Seasonal changes of the age composition and growth rate of a prosobranch, Heminerita japonica (Dunker), which is living on the surfaces of rock in the intertidal zone, were studied on the population of Imaura coast of Toba City on the Shima Peninsula. The samples were monthly collected from an intertidal rocky shore during the period from July 1977 to August 1980. The age composition and growth rate were analyzed by the method using probability paper based on the frequency distribution of shell length in each sample. The population of this species consists of four age groups of 0, 1, 2, and 3 years old. But, most individuals of 3 year-class die within several months after spawning in summer. The youngest snails start to live as a benthos from August, but, when the air temperature from spring to early summer is low, the time of larval settlement is retarded. In all age groups, the growth continues from April to November, but the growth is retarded during the winter when the air temperature is lower than 11℃. The growth rate of individuals of 1 and 2 year-classes is retarded in early summer because of spawning. Snails of 0 year-class reach the mean shell length of 4.5-4.75 mm in December, snails of 1 year-class 9.6-11.2 mm in December, snails of 2 year-class 13.3-14.6 mm in December and snails of 3 year-class 13.5-14.6 mm in July.
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