ANNUAL VARIATION IN THE GROWTH-RATE OF YOLDIA-NOTABILIS (BIVALVIA, NUCULANIDAE) IN OTSUCHI BAY, NORTHEASTERN JAPAN, ANALYZED USING SHELL MICROGROWTH PATTERNS

Annual variation in the growth rate of the protobranch bivalve Yoldia notnbilis (collected in Otsuchi Bay, northeastern Japan between 1989 a nd 1991) was determined by measuring shell length at successive growth lines. Factors affecting growth rates were assessed by examining long -term environmental data. Shell cross-sections of Y. notabilis showed a clear pattern of internal growth lines which formed simultaneously w ith the annual external lines on the outer shell surface. Hence, they were used as an age marker. A maximum lifespan of 17 yr was determined for this species, and a sigmoidal growth curve was obtained by the in ternal growth line analysis. Examination ofthe growth lines also revea led a large annual variation in growth rate, fluctuating as much as 32 -fold during the past 9 yr. The variation correlated negatively with w ater temperature and positively with chlorophyll a content in the wate r column. It is suggested that the variation in annual growth rate is dependent on food supply during the spring phytoplankton bloom which v aries from year to year according to the flow of the cold Oyashio curr ent each spring.