Intraspecific differences in habitat depth in a deep-sea isopod, Bathynomus doederleini (Crustacea: Isopoda: Cirolanidae), off the west coast of Kyushu, Japan

The giant deep-sea isopod, Bathynomus doederleini, is a benthic scavenger distributed in the northwestern Pacific. Despite its ecological importance, little is known about its habitat use and intraspecific variation in body size in relation to depth. In this study, we examined the habitat depth, size structure, and distributional limits of B. doederleini off the western coast of Kyushu, Japan, using baited traps deployed at depths ranging from 151 to 821 m. A total of 1,152 individuals were collected, with the highest catch per unit effort (CPUE) observed between 300 and 500 m. CPUE declined sharply below 700 m, likely due to thermal constraints and interspecific competition. Body size distribution varied significantly with depth: minimum body size increased with depth, while maximum body size remained constant. Smaller individuals were more abundant in shallower, warmer waters, suggesting ontogenetic habitat segregation possibly driven by metabolic and competitive factors. No brooding individuals were captured, supporting previous findings that reproductive females avoid baited traps. These results suggest that B. doederleini forms a reproductively active population in the East China Sea, with ecological adaptations to thermal conditions and depth-related niche partitioning. This study highlights the importance of trap type and environmental gradients in understanding deep-sea species ecology.