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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 indicate 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 encompassed 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.

This study investigates shell growth patterns and the reproductive cycle of the Mediterranean mussel Mytilus galloprovincialis in Tokyo Bay, central Japan. A follow-up study of marked mussels within the intertidal zone from July 2007 to September 2008 indicates a remarkably fast annual shell-growth rate and a short life span, reaching the maximum shell length (ca. 70 mm) at age 3 or 4 years. Histological examinations of mussel gonads collected fortnightly revealed that the Tokyo Bay population has a spawning season between late autumn and spring, and attains sexual maturation several months after recruitment. Shell growth patterns for individual mussels show seasonal variations, with high shell-growth rates during spring and summer, and minimal growth from late autumn to early spring. Slow shell growth during winter is possibly controlled by combined environmental (lower air and seawater temperatures, and scarce phytoplankton) and physiological (the reproductive effort expended in gametogenesis and spawning) factors.