Pigment cells representing polychromatic colony color in Botrylloides simodensis (Ascidiacea, Urochordata): Cell morphology and pigment substances

The colonial ascidian Botrylloides simodensis displays multiple body colors - yellow, orange-red, violet, black and white - in a clonal colony. The colors are due to pigmented blood cells that exist particularly around a branchial siphon or on an atrial languet of individual zooids. These pigment cells are distributed in mesenchymal space or vascular lumen, and many of them are loosely bound to the epithelium. In the space, there are also colorless blood cells that are circulating with blood. When the colorless blood cells are isolated and cultured, some of the cells produce colored substances and change to pigment cells. Therefore, it is presumed that the pigment cells are derived from colorless blood cells. The pigment cells in this ascidian have a spherical shape with no dendrites and contain many granules. Electron microscopic observation showed that there are several different types of granules, and all types of granules are similarly packed in a large vacuole in the cytoplasm. Chemical analysis disclosed that the pigmentary tissues contain carotenoids, pteridine, and purines that are known pigments in vertebrates. The main components of black and violet pigment cells are still unidentified. The former may be a melanin-like substance, but a significant dose of eumelanin or phaeomelanin was not obtained. Although ascidian pigment cells show a certain extent of similarity to vertebrates in their pigments, their cell structure is quite different from that in vertebrates. Because of the phylogenetic relationship between ascidians and vertebrates, it is assumed that ascidian pigment cells might be a primitive type of those in chordates, although these ascidian cells seem to have a unique origin and function.