Climate-driven changes in marine environments can significantly alter the physiological performance and disease susceptibility of bivalve mollusks. This study re-evaluated the reproductive cycle and parasitic infection dynamics of the Manila clam, Ruditapes philippinarum, at a sand beach in Jeju Island. Adult clams were collected monthly from May 2019 to April 2020 and analyzed using histology and Ray’s fluid thioglycollate medium (RFTM) assays.　Over the 20-year interval, the mean sea surface temperature (SST) rose from 17.1°C to 17.8°C. The major spawning season (August–September) was delayed compared to the earlier study (July–August). Furthermore, the condition index (CI) was notably lower in the 2019–2020 cohort than in the 2001–2002 cohort. Histological analysis revealed a significant increase in Perkinsus olseni prevalence (73.3–96.7%) compared to the past (6.0–86.0%), with high infection intensities persisting year-round. While the elevated parasite burden is attributed to the slight but influential rise in SST, the decline in CI and delayed spawning are likely associated with low food availability (chlorophyll-a) in the Jeju coastal waters. These findings suggest that long-term environmental shifts are exerting cumulative stress on Manila clam populations through altered reproductive timing and intensified parasitism.

Sea Surface Temperature (SST), Ruditapes philippinarum, Perkinsus olseni, phenology, benthos