MONITORING Perkinsus olseni INFECTION INTENSITY AND PREVALENCE IN JUVENILE MANILA CLAM Ruditapes philippinarum TRANSPLANTED IN INTERTIDAL MUD FLAT AND A CAGE SUSPENDED IN WATER COLUMN

Perkinsus olseni has been reported as a major pathogen in Manila clams in Korean waters, causing mass mortalities and subsequent declining in the clam landings. To understand impacts of P. olseni infection on growth, we experimentally transplanted uninfected juvenile clams (11.6 mm in shell length) into a muddy intertidal where high level of P. olseni infection has been reported. The juvenile clams were also placed in a net cage and suspended in the water column (2-3 m depth) near the tidal flat. The infection intensity and prevalence of the clams were monitored biweekly over 103 days from June to September 2011 using Ray's fluid thioglycollate medium assay (RFTM) and the 2M NaOH digestion. At the intertidal mud flat, the first infection observed 30 days after the transplantation, with prevalence and intensity of 25% and 12,800±1,500 cells/g tissue. From 30 days to 58 days, the intensity and prevalence increased dramatically to 90% and 880,000±55,000 cells/g tissue, respectively. At the end of the experiment in September, the prevalence reached 100%, with a mean intensity of 1,210,000±64,000 cells/g tissue at the tidal flat. The mortality at the tidal flat varied from 0 to 47% at the end of the study. In contrast, the first infection was confirmed 58 days after the transplantation in the suspended culture, with a mean infection intensity of 620±100 cells/g tissue. The prevalence and infection intensity also increased markedly in the suspended cultured clams, from 58 days to the end of the experiment (103 days after), with the prevalence and intensity as 65% and 69,000±8,000 cells/g tissue. Mortality of the clams in the suspended culture was much lower, compared to the bottom culture, 4% at 71 day and 16% at the end of the experiment. Shell length of the juveniles in the suspended culture increased from 11.6mm to 29.0mm over 103 days of the culture, while shell length of the clams in the bottom increased to 18.0mm over 3 months period. The observed difference in the growth between the suspended and the bottom cultured clams in this study was in part, explained by the difference in level of P. olseni infection, which may disturb the early growth of Manila clams used in this experiment.