Annual shell growth patterns of three venerid bivalve mollusk species in the subtropical northwestern Pacific as revealed by sclerochronological and stable oxygen isotope analyses

Annual shell growth patterns of the three venerid bivalve species, Gafrarium pectinatum, Pitar citrinus, and Katelysia japonica were investigated based on the results of sclerochronological and stable oxygen isotope analyses of live-caught specimens from the intertidal zone of Iriomote Island, southern Ryukyu Archipelago. In the study area, these three species temporally stopped shell deposition, when sea surface temperature (SST) dropped to 23–26 °C, during the first three years. However, the shutdown temperature for shell growth increased slightly to higher than 26 °C after 6 years old for G. pectinatum combined with a shortening in the length of shell growing period. Seasonal changes in daily shell growth in these species were controlled mainly by SST and primary production. Shell δ18O-derived summer temperatures recorded in the annual increments were higher by 3–5 °C than the highest SST records of the habitat. This data mismatch might be caused by an abrupt decrease in seawater δ18O values during the summer and fall typhoon seasons because of the influx of fresh water into the study area from nearby rivers. This study suggests that in the study area the annual shell growth patterns and shell δ18O values in the three species examined were controlled by mutually related biological and environmental factors such as ontogenetic age and seasonal changes in SST, salinity and primary production.