Abstract Bathynomus doederleini Ortmann, 1894, a deep-sea isopod, is a benthic scavenger in the northwestern Pacific. Little is known about its habitat and intraspecific variation in body size in relation to depth. We examined habitat depth, body-size distribution, and range limits of B. doederleini along the western coast of Kyushu, Japan using baited traps deployed at of 151–821 m deep. A total of 1,152 individuals were collected, with the highest catch per unit effort (CPUE) between 400 and 500 m. CPUE declined sharply below 700 m, likely due to thermal constraints and interspecific competition. Body size varied significantly with depth. Minimum body size increased with depth, whereas 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 tend to be underrepresented in baited traps. Results show that B. doederleini exhibits depth-related niche partitioning. Our study area likely represents the northernmost distribution limit of the species in the East China Sea. The study highlights the importance of trap type and environmental gradients in understanding the ecology of deep-sea species.
Anzai et al. (Wed,) studied this question.