MICROGROWTH PATTERNS AND SHELL ISOTOPE RATIOS OF THE DEEP-SEA HYDROTHERMAL VENT BIVALVE MOLLUSK BATHYMODIOLUS BREVIOR FROM THE NORTH FIJI BASIN, PACIFIC OCEAN

Bathymodiolus brevior Cosel, Métivier & Hashimoto 1994, a bivalve mollusk living at deep-sea hydrothermal vents, exhibits daily microgrowth structures in its shell. This interpretation is substantiated by various lines of evidence. (1) Similar shell portions of contemporaneous specimens from the same locality contain almost the same number of microincrements; (2) The number of microincrements coincides with the expected number of days in which shell portions of Bathymodiolus spp. shells form; (3) The width of such microincrements compares well to daily growth rates estimated for the close relative, B. thermophilus Kenk & Wilson 1985; (4) Different specimens from the same site show similar microgrowth curves. In addition, we found support for tide-controlled shell growth. Daily shell growth rates fluctuate on a fortnightly basis. Some shells portions also revealed the typical tide-controlled microgrowth pattern commonly observed in intertidal bivalves. Based on the analyses of lunar daily growth increments, a growth curve for B. brevior was computed which enables estimation of ontogenetic age from shell length. According to this equation, B. brevior reaches its maximum shell length of 14cm at about age eighteen. Shell isotope analyses suggest that some major shell growth interruptions or retardations are related to extremely active hydrothermal venting activity. However, shell growth also stopped during periods of low venting implying physiological controls on shell formation. Results of the present study demonstrate that shells of B. brevior function as calendars and environmental data loggers that can complement or partly substitute long-term observations of venting systems.