INCREASE IN Perkinsus olseni INFECTION IN MANILA CLAMS Ruditapes phillippinarum TRANSPLANTED FROM LOW TO HIGHER INFECTION AREA ON THE WEST COAST OF KOREA  

In this study, we experimentally transplanted Manila clam Ruditapes philippinarum from a low Perkinsus olseni infection to higher infection area to understand effect of clam density and progression of the infection in the clam body. Two to 3 years old adult clams with low level of P. olseni infection (25% prevalence and the intensity up to10⁴ cells/g tissue) were transplanted to a tidal flat where the infection has been reported as 100% prevalence with the intensity up to several million cells/g tissue. To test effect of the density on P. olseni infection, clams from the low infection area were mixed with native clams heavily infected with P. olseni and raised over 107 days from June to September, as the seawater temperature ranged 17 to 27°C. To monitor the progression in the infection, P. olseni density in the gill, mantle, adductor muscle, siphon, foot and visceral mass of each transplanted clam were determined using Ray's FTM and 2M NaOH digestion assay. From the beginning to 23 days, the prevalence in the transplanted clams increased to 50-76%. In July 23, 49 days after the transplantation, all transplanted clams showed 100% prevalence, while prevalence of the control clams remained 67%. The infection intensity in the transplanted clams increased dramatically, from 34 days (3,100-6,500 cells/g tissue) to 62 days (253,000-997,000 cells/g tissue). At the end of the experiment, 107 days after transplantation, intensity of P. olseni in the transplanted clams ranged 1.42 million -1.88 million cells/g tissue), which was comparable to the intensity of the native clams (3.33 million cells/g tissue) at the transplanted area. No clear effect of clam density on the prevalence or the infection intensity was observed. Mortality of clam observed 2 months after transplantation and the mortality reached 10% at the end of the study. Among different tissues of clam, P. olseni density was highest in the gill tissues, while the density was lowest in the adductor muscle. Fast increase in P. olseni infection intensity in the gill tissues and comparatively slow incline of the intensity and prevalence in the foot and the adductor muscle suggested that the main portal for P. olseni in clam is the gill tissues and as the infection progressed, P. olseni migrated to the muscular tissues, such as the foot and adductor muscle.