Annual shell growth pattern of the Stimpson's hard clam Mercenaria stimpsoni as revealed by sclerochronological and oxygen stable isotope measurements

Mercenaria stimpsoni (Stimpson's hard clam) is a shallow-water bivalve species distributed along mid- to high-latitude coasts of the Northwest Pacific influenced by the Oyashio cold current. As this animal can live for decades, oxygen isotopes of its shell can potentially provide long-term seawater temperature and salinity data. However, little is known about the life history traits of M. stimpsoni, which hampers their use in paleoclimatology. Thus, we investigated the growth patterns of three live-caught M. stimpsoni individuals from Funakoshi Bay, especially focusing on juvenile ontogenetic stages. Sclerochronological and high-resolution oxygen isotope analyses suggest that young (below age 10) specimens of this species grow predominantly between spring and fall, but ceased growing during the cold season when sea surface temperature drops below approximately 10 °C. The results also show that ontogenetically older (> 10 yr) specimens grow shell material almost exclusively during summer. Oxygen isotopes in ontogenetically younger portions of the shells are a good indicator of past summer temperatures.