The sexuality of oysters is unique in that individuals with diverse sexual expressions may co-occur in a single species. However, little information is available on the sexual expressions in natural populations of oysters, which impedes to discuss the adaptive significance of the diverse sexual systems. In this study, we conducted field samplings and in situ experiments to examine sexual expression of Crassostrea gigas.
 First, during the reproductive season (spring to autumn) of 2012, the sex ratios was investigated in relation to shell length, sampling month, and gregariousness (whether they were solitary or clumped when collected). Almost all mature individuals were either males or females, with simultaneous hermaphrodites encountered very infrequently. The proportion of males was found to decrease with increasing shell length, although many large individuals remained male. The proportion of males was greater in clumped than solitary individuals.
 Second, we observed annual changes of sexual expression by following the same individuals in the field. The sex of collected specimens was identified by biopsy, and they were then returned to the sites of their collection during the reproductive season of 2013 with or without changing their gregariousness. When the individuals were re-collected the following year, sex change in both directions was observed. The frequency of sex change from male to female was higher when the oysters were placed solitarily.
 We concluded that C. gigas have a tendency of protandrous hermaphroditism and the ability to change sex in both directions. These findings suggest that sexual expression in C. gigas is labile, in that not all change sex from male to female at a fixed size. The sexual expression was also affected by the presence of neighboring individuals. Such labile sexual expression may be adaptive for sessile organisms.