Environmental and physiological controls on shell microgrowth pattern of Ruditapes philippinarum (Bivalvia: Veneridae) from Japan

The reproductive cycle and shell microgrowth patterns of the venerid bivalve Ruditapes philippinarum from Matsukawa-ura, a small inlet facing the Pacific Ocean, northeastern Japan were examined. Histological examination of the gonads revealed that spawning in this species occurred twice in 2005, once between late June and early August and the other between late September and early October. Comparison of the shell microgrowth patterns with the developmental stages of the gonad in each individual revealed that in spawning individuals (mature and spawning stages), the mean lunar-day growth rates were significantly smaller than those in individuals which were not in spawning condition (spent, recovery and growing stages). In non-spawning individuals, the mean lunar-day growth rates were positively correlated with seawater temperature up to 20°C. However, in spawning individuals, no correlation was observed between shell growth and seawater temperatures >16°C. These facts suggest that physiological stress during reproduction has a negative influence on shell growth and results in spawning breaks. The present study indicate that spawning breaks can be used to identify the timing of sexual maturity and to count the number of spawning events occurring per year in extant and fossil bivalves.