Growth, histology, ultrastructure and expression of MITF and astacin in the pigmentation stages of green, white and purple morphs of the sea cucumber, Apostichopus japonicus

The body colour of sea cucumber, Apostichopus japonicus (Selenka), is an important trait affecting the price and taste of its products; the rare white and purple colour types are favoured and are more expensive in China. Identification of factors that may contribute to the differences among white, green and purple sea cucumbers will provide a scientific basis for improving the cultivability of the rare colour morphs. In this study, the growth, histology, ultrastructure and expression of microphthalmia-associated transcription factor (MITF) and astacin of two newly discovered white and purple colour morphs were investigated and compared with the common green morph, in the same breeding environment. At 50days old, the growth and survival rates and the extent of pigmentation were the highest in purple, then green colour morphs and were lowest in white morphs. The white morph had fewer, and less developed, epidermal melanocytes compared with the green and purple colour morphs. MITF and astacin expression, which are necessary for the formation of melanin and astaxanthin, respectively, were highest in purple morphs and lowest in white morphs. In addition, there were significant differences in growth rates and expression of MITF and astacin among early, mid and late pigmentation stages of each colour morph. The results suggested that growth and survival rates were highest in purple sea cucumber and lowest in white sea cucumber. Epidermis thickness and melanin content may be the newfound factors contributed to the differences in growth and survival rate among three colour morphs.