Hypermastus tokunagai is a eulimid gastropod infecting the sand dollar Scaphechinus mirabilis. The eulimid is easily detached from the host and thus regarded as a temporary parasite. In this study, the growth and reproductive cycle of H. tokunagai were investigated using specimens collected from S. mirabilis in the western Seto Inland Sea, Japan. Newly recruited individuals of H. tokunagai appeared in early autumn and grew rapidly, while growth was retarded from mid-autumn to winter months. A second rapid growth occurred the following summer. Based on the growth pattern, the life span of H. tokunagai was estimated to be more than 20 months. This longer life span compared to other eulimids could be due to the accessibility and population stability of its host S. mirabilis. Maturation of H. tokunagai occurred within a short period in the summer, and was synchronized with the second growth phase. Hypermastus tokunagai showed size dimorphism between sexes and an investigation on change in shell shape with growth revealed that this characteristic can be attributed to the female's acquisition of a larger shell within a short time during its maturation period.
