Growth and reproductive cycle of <i>Hypermastus tokunagai</i> (Caenogastropoda: Eulimidae), an ectoparasite of the sand dollar <i>Scaphechinus mirabilis</i> (Clypeasteroida: Scutellidae) in the Seto Inland Sea, Japan

<jats:p><jats:italic>Hypermastus tokunagai</jats:italic> is a eulimid gastropod infecting the sand dollar <jats:italic>Scaphechinus mirabilis</jats:italic>. The eulimid is easily detached from the host and thus regarded as a temporary parasite. In this study, the growth and reproductive cycle of <jats:italic>H. tokunagai</jats:italic> were investigated using specimens collected from <jats:italic>S. mirabilis</jats:italic> in the western Seto Inland Sea, Japan. Newly recruited individuals of <jats:italic>H. tokunagai</jats:italic> 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 <jats:italic>H. tokunagai</jats:italic> 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 <jats:italic>S. mirabilis</jats:italic>. Maturation of <jats:italic>H. tokunagai</jats:italic> occurred within a short period in the summer, and was synchronized with the second growth phase. <jats:italic>Hypermastus tokunagai</jats:italic> 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.</jats:p>