as the "medial caudal muscle") between left and right bifurcated caudal fin skeletons. Our immunohistochemical analyses revealed that the medial caudal muscle showed laterally biased distribution patterns of the slow and fast muscle fibers. Similar distribution patterns were also commonly observed in several deep muscles of wild-type goldfish as well as zebrafish, suggesting that these muscle fiber distribution patterns are formed by the same molecular developmental mechanisms even though their morphologies are highly diverged. These findings provide empirical evidence to consider how the histological features of a newly emerged morphology are influenced by selective pressures and pre-existing developmental mechanisms.