Divide or conquer? How hadal trenches and island arc ridges shape deep-sea isopod biogeography and diversity in the North Pacific

Understanding whether deep-sea trenches and island arcs ridges function as barriers to dispersal or as drivers of genetic differentiation is essential for clarifying the processes that generate and maintain biodiversity in the deep ocean. Herein, this question was investigated across three hadal trench systems (Aleutian, Japan, and Kuril-Kamchatka) and two island arc systems (Aleutian Islands and Kuril Islands) in the North Pacific, focussing on the integrative biogeographical and population genetic patterns of two deep-sea isopod families, the epibenthic Haploniscidae and endobenthic Macrostylidae. Haploniscids exhibited greater dispersal flexibility than macrostylids, resulting in wider distributional and bathymetric ranges, also inhabiting marginal seas. Still, trenches and especially island arc ridges acted as major barriers limiting faunal exchange, though barrier-crossing species displayed only weak genetic differentiation across these barriers. Instead, most species exhibited increasing genetic differentiation with geographic distance, suggesting Isolation by Distance. Species turnover rates for both families hinted at a bathymetric ecocline extending from mid abyssal to upper hadal depths, below which hadal endemic fauna took over and faunal turnover stagnated. Bathymetric structuring among closely related sister species within multiple species complexes indicated that depth might represent a major contributor to differentiation processes, detectable on an interspecific rather than an intraspecific level. Our findings showcase that areas of topographic complexity give rise to diverse biogeographic patterns for benthic isopod fauna in the deep sea. More elaborate population genetic investigations are necessary to disentangle the complex interplay of bathymetric gradients, geographic distances, topographic structures and isopod life histories that shapes genetic differentiation and speciation processes in the deep sea.