Coloration and Chromatophores of the Domino Damsel, Dascyllus trimaculatus

The general background hue of the skin of the domino damsel (Dascyllus trimaculatus; Pomacentridae) is velvet-black, but three white spots, one on the median parietal region and the other two around the base of the dorsal fin, form a characteristic, conspicuous pattern for this species. In the dark background region, dermal melanophores form a dense monolayer, while in the white spots, a thick accumulation of non-dendritic iridophores is present in the dermis, which is lined with a layer of melanophores. The melanophores in the velvet-black area extend their processes parallel to the plane of the skin, and respond to various stimuli by aggregating or dispersing their melanosomes, which result in the hue changes there. Iridophores in the white spots contain many piles of thin light-reflecting platelets which are responsible for the whiteness of the spots through the multilayered thin-film interference phenomenon. In response to certain stimuli, the spectral reflectance peak shifts to some extent, and such responses may be due to simultaneous changes in the distance of platelets in those piles. The melanophores underlying the white spots extend their cellular processes through spaces among iridophores in order to cover them. When melanosomes migrate peripherally into the tips of the dendrites, they cover the underlying iridophores and the spot darkens; the whiteness reappears when the melanosomes move back down the dendrites into the perikarya of the melanophores.