Cellular basis of metallic iridescence in the Siamese fighting fish, Betta splendens

Abstract

The ultrastructural morphology of chromatophores that gives rise to the highly valued metallic iridescence in the Siamese fighting fish (Betta splendens) was examined by transmission electron microscopy (TEM). Melanophores, iridophores, xanthophores, erythrophores, and leucophores were observed in the epidermis and dermis of B. splendens. Specific combinations of these chromatophores formed the pigmentation patterns of the dark blue, ultra-marine, turquoise-bronze, and golden strains. Under the TEM, large spherical melanosomes appeared which were highly electron dense in the cytoplasm of melanophores. These cells also possessed slightly electron dense organelles which could be partially melanized pre-melanosomes. The wide spectrum of metallic hues on the body of B. splendens is attributed to refraction, reflection, and thin-film interferences from reflecting platelets in iridophores that are closely associated with other chromatophore types. The designation "irido-melanophore unit" is proposed for one such close association which comprises an iridophore underlined by a melanophore. Varied inclination angles and thicknesses of reflecting platelets are elucidated to cause the iridescent sheen. Thin platelets oriented at small angles to the long axis of a cell produce purplish-blue hues whilst numerous thick and thin platelets at larger angles generate blue, green, and silvery-golden sheens.