Oxygen Isotope Sclerochronology of the Giant Honeycomb Oyster Hyotissa hyotis off Jeju Island, Korea

The giant honeycomb oyster, Hyotissa hyotis (Linnaeus, 1758), is a large tropical gryphaeid bivalve that has recently formed a dense population in the temperate coastal waters off Jeju Island, Korea, likely facilitated by rapid ocean warming. Understanding the age and growth characteristics of this species is fundamental to assessing its ecological adaptation and emerging value as a shellfish resource in this temperate marginal habitat. We conducted a single-specimen, high-resolution oxygen isotope (δ18O) sclerochronological study of a giant oyster (shell height, 13.5 cm) collected from the subtidal rock bottom (depth, approximately 10 m) off the southern coast of Jeju Island. To circumvent analytical challenges posed by the vesicular honeycomb shell microstructure that is characteristic of the family Gryphaeidae, we targeted dense foliated calcite in the umbo (ligament area) and obtained 53 microsamples along a 20.8 mm hinge axis transect for δ18O analysis. The resulting profile revealed distinct seasonal cycles that, together with in situ sea surface temperature (SST) records from the nearby Seogwipo tide station (2000–2020), define a provisional chronology of approximately nine years (mid-2009 to September 2018) and annual hinge extension rates for eight complete years (2010–2017), together with partial first- and final-year intervals. Among the complete annual increments, hinge extension ranged from 1.6 to 3.2 mm year−1 (mean, 2.25 mm year−1), with pronounced interannual variability and a marked increase in 2017, which temporally coincided with elevated summer SSTs at Seogwipo station. The seasonal δ18O amplitude of the early record (~2.7‰) exceeds the amplitude expected from the local seasonal temperature range alone (~2.0‰), suggesting an additional contribution from seasonal variability in seawater δ18O or other unresolved influences. A comparison with published data for a single H. hyotis specimen in the northern Red Sea indicated broadly comparable hinge growth rates, although this comparison is limited by differences in specimen size, age, and increment definition between the two studies. Although based on a single specimen, this pilot study suggests that the H. hyotis umbo may represent a high-resolution archive of individual growth and seasonal hydrographic variability and provides a proof-of-concept baseline and preliminary age-at-size information for future multi-shell, multi-site sclerochronological investigations under tropicalization in the northwestern Pacific.