Molecular detections of 14 amphidomatacean species (Dinophyceae) and their temperature-associated distribution in subarctic waters off eastern Hokkaido, Japan

Abstract To better understand the diversity and ecological characteristics of the amphidomatacean dinoflagellates around Japan, their genetic diversity was examined using ITS2-based environmental DNA (eDNA) metabarcoding and cell densities of toxigenic species Azadinium poporum and Az. spinosum were estimated using LSU rDNA-based quantitative PCR (qPCR). Samples were collected in the offshore waters off the coast of Hokkaido, Japan, during the summer in 2023 and 2024. eDNA metabarcoding revealed the presence of 14 species (11 Azadinium and three Amphidoma), including Azadinium concinnum previously not found in the western North Pacific. The presence of Am. languida was also detected based on the comparison of ITS phylogeny with the toxigenic strains. Five species predominantly detected in the 2024 cruise exhibited their temperature-associated distribution in the species and intraspecific group (ribotype) levels; Am. fulgens, Az. dexteroporum, and Az. spinosum (ribotype A) were found in the warm-water (warming Kuroshio-Oyashio mixed water), while Az. dalianense, Az. polongum, and Az. spinosum (ribotype C) in the cold-water (surface-warmed subarctic water). Moreover, species-specific qPCR analysis of two toxigenic species revealed the predominant distribution of Az. spinosum in the pelagic western North Pacific for the first time, while Az. poporum was rarely detected.