Bone-eating worms and wood-eating bivalves: characterising the ecology of deep-sea organic falls from multiple ocean basins

Abstract

Large organic inputs to the deep seafloor such as the remains of whales or pieces of wood are termed ‘organic falls’. Despite over 30 years of research on these interesting deep-sea habitats, we still have only a basic understanding of their taxonomic composition and for some ocean basins, no natural or experimental studies have ever been conducted. The degree of connectivity between these isolated habitats, as well as how quickly organic matter is remineralised by specialist organic-fall fauna (e.g bone-eating Osedax worms and wood-eating Xylophaga molluscs) is poorly known.

In this thesis, I report the discovery of the first Antarctic whale fall and the diverse assemblage of fauna encrusting it (Chapter 2). The microdistribution of fauna on the whale bones provided evidence for the ‘oil-gradient’ hypothesis that more lipid-rich bones support a greater abundance of sulfophilic bacterial mats, which are also correlated with the abundance of grazing fauna. The abundance of Osedax species on bones however, showed a negative correlation with the bacterial-mat cover, and hence the greatest abundance was on bones predicted to have the lowest lipid content. The Osedax species discovered were investigated in detail (Chapter 3) and revealed two new species and a third previously-known species; Osedax rogersi sp. nov., Osedax crouchi sp. nov. (described in this thesis and associated paper) and Osedax antarcticus. The new species, O. crouchi as well as another new species, Osedax nordenskioeldi sp. nov. (also described in this thesis and associated paper) and Osedax antarcticus were also found on implanted whale bones off Smith Island in the Bransfield Strait. These two localities are approximately 1800 km apart demonstrating the remarkable dispersal capability of species within this genus.

As well as the Antarctic study, I report on wood and bone-colonisation experiments on the Southwest Indian Ridge at two seamounts. A large number of species were found colonising the deployments; 53 species at Coral Seamount and 38 species at Atlantis Bank seamount with only 11 species in common and several putative new species present. Apart from Xylophaginae and Idas bivalves, few organic-fall specialists were present, possibly as there were major differences between the two seamounts suggesting that there were barriers to dispersal (Chapter 4). The wood deployments from each seamount were investigated in further detail using X-ray micro-computed tomography to examine the nature of intact Xylophaga borings, the comparative abundances and population size structures of the species, their rates of growth and their consumption rates of wood (Chapter 5). Two more sets of samples from the Mid-Cayman Spreading Centre and the Tongue of the Ocean, Bahamas were scanned also. The wood at each deployment site was colonized by a different species of Xylophaga. This novel analysis has shown that an individual Xylophaga can bore between 0.235 and 0.606 cm3 of wood per year depending on the species, emphasising the importance of the genus Xylophaga with regard to wood remineralisation in the deep sea and its role as an ecosystem engineer.

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ORCID for J. Copley: orcid.org/0000-0003-3333-4325

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