Environmental controls on daily shell growth of Phacosoma japonicum (Bivalvia: Veneridae) from Japan

This study examined the environmental factors controlling the daily shell deposition of the intertidal bivalve Phacosoma japonicum from Seto Inland Sea, west Japan, and Tokyo Bay, central Japan. Sclerochronological analyses of microgrowth patterns in marked-and-recovered specimens in- dicate that a pair of 2 etch-sensitive increments and 2 etch-resistant lines is formed every lunar day (duration 24.8 h). The accretionary pattern of the lunar day growth increments (LDGIs) reflects tidal cycles. Prominent growth lines were formed during spring tides, when the bivalves were subaerially exposed, and weak ones were deposited during neap tides, when they were continuously submerged. The bivalves stop secreting shell carbonate during winter and early spring. The time interval encom- passed by the winter break in the specimens from Tokyo Bay lengthened as the shells grew older. Although seawater temperature is the main controlling factor for shell growth, a number of mutually related environmental factors such as salinity and food availability also affect shell growth. In Tokyo Bay, the broadest LDGIs were deposited between temperatures of 21 and 24°C. Our findings provide a basis for the interpretation of the temporal changes in shell microgrowth patterns in terms of environmental conditions of extant and fossil P. japonicum specimens.