Biodiversity of the abyssal and hadal zone in the Nova Canton Trough (Central Pacific Ocean; 2751-7989 m): a visual inventory.

Abstract

Comprehensive biodiversity inventories of abyssal and hadal ecosystems remain rare due to the financial and logistical challenges associated with operating in the deepest parts of the ocean. However, deep-sea exploration programmes are increasingly generating extensive video datasets offering new opportunities to document biodiversity across broad depth ranges. Here, we present the most extensive visual biodiversity inventory assembled from an abyssal-hadal system to date, using imagery collected in the Nova Canton Trough (Central Pacific Ocean), combining observations from 99 baited lander deployments and 16 submersible transects conducted between ~ 3000 m and ~ 8000 m depth. Across more than 1500 hours of lander video and 136 hours of submersible video, ~ 30,000 individual organisms belonging to the kingdoms Animalia and Protista were recorded and grouped into 325 morphotaxa, spanning a wide range of benthic and demersal taxa. The integration of baited and non-baited visual platforms provided complementary observations of mobile scavengers and sessile or low-mobility benthic taxa. Combined observations reveal a depth-structured transition from a diverse abyssal biota through a broad abyssal-hadal transition zone to a distinctive hadal assemblage. Notable findings include the first population of hadal snailfish (Pseudoliparis sp.) reported outside a subduction trench, high density and diversity of xenophyophores across the depth range and several putative bathymetric range extensions, including first hadal records for several groups within the Animalia and Protista. At phylum level, these include live Brachiopoda and direct observations of protists belonging to the Cercozoa. The orders Corallimorpharia (Phylum Cnidaria, Class Hexacorallia) and Nacromedusae (Phylum Cnidaria, Class Hydrozoa) are extended by over 1200 m, with the former being the first hadal record. The families Amathillopsidae (Phylum Arthropoda, Order Amphipoda), Candelabridae (Phylum Cnidaria, Order Anthoathecata), Hesionidae (Phylum Annelida, Order Phyllodocida), Onuphidae (Phylum Annelida, Order Eunicida) and Umbellulidae (Phylum Cnidaria, Order Scleralcyonaceae) are also depth-extended, with the first three representing first records at hadal depths. The Nova Canton Trough also differs from most previously studied hadal systems in being a tectonically quiescent fracture zone rather than a subduction trench. This makes it an important reference system for evaluating whether depth-structured faunal zonation is consistent across different geological contexts. Its position in the central Pacific provides another important biogeographic relevance connecting existing hadal studies focused on the north and south Pacific. By combining observations with curated reference imagery and identification notes, this study establishes a biodiversity baseline for the Nova Canton Trough and provides a visual framework to guide future taxonomic identification, ecological studies and targeted biological sampling, particularly at lower abyssal and hadal depths where biodiversity remains most poorly resolved.