A series of submersible surveys to nearly 10 kilometres in the deepest parts of Japan's ocean trenches have revealed seafloor biodiversity is shaped by depth, food supply, seismic activity and the seafloor's structure.
Dr Denise Swanborn, from The University of Western Australia's School of Biological Sciences and Minderoo-UWA Deep-Sea Research Centre, was lead author of the study published in the Journal of Biogeography.
Researchers analysed video from six crewed submersible dives at hadal depths of 6,939 metres to 9,775 metres in three Japanese subduction trenches – the Japan, Ryukyu and Izu-Ogasawara trenches.
The video observations led to the documentation of nearly 30,000 organisms, spanning 70 morphological groups in 11 taxonomic categories across eight habitat types.
"This provided one of the most detailed observations of seafloor biodiversity and habitats at these depths," Dr Swanborn said.
"We found differences in community composition and diversity between trenches, linked to depth and nutrient input from surface waters."
The nutrient-rich Japan Trench supported high abundances of sea cucumbers and other deposit feeders at 7.5 kilometres depth.
The same depth in the more food-limited Ryukyu Trench was dominated by very different communities where sea cucumbers were nearly absent and instead brittle stars where found.
In the Izu-Ogasawara Trench on a dive area near the Boso Triple Junction to a depth of about nine kilometres, extensive crinoid (sea lilies) meadows were discovered.
"Within trenches, at the same depth band, differences in historical seismic disturbance and seafloor stability created different communities," Dr Swanborn said.
"For example, historically seismically active areas in the Japan Trench were dominated by low-diversity, organisms that had adapted to their environment, while the more stable under-riding slope supported more diverse communities.
"Hadal video surveys of this scope are exceptionally rare and our findings of how depth, regional setting and seafloor disturbance interact to structure biodiversity has provided the foundation for future hypothesis-driven ecological research in the deepest parts of the ocean."
