A deep abyssal natural wood fall in the Northwestern Pacific and its associated fauna

Plant remains from land, particularly sunken wood, represent significant input of organic matter into the generally oligotrophic deep sea, leading to the formation of macrofaunal assemblages around them. Following the onset of decomposition by wood-boring bivalves, microbes and fungi the wood eventually becomes an anoxic and reducing environment attracting chemosynthetic fauna. Wood falls have primarily been studied by either opportunistic sampling by trawls or artificial deployments, while in situ observations of natural systems remain scarce. Here, we report a natural late-stage wood-fall community at 5505 m depth in the Northwestern Pacific abyssal plain by a manned submersible. Our observations revealed details in faunal distribution that cannot be obtained by trawled material, including the presence of associated xenophyophores. We identified a total of 26 animal species and one xenophyophore from the wood fall including both wood-associated and chemosymbiotic taxa, although cocculiniform limpets were conspicuously missing. Living xylophagaid bivalves were also not observed, but the wood was heavily burrowed and they were clearly once present. This rare finding improves our understanding of deep abyssal wood falls at the senescence stage, and highlights the importance of direct seafloor observations.