Integrating eDNA and trawl surveys reveals fish diversity and functional redundancy in the Haima cold seep area

Cold-seep ecosystems support distinctive biological communities, yet the functional and phylogenetic diversity of their fish assemblages remains poorly understood. We combined environmental DNA (eDNA) metabarcoding and trawl surveys to assess fish taxonomic, functional and phylogenetic diversity in the South China Sea. Surveys covered the Haima cold seep area, an adjacent area and a non-seep control area. Shannon diversity, functional richness and phylogenetic diversity were significantly higher in the cold seep and adjacent areas than in the control area. Mean functional richness and phylogenetic diversity in the cold seep area were 0.137 and 9.05, respectively, compared with 0.028 and 5.36 in the control area. Despite this higher diversity, 65 of the 87 functional entities in the cold seep area (74.7%) were represented by single species. Of these species, 36 (55.4%) contributed less than 0.1% of eDNA reads, although low read proportions do not necessarily indicate low population abundance. Functional and phylogenetic diversity in the cold seep area reached their highest observed values at 750 m. These findings indicate that high fish diversity in the Haima cold seep area coexists with limited redundancy across many trait combinations. Biodiversity assessment and conservation of cold-seep ecosystems should therefore integrate taxonomic, functional and phylogenetic diversity while accounting for functional redundancy.