Treatment effects during the post-operative care on the rate of pearl-sac formation in the pearl oyster Pinctada fucata

Abstract

The pearl culture industry using the Akoya pearl oyster Pinctada fucata in Japan has been suffering from low productivity due to the proportion of low-quality pearls, which are not round in shape and have blemishes on the surface, is fairly high with over 70% of pearls effected. Recent studies indicate that the immersion treatment of oysters in low salinity seawater (25psu) in a tank for 8days just after the implantation of nuclei has a positive effect on the production of high-quality pearls compared to the suspension of oysters in the sea (33psu), so-called the “conventional method”, throughout two weeks of the post-operative care. However, a mechanism to increase the proportion of high-quality pearls in low salinity seawater is not fully known. In this study, we compared the periods from the implantation of nuclei to the completion of pearl-sac formation among implanted pearl oysters nursed in tanks containing low salinity (25psu) or normal salinity (33psu) seawater in tanks and in the sea (33psu). After the immersion of the implanted oysters in 25 or 33psu seawater in the tanks for 8days, animals were suspended from a raft in an inland bay for the remainder of post-operative care period. The implanted oysters treated with the conventional method were all fastened from the raft throughout the post-operative care period. The oysters from the three treatments were 10% formalin-fixed at periodic intervals and thin slices of the nucleus-implanted part of body were prepared for the examination of the process of pearl-sac formation. The oysters suspended from the raft in the sea throughout the post-operative care period showed the shortest interval from the implantation to the pearl-sac formation, followed by those in normal salinity and low salinity treatments, respectively. This trend corresponds well to that in the proportion of high-quality pearls among the three treatments. The results of the present study suggest that the delay of pearl-sac formation is most likely one of the important factors in preventing deformities and blemishes of pearls in future culture production.