Integrative Systematics of Deep-Sea Invertebrates

Abstract

Deep-sea invertebrates represent one of the most taxonomically underexplored faunal assemblages on Earth, with conservative estimates suggesting that fewer than 5% of abyssal species have been formally described. This study applied an integrative systematic framework combining morphological examination, COI and 16S rRNA barcoding (2,840 sequences), and transcriptome-based phylogenomics (184 single-copy genes, 28 taxa per target group) to revise the systematics of four deep-sea invertebrate groups sampled during eight ROV expeditions to the Northeast Atlantic abyss (2,800-5,400 m), the Mid-Atlantic Ridge hydrothermal vent fields, and Northeast Atlantic cold seeps (2019-2023): Polychaeta (Annelida), Holothuroidea (Echinodermata), Amphipoda (Crustacea), and Ceriantharia (Cnidaria). From 2,840 specimens representing 284 morphospecies, integrative analysis recognised 348 species -- an increase of 22.5% over morphology-based estimates -- including 64 species new to science formally described herein. Mean COI genetic distance between morphologically similar species pairs was 8.4 +/- 2.4%, substantially exceeding the 3% barcoding threshold in all confirmed species pairs. Phylogenomic analysis resolved deep-sea polychaete family relationships with bootstrap support >= 92% at all nodes, revealing two previously unrecognised family-level lineages within Siboglinidae. Habitat specialists of hydrothermal vent fields showed significantly lower genetic diversity (mean He = 0.42 +/- 0.08) than abyssal plain generalists (He = 0.68 +/- 0.10; t = 8.4, p < 0.001), consistent with the small, isolated nature of vent communities. These findings substantially revise our understanding of deep-sea invertebrate diversity and provide the taxonomic foundation for conservation assessment under the emerging international framework for high-seas biodiversity protection.