Comparison of Marteilioides chungmuensis infection in the Pacific oyster Crassostrea gigas cultured in different conditions

Experiments were conducted to ascertain if there is any link between infection with Marteilioides chungmuensis, an ovarian parasite of Pacific oyster, Crassostrea gigas and the gonad development of the host. Oyster spat were maintained for 1 year at three different daily time periods in seawater: intertidal zone group (IG) with a daily average of 6 h in seawater, submerged group (SG) with 24 h in seawater and transferred group (TG) with the first 6 months in the IG and the latter in the SG. Bi-monthly samplings were made, and gonad development and parasite prevalence among the different groups were compared. The lowest prevalence of infection was detected in the IG oysters, while the highest was in the SG. With respect to the sex ratio, the IG oysters showed a significantly lower female ratio than the SG. Since spore formation of M. chungmuensis occurs in female oysters only, the lower prevalence in the IG was considered to be due to the smaller number of females. By controlling the sex ratio, as by changing the culture method, reduction in prevalence of M. chungmuensis infection can be achieved.