Life cycle and seasonality of Idarcturus trispinosus Kim & Yoon, 2021 (Isopoda: Valvifera: Arcturidae) in Izu Oshima, Japan

The life cycle and ecology of species of the family Arcturidae Dana, 1849 (Isopoda: Valvifera) remain largely unknown. Monthly quantitative sampling of Idarcturus trispinosus Kim & Yoon, 2021 on the coast of Izu Oshima, Japan from December 2023 to February 2025 examined the morphology and body length of 1,243 individuals. We identified the developmental stages in the life cycle based on external morphology. Males mature after passing through three manca stages, whereas females mature after passing through three manca and two immature stages. While males die without molting after reaching maturity, females may molt again after reaching maturity. The brood size of adult females ranged 1–27 and showed a positive correlation with body length, with some large-size adult females having small brood sizes. The number of individuals was low in winter and increased from spring through fall. Ovigerous females were found year-round, but their proportion was high in most months from spring through fall, and the mean brood size and egg size were larger during this period. The protrusion of pereonite 4 (the basis of the oostegite for egg-laying) became more pronounced with increasing body length and degree of maturity. Body pigmentation blended in with the red-algae habitat. Most developmental stages were brown but the proportion of red individuals increased in large adult females.