Diversity and distribution of megafauna community structure from abyssal to hadal zone in the Mariana Trench

The Mariana Trench has attracted research studies from various directions, mostly to the CHALLENGER DEEP. Yet, the bathymetric diversity trend across this geographical region and the corresponding ecological drivers remain vague. In this study, we examined the megafauna community structure and the environmental factors influencing their distribution patterns across the full span of ocean depths (4066 – 10900 m) in the Mariana Trench. An effective dataset was generated from twenty-three dives of the human-occupied vehicles “JIAO LONG” and “FEN DOU ZHE” video transects, which covered a distance of approximately 20 km. A total of 1,233 megafauna individuals were recorded from 144 morphospecies, identified into nine phyla, of which Echinodermata was the dominant fauna group in abundance and diversity. Along with other taxon groups, Holothuroidea and Cnidaria exhibited wide distribution across depths and were the common taxa characterising extreme depths (>10 km). Clustering analysis revealed eight distinct assemblages, of which five and one were exclusively from hadal and abyssal zones, respectively, whereas two assemblages were mixed groups. The result of the PERMANOVA test indicated significant differences in species composition between the abyssal and hadal zones. Environmental parameters of depth, slope, and latitude were identified as significant explanatory variables shaping faunistic components. By offering a non-invasive approach to studying bathymetric diversity patterns, this study accentuates a comprehensive view of the world's deepest megafauna communities and highlights megafauna transition depth between abyssal and hadal zones in the Mariana Trench, serving as a reference guide for future hadal zone ecological studies.