Community structure of benthic amphipods in subtidal bottoms of Akkeshi Bay, northeastern Japan: effects of environmental gradients and biogeographic affinities

Abstract Gammaridean amphipods are a major component of macrobenthic communities, but information on their distribution and ecology in subtidal habitats of northern Japan is limited. We present the species composition and community structure of benthic amphipods in the subtidal bottoms (5–32 m deep) of Akkeshi Bay, eastern Hokkaido, Japan, and examine whether spatial variability of amphipod assemblage is related to local environmental gradients and biogeographic affinities of the component species. Amphipods were collected at nine sites in 2020 and identified to 40 species consisting of 21 families. Similarity analyses showed that the amphipod community differed among the three habitats: (1) marine soft-bottom substrate, (2) gravelly estuarine substrate, and (3) marine hard-bottom substrate. Distance-based redundancy analysis revealed that variation in the soft-bottom community was related to that of depth and sediment composition. The biogeographic affinities of 15 identified species were categorized as two groups, “Arctic and surroundings” and “Northwestern Pacific,” by similarity analysis with global occurrence records in marine ecoregions. Differences in species composition were related to the biogeographic affinities of the component species, with species grouped as Arctic and surroundings occurring at deeper sites than the Northwestern Pacific species. Changes in community structure along the depth gradient reflected variations in environmental factors, such as temperature, as well as differences in the suitable temperature ranges of the species as determined by their biogeographic history. This study highlights the importance of considering both environmental gradients and macroecological features of component species in interpreting variations in amphipod community structure on the local scale.