Macrobenthic fauna in the shallow coastal zones of Nanao Bay, Ishikawa Prefecture: records of threatened species and habitat function of seagrass beds

Macrobenthic fauna were investigated at nine sites in shallow coastal habitats, including bare flats, seagrass beds, reed marshes, and hard substrate reefs, along the coast of Nanao Bay, Noto Peninsula (Japan). In the 2017 and 2018 surveys, we identified 296 taxa, including 40 Red List species. The actiniarians Paracondylactis hertwigi and Paranthus sociatus are the first records, and the gastropod Cerithium coralium is the northernmost records along the Japan Sea coast of Honshu Island. Of the 296 taxa, 166 occurred only at one site, indicating high β-diversity in the bay. Cluster analysis revealed that the macrozoobenthic assemblage structure differed significantly among the sites and/or years. Randomized block design sampling conducted in 2018 showed that macrozoobenthic structures differed significantly between bare flats and Zostera marina beds at the Wakura and Ninzaki sites. Several taxa, including the gastropod Alaba picta and the polychaetes Scoletoma nipponica and Prionospio krusadensis occurred more densely in Z. marina beds, indicating the contribution of seagrass to spatial heterogeneity in the macrozoobenthic assemblage structure. The macrozoobenthic diversity is comparable with that reported for major tidal flats along the Pacific Coast of central to southwestern Japan, indicating that the shallow coastal zones of Nanao Bay act as important habitats for macrozoobenthos, including endangered intertidal species. Habitat variability such as bare soft bottoms, hard substrates, seagrass beds, and reed marshes would contribute significantly to the α-, β-, and γ-diversity of macrozoobenthos in the bay system.