New insights from the deep: Meiofauna in the Kuril-Kamchatka Trench and adjacent abyssal plain

Community and abundance patterns of meiofauna and Harpacticoida in the Kuril-Kamchatka Trench, that trench's slope and adjacent abyssal plain which is located in the North-Western Pacific were investigated in relation to environmental variables. Overall, Nematoda was the dominant taxon of the meiofauna community (93%), followed by Copepoda (3%). Nineteen harpacticoid families and 1 subfamily were found, whereby the family Ameiridae was most abundant in the study area. Within eight selected (sub)families (Argestidae, Aegisthidae, Ancorabolidae, Canthocamptidae, Cletodidae, Cylindropsyllinae, Parameiropsidae, Zosimeidae) 27 genera were found, with Zosime and Mesocletodes being most abundant. Dispersion effects characterized the meiofauna community abundances, while the Harpacticoida communities in the hadal and abyssal were distinct. The main contributor to the dissimilarities observed were Nematoda, Ameiridae, Zosime and Mesocletodes. Food availability, sediment composition and water depth are environmental drivers structuring the investigated communities in the abyssal and hadal. For the trench slope, no correlations with the investigated environmental variables were found. Very likely variables other than the investigated ones act as environmental drivers for the trench slope (e.g. topography). Furthermore, a tendency of meiofauna to increase in total abundances was detected with increasing depth in the deeper stations starting at 6000 m. In contrast, total Harpacticoida abundances decrease with depth. Considering the abundance of Harpacticoida families and genera individually, the abundance of some taxa decreased in the trench slope but increased in the trench floor (Zosimeidae, Miraciidae, Argestidae, Parameiropsis, Zosime). Diversity indices based on Harpacticoida genus and family level were not distinguishable with the exception of equitability (J) at the genus level (abyssal vs hadal; trench slope vs hadal).