Hadal copepods in and around: A metabarcoding Survey of meiofauna in the Aleutian trench and adjacent regions

• This is the first detailed metabarcoding study conducted in a hadal trench. • The Aleutian trench does not inhibit distribution of harpacticoid species. • Some detected harpacticoids show broad distributions, ranging from the CCZ to deep-sea areas around the Aleutian Islands. Hadal trenches remain among the least explored marine environments, with much of their faunal diversity still unknown. It is still debated whether trenches act as dispersal barrier for benthic organisms potentially leading to distinct faunal communities within and around them. This study aimed to investigate the meiofauna diversity in and around the Aleutian trench using a multi-gene metabarcoding approach. Two genetic markers, CO1 and 18S, were used to assess the potential uniqueness of hadal, abyssal and bathyal habitats. To improve the accuracy of taxonomic assignments, a curated reference library of harpacticoid species was created. All sampling sites revealed unique meiofaunal communities; however, both markers consistently detected a clear separation between hadal and abyssal assemblages. While certain taxa were underrepresented in the CO1 dataset compared to 18S, likely due to primer bias, the overall patterns in community structure remained consistent. Besides depth, Chl- a (for CO1) and TOC (for 18S) emerged as key environmental factors influencing meiofauna community composition. Notably, eight COI OTUs were detected across sites spanning the trench, five of which were also detected in the Clarion-Clipperton Zone, suggesting that the trench does not necessarily act as a strict barrier to dispersal. One of these OTUs was assigned to genus Dorsiceratus − a taxon rarely encountered in previous deep-sea surveys − demonstrating the potential of metabarcoding, especially when combined with curated reference libraries, to reveal hidden biodiversity.