Taxonomy, ecology, and evolution in the Eastern Pacific abyss

Abstract

The abyssal plains (3000–6000 m depth) underlie over two thirds of the world’s oceans. Once considered a vast contiguous landscape, advances in deep-ocean exploration have revealed the abyssal seafloor to be geologically and ecologically varied—harbouring biodiverse communities influenced by complex interactions between deterministic and stochastic assembly processes. However, the abyssal plains are also home to a wealth of mineral resources, the extraction of which presents an emerging threat to the diverse communities they coexist with. As such, there is a growing need for integrative assessments of both baseline patterns and the mechanisms maintaining abyssal diversity, to both monitor change and inform conservation efforts. Using the Clarion-Clipperton Zone (CCZ), Eastern Pacific Ocean—the world’s largest frontier for deep-sea mining—as a model ecosystem, this thesis explores the patterns and drivers of invertebrate diversity across the abyssal seafloor, from single species through to local and regional scales. Two species of scavenging amphipod, and a new genus and two species of myzostomid annelid are described using combined morphological and molecular methods, highlighting the value of an integrative taxonomic approach in assessing both biogeography and evolutionary history. These are the first species of Myzostomida known from abyssal depths, and the expanded phylogenetic hypothesis demonstrates unique evolutionary transitions between life history states. A regional-scale analysis of polychaete taxonomic and phylogenetic diversity suggests that environmental filtering rather than dispersal limitation is the primary driver of local species assembly, with an assessment of population-level connectivity in a subset of six abundant species highlighting broad ranges and high haplotype diversity. Additionally, this study provides the first evidence that polychaete diversity within the CCZ is higher than in other comparable deep-sea sedimented regions. Finally, the first spatio-temporally replicated assessment of deep-sea mining impacts on seafloor diversity was conducted, finding significant negative effects on abundance and diversity, and increased multivariate dispersion in community composition, against a backdrop of natural temporal variability. This thesis highlights the abyssal seafloor as a vast and dynamic ecosystem with an equally vast potential for novel taxonomic, ecological, and evolutionary discovery—should we act to protect it first.

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Identifiers

ORCID for Eva Catherine Dawson Stewart: orcid.org/0000-0001-8383-5705

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

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