Response of daily and annual shell growth patterns of the intertidal bivalve Phacosoma japonicum to Holocene coastal climate change in Japan

It is well known that the latitudinal distribution of shallow-marine mollusks around the Japanese Islands has changed episodically during the Holocene in response to climate change; however, few studies have investigated the biotic response of individual species to Holocene climate change at daily to annual scales of temporal resolution. Holocene shells of the venerid Phacosoma japonicum recovered from various sites in Japan preserve lunar daily and annual growth increments in the outer shell layer and hinge plate, as also observed in modern shells. Sclerochronological analysis of 29 shells with known ¹⁴C ages revealed that the life history traits (e.g., length of the growth period and the mean lunar daily growth rate at a given age) have changed markedly over the past 8000 years. The intra-annual patterns of lunar daily increments and shell oxygen isotope records of the fossil shells can be compared with those of modern specimens from various locations, which are constrained by annual patterns of seawater temperature and summer-autumn precipitation rates. Furthermore, the temporal trends recorded in the fossil shells show a strong correlation with climate change in mid-latitude parts of East Asia over the past 8000 years, indicating that P. japonicum has changed its life history traits in response to Holocene terrestrial and coastal climate change.