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̶ Population structure, growth and reproductive activities were investigated over one year for the boring isopod Sphaeroma sieboldii Dollfus, 1889 for a population burrowing in sand-stone. The occurrence of heterosexual pairs peaked in spring, followed by the peak occurrence of ovigerous females and females cohabiting with early juveniles in the summer. This suggests the reproductive season is from spring to summer. For adult males that had penes, three reproductive stages were recognized: large-sized males with mature appendices masculinae, intermediate-sized males with immature appendices masculinae, and small-sized male without appendices masculinae. Most of the males pairing with females were large-sized males with mature appendices masculinae, while most of the pairing females were smaller than the partners, carrying small, non-functional oostegites. Males with mature appendices masculinae were frequent from winter to summer, covering the reproductive season. Size-assortative pairing was found in the peak season of heterosexual coupling. Sex ratios were female-biased in all months. Observation of the interior reproductive organs confirmed that adult females had ovaries, while most males had both ovaries and male reproductive tracts. Some males with mature appendices masculinae were found to have degenerated ovaries. These findings suggest that protogynous sex change occurs in S. sieboldii.

Life history of an intertidal boring isopod, <i>Sphaeroma</i> <i>sieboldii</i> Dollfus, 1889