| 06 May 2025
Hidden behind the scene – high diversity, low connectivity of deep-sea Amphipoda in the polymetallic nodule fields in the Northeast Pacific
Abstract. The Clarion-Clipperton Zone (CCZ) situated in the central east Pacific holds major portions of manganese nodule deposits and is therefore subject to intense exploration for future deep-sea mining. However, mining rises multiple concerns. Among others about its direct or indirect impact on abyssal environment. The more, proper evaluation of deep-sea protection plans to be applied there is still hindered by insufficient knowledge of the abyssal fauna diversity and their assemblages.
Amphipoda are speciose and abundant in all marine habitats and were proven to be important food source for higher trophic levels. These crustaceans are brooders with no planktonic larval stage, so migration of species depend only on swimming capacities of adult animals. In the CCZ macrofauna-sized and relatively mobile, epibenthic Amphipoda, not collected with commonly used box corers or multicorers, remained unstudied. The present work aims to fill in this gap in knowledge by: 1) characterizing the diversity of amphipods across the CCZ and one Area of Particular Environmental Interest (APEI), 2) studying the amphipod species distribution and assemblages as well as 3) inferring the connectivity between eastern and western parts of the CCZ and other deep-sea regions.
The material was collected with epibenthic sledge from three contractor zones (UK, Singapore, Germany) and from APEI-06. Specimens were sorted into families and preferably morphospecies and subjected to cytochrome c oxidase subunit I gene (COI) barcoding.
Within total of 708 individuals 23 known and one undescribed families were identified. The barcoding allowed to obtain 581 sequences that were ascribed to 207 Molecular Operational Taxonomic Units (MOTUs). The families that dominated barcoded material in terms of abundance and species richness were: Pardaliscidae (123 ind., 44 MOTUs), Eusiridae (122 ind., 32 MOTUs) and Synopiidae (74 ind., 31 MOTUs). Only 17 known species were identified; another 48 are considered as new to science. Almost 70% of MOTUs were singletons or doubletons and 186 MOTUs were unique for the present study. The remaining 21 taxa appeared to be broadly distributed. Among the MOTUs recorded only in the CCZ majority was distributed within 400 km distance. Just six species reached or exceeded 1000 km distance of occurrence. Additionally only one species was shared between contractor areas and the studied APEI that was supposed to be preserved area representative for the mining zones. The cluster analysis of the fauna collected during three expeditions to the German contractor areas revealed almost no similarity between the West and East part of the CCZ. Additionally, within the eastern part of the studied region geographic proximity appeared to have no influence on station clustering.
The study confirmed low abundance and high species richness of deep-sea amphipod fauna of the CCZ and their highly variable assemblages. Moreover, it has shown weak connectivity between eastern and western parts of the region as well as between the contractor zones, APEIs and other deep-sea regions of the World. All the above suggest high vulnerability of the assemblages studied and, with the present state of knowledge, weak representativeness of Areas of Particular Environmental Interest for the mining zones. In order to better understand the deep-sea biodiversity and develop Regional Environmental Management Plans for the whole CCZ area two main recommendations are to be made: 1) to conduct more intensive sampling program of APEIs parallel to monitoring studies of contractor zones with the goal to better assess their usefulness as reservoirs of biodiversity for the mining areas, 2) to include the epibenthic sledge among standard gears used for sampling in order to avoid missing important part of deep-sea fauna in the monitoring studies.
This study represents a significant contribution to the understanding of deep-sea biodiversity in the Clarion-Clipperton Zone (CCZ), a region targeted for potential deep-sea mining. The authors focus on an often-overlooked benthic group, Amphipoda, by using material sampled primerly with an epibenthic sledge. DNA barcoding is employed to reveal an impresive amphipod diversity, including a high proportion of potentially new species as well showcase the limited connectivity between CCZ areas. The research is methodologically sound, well-structured, and clearly addresses a major gap in current knowledge of the group Amphipoda, while emphasing the value of the sampling gear (EBS).
That said, I urge the authors to exercise greater caution in their conclusions regarding the effectiveness of APEI-6 as a biodiversity reservoir. While their inference aligns with previous literature, it is based on very limited sampling—only six specimens were collected from APEI-6—which restricts the strength of the conclusion.
Additionally, attention should be given to the species delimitation approach using BINs (Barcode Index Numbers). In its current form, the BIN-based delimitation appears inconsistent with the presented NJ (Neighbor-Joining) tree, and I urge the authors to carefully re-examine this analysis. Furthermore, the manuscript would benefit from thorough language editing. There are numerous grammatical issues, such as missing or unnecessary articles and awkward sentences, that undermine the clarity of the work. More specific comments are given to the pdf attached.
For the above reasons I recommend this article for publication after its being revised, as it provides robust data that should inform both the scientific community and assist to policy decisions related to deep-sea mining in the CCZ .