Seasonal changes in the assimilation energy of Neptunea arthritica in Usu Bay, Hokkaido were examined. In adults, energy directed to growth was largest in summer, and energy accumulated as reproductive output was largest in autumn. The assimilation energy levels that sustained metabolism during those periods were considerably higher than during winter and spring. The assimilation energy levels in immature individuals showed less variation than in adults.
Net growth efficiency exhibited a convex curve with a peak (49.4%) at 2 years old. In both males and females, the proportion of assimilation energy utilized for reproductive output increased with age. The proportion of investment in females was 3 to 6 times that of males, owing to the production of egg capsules. The production efficiency of adults remained relatively constant at 35 to 40%, except for 5 years male, which showed 25.0%.
