Deep-sea benthic communities in the largest oceanic desert are structured by the presence of polymetallic crust

Juliette Delavenne,Philippe Maestrati,Louise Keszler,Sarah Samadi,Magalie Castelin,Pierre Lozouet

doi:10.1038/s41598-019-43325-0

Abstract

Based on the specimens collected during three deep-sea cruises, and deposited at the Muséum National d’Histoire Naturelle (MNHN) in Paris, we analysed the diversity of benthic communities within the EEZ of French Polynesia. The literature and the MNHN database allowed us to inventory 471 species of invertebrates, among which 169 were newly described. We mainly found data for Mollusca, Crustacea, Brachiopoda and Crinoidea. We also found samples from other taxa, which still remain unidentified within the collections of the MNHN. Although this inventory is incomplete, we demonstrate that the deep waters of French Polynesia host unique benthic communities and endemic species. Using diversity and multivariate analyses, we show that the deep-sea benthic communities are structured by depth, habitats, geography and also by the presence of polymetallic crust. Furthermore, by focusing on the molluscs of the central area of French Polynesia, we show that the spectrum of shell size differs among deep-sea habitats. Specifically, shells tend to be smaller on encrusted seamounts than on island slopes. Together with the size range of organisms, low abundance, rarity and endemism designate these habitats as sensitive. These results should thus be taken into account in the evaluation of the expected impact of mining activities on biological communities.

Highlights

Deep-sea waters represent more than 90% of the habitable volume on earth (Levin and Le Bris 2015), and host the most poorly known habitats on earth[1]
The Venn diagram (Fig. 3c) shows that 2.3% of the whole diversity was shared among the three surveys, and that more species were shared between MUSORSTOM 9 and BENTHAUS (21) than between MUSORSTOM 9 and TARASOC (12)
Despite the intrinsic limitations of the data derived from naturalistic explorations, our analyses show that geography, depth, topography and the substrate type structure the deep-sea communities of French Polynesia; both at the scale of the Exclusive Economic Zone (EEZ) (Fig. 4a,b), and at the scale of the sub-region explored by the TARASOC cruise

Summary

Introduction

Deep-sea waters represent more than 90% of the habitable volume on earth (Levin and Le Bris 2015), and host the most poorly known habitats on earth[1]. Among the specific features of the area, it is worth noting that French Polynesia is isolated in the central Pacific, in the middle of the South Pacific Gyre, often considered as the Earth’s largest oceanic desert from the sea surface to the deep sea floor[17]. Within this context, our aims were (i) to gather available data on the deep-sea benthic communities of the French Polynesian EEZ; and (ii) determine if the observed polymetallic crusts, occurring mostly on seamounts, are associated with unique benthic communities. Total Below 200 m Beam Trawl (CP) Waren Dredge (DW) Rock Dredge (DR) Traps (CAS) Total Mollusca Crustacea Crinoidea Brachiopoda Total Mollusca Crustacea Crinoidea Brachiopoda

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Results

Discussion

Conclusion