Pigment Distribution and Secretion in the Mantle of the Pacific Oyster (Crassostrea gigas)

The color of Mollusca shells is one of the most important attributes to consumers. At the cellular level, black color is mainly from the melanin produced by melanocytes. The melanosome is a specialized membrane-bound organelle that is involved in melanin synthesis, storage, and transportation. How the complex pigmentation process in the Crassostrea gigas is established remains an open question. The objectives of this studies are to examine the morphological characteristics of melanosomes or melanin of mantle pigmentation in the Pacific oyster, thereby investigating its contribution to shell color. The results show that pigmented granules of the mantles vary among the three lobes, and the melanosomes at different stages are enriched in distinct cargo molecules, which indicate the remarkable difference between the marginal mantle and central mantle. Examination of mantle histology reveals that the mantle margin of the oyster is characterized by three different folds, including the outer secretory, middle sensory, and inner muscular fold. Ferrous ion chelating assays against the tyrosine hydroxylase indicate that a large amount of melanin is localized in the inner surface of the middle fold. Transmission electron microscopy analyses show that the mantle edge is composed of tall columnar and cuboidal epidermal cells and some pigmented melanocytes intersperse among these cells. The numbers of melanosomes among the three lobes are different. In the inner fold and the middle fold of the mantle, some single dispersion, or aggregation of melanosomes with different degrees of melanization are found in the outer surface. Numerous melanosomes are distributed in the epithelium of the outer fold of the mantle, and mainly are at the apical microvillar surface near the lumen. However, melanosomes are occasionally observed in the central mantle, and they are relatively less. This work provides new insights into the process of melanin deposit in the mantle and shell pigmentation in C. gigas.