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 the natural but also domesticated conditions. One of the examples of the drastic morphological changes in body shape under the artificial condition (i.e. with significant human intervention) is the ornamental goldfish (Carassius auratus). Ornamental goldfish were originally derived from wild crucian carp. Over time, they diversified into numerous strains with strikingly different body shapes. 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 significangoldfish in relation to naturaltly 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 discussed the adipose accumulation in Pearl scale goldfish in relation to natural examples, aiming to investigate possible links between function, morphology, and histology.