Histological divergence underlying globular body shapes in ornamental goldfish

Body shape diversity in vertebrates reflects a complex interplay between functional demands, environmental constraints, and internal developmental mechanisms. Various environments have promoted diverse morphological adaptations not only under natural but also domesticated conditions. One of the most remarkable examples of artificially induced morphology is found in the domesticated ornamental goldfish (Carassius auratus), which has diversified into numerous strains with strikingly different body shapes through prolonged human selection. In this study, we compared the body shapes of representative goldfish strains: the single-tail common goldfish (wild-type), Ryukin, Oranda, Pearl scale, and Ranchu. Our analysis revealed that the Ryukin and Pearl scale strains exhibit significantly greater body circularity in dorsal view compared to the other strains. Further anatomical and histological analyses showed that Pearl scale goldfish possess a thicker lateral body wall along with increased adipose tissue accumulation and reduced muscle fiber density, unlike Ryukin goldfish. These findings suggest that similar globular body shapes in different goldfish strains have arisen through distinct developmental pathways, exemplifying morphological convergence accompanied by histological divergence. We further discuss adipose accumulation in Pearl scale goldfish in relation to natural examples, providing insight into how function, morphology, and tissue organization may be interlinked in the evolution of globular body shapes.