Effects of size and gregariousness on individual sex in a natural population of the Pacific oyster Crassostrea gigas

Crassostrea oysters show intraspecific variation in sexuality, including dioecy as well as sequential and simultaneous hermaphroditism, but their sexuality under natural conditions has rarely been reported. We examined sexuality and the effects of body size and gregariousness on sexuality in a natural population of the Pacific oyster C. gigas in southern Japan. First, monthly samplings were made during their reproductive season (May -August 2012) to study the sex of wild individuals. The proportion of males was found to decrease with increasing shell length (SL) and was greater in gregarious than solitary individuals. Simultaneous hermaphrodites were encountered infrequently (c. 1%). The negative relationship between the proportion of males and SL was also observed among the new cohort that had settled within one year. Next, we directly observed sex change by inspecting the sex of the same individuals through biopsy in two consecutive years (2013-2014; long-term experiment) or within a reproductive season (May-August 2014; short-term experiment) in the same population. Manipulation of the gregariousness (whether solitary or gregarious) was also conducted at the beginning of the experiments. When the individuals were re-collected, sex change in both directions was observed both in the long- and short-term experiments. Moreover, sex change from male to female was promoted when individuals were made solitary rather than gregarious. In summary, our study has shown that C. gigas has a tendency for protandry and has the ability to change sex in both directions under natural conditions in an area where they are native. Although within-season sex change of Crassostrea species has heretofore been unknown, C. gigas can change sex even in the short term. We suggest that such plasticity and the resulting diverse sexuality may be adaptive for sessile organisms such as oysters.