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

ABSTRACT

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.

KEYWORDS:

• Abyssal plain
• chemosynthesis
• organic fall
• Provanna
• sunken wood

Acknowledgements

We are grateful to the captain and crew on-board R/V YOKOSUKA during the research cruise, and extend our gratitude to the DSV SHINKAI 6500 team for their efforts in sample collection – especially Satsuki Iijima who was the chief pilot of the dive. Sakura Uchiumi (Tokai University) and Satoshi Okada (JAMSTEC) are thanked for their help in sorting the wood-fall fauna. Itaru Kobayashi (Misaki Marine Biological Station, the University of Tokyo), Akito Ogawa (National Museum of Nature and Science, Tsukuba), Hayato Tanaka (Tokyo Sea Life Park), Naoto Jimi (Sugashima Marine Biological Laboratory, Nagoya University) and Julia D. Sigwart (Senckenberg Research Institute and Museum, Frankfurt) are acknowledged for their help with specimen identification. Helpful comments from anonymous reviewers improved an earlier version of this paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability

Original seafloor images used in this study have been deposited in Figshare under the 10.6084/m9.figshare.24156405. All other data generated or analysed during this study are included in this published article.

Author contributions

CC conceived and designed the project. HN and CC collected, sorted, identified and interpreted the wood fall assemblage. NI carried out microscopic wood anatomy and identified the wood type. CC drafted the original manuscript which was edited by NI and HN. All authors agreed with the submission and publication of the present manuscript.