Abstract:
Previous studies have shown that the apicomplexan protist
Perkinsus olseni induces both sublethal and lethal effects on the
Manila clam (Ruditapes philippinarum) along the west and south
coasts of Korea, including Jeju Island. In this study, we assessed
the prevalence and infection intensity of P. olseni in Manila clams
from sand beaches on Jeju Island in July 2022, using Ray's fluid
thioglycollate medium assay (RFTM) and histology. Condition
index (CI) as the proxy of the fitness of clams (i.e., the ratio of
dry or wet tissue weight to the shell weight) collected in July
2022 ranged from 74.5 (Jongdal-ri) to 122.9 (Segwipo), or 0.27
(Moseulpo) to 0.62 (Segwipo). The mean infection intensity,
measured as P. olseni cells per gram of gill tissue in clams
collected from six sites, ranged from 0 (Geumneung, Gimnyeong,
and Moseulpo) to 56,000 cells/g of gills (Seongsan), with
prevalence rates (i.e., the percentage of infected clams) ranging
from 0 to 47%. Similarly, infection intensity in total tissue ranged
from 0 (Geumneung and Gimnyeong) to 47,870 cells/g of tissue
(Seongsan), with prevalence rates between 0 and 83%.
Histological analysis corroborated these findings, revealing low
infection intensity, with infection scores ranging from 0 to 1.0
across the six sites. Despite rising sea surface temperatures in
Jeju Island over recent decades due to global warming, P. olseni
infection prevalence and intensity appear to have remained stable. The low density of Manila clams in Jeju Island may partially
explain the observed low infection intensity and prevalence.
