Insights into the abundance and diversity of abyssal megafauna in a polymetallic-nodule region in the eastern Clarion-Clipperton Zone

Abstract

There is growing interest in mining polymetallic nodules in the abyssal Clarion-Clipperton Zone (CCZ) in the Pacific. Nonetheless, benthic communities in this region remain poorly known. The ABYSSLINE Project is conducting benthic biological baseline surveys for the UK Seabed Resources Ltd. exploration contract area (UK-1) in the CCZ. Using a Remotely Operated Vehicle, we surveyed megafauna at four sites within a 900 km(2) stratum in the UK-1 contract area, and at a site ~250 km east of the UK-1 area, allowing us to make the first estimates of abundance and diversity. We distinguished 170 morphotypes within the UK-1 contract area but species-richness estimators suggest this could be as high as 229. Megafaunal abundance averaged 1.48 ind. m(-2). Seven of 12 collected metazoan species were new to science, and four belonged to new genera. Approximately half of the morphotypes occurred only on polymetallic nodules. There were weak, but statistically significant, positive correlations between megafaunal and nodule abundance. Eastern-CCZ megafaunal diversity is high relative to two abyssal datasets from other regions, however comparisons with CCZ and DISCOL datasets are problematic given the lack of standardised methods and taxonomy. We postulate that CCZ megafaunal diversity is driven in part by habitat heterogeneity.

Figure 1. Locations of megafaunal surveys during ABYSSLINE cruise 01 (AB01) in the eastern Clarion-Clipperton Zone.

(a) The location of the UK-1 exploration contract area in the Pacific Ocean. (b) The location of UK-1 Stratum A in relation to the UK-1 contract area and the AB01 ROV dive site, EPIRB, which was approximately 250 km east of the UK-1 contract area. (c) A bathymetric map of UK-1 Stratum A with the locations of ROV dives indicated by circles with the dive number. All maps were created by Seafloor Investigations Ltd. for the ABYSSLINE Project using ArcGIS software (https://www.arcgis.com/features/). (c) was created using unpublished ship-based bathymetry collected during AB01.

Figure 2. Dominant megafauna observed in the eastern CCZ.

(a) Psychropotes cf. semperiana; (b) Ophiomusium cf. glabrum; (c) Xenophyophore plate-like morphotype 1: Psammina sp. in situ on seafloor; (d) Xenophyophore plate-like morphotype 1: Psammina sp. in situ close up; (e) Xenophyophore plate-like morphotype 8/9; (f) Calyptrophora persephone; (g) Relicanthus sp.; (h) Abyssoprimnoa gemina. Image attribution: (a,c,e,g) - DJ Amon & CR Smith, University of Hawai’i; (d) – AJ Gooday and A Goineau, National Oceanography Centre, Southampton; (b,f,h) – AG Glover, TD Dahlgren & H Wiklund, Natural History Museum, London & Uni Research.

Figure 3. Megafaunal morphotype richness and individual abundance observed during AB01.

(a) Megafaunal morphotype richness by phylum or other grouping in the case of xenophyophores. (b) Megafaunal abundance by phylum or other grouping in the case of xenophyophores. ‘Unknown’ refers to morphotypes that could not be assigned to a phylum.

Figure 4. Chao-1 estimate of species richness (±s.d.) of epibenthic megafauna with increasing number of ROV dives during AB01 in the eastern CCZ.

Circles = all megafauna surveyed during AB01 (five ROV dive sites); crosses = megafauna from only the UK-1 contract area (four ROV dive sites); triangles = megafauna observed only on hard substrata (nodules); squares = megafauna occurring on sediments or both sediments and nodules.

Figure 5. The relationship between species richness and nodule cover during AB01.

Chao-1 species richness (±s.e.) in areas of low nodule percent cover (<15% cover) is represented by the black line, Chao 1 species richness (±s.e.) in areas of medium nodule percent cover (15–50% cover) is represented by the grey line. Ugland (UGE) species accumulation in areas of low nodule percent cover (<15% cover) is represented by the light green line and Ugland (UGE) species accumulation in areas of medium nodule percent cover (15–50% cover) is represented by the dark green line. Bootstrap species richness in areas of low nodule percent cover (<15% cover) is represented by the dark blue line, Bootstrap species richness in areas of medium nodule percent cover (15–50% cover) is represented by the light blue line. Jacknife 2 species richness in areas of low nodule percent cover (<15% cover) is represented by the yellow line, Jacknife 2 species richness in areas of medium nodule percent cover (15–50% cover) is represented by the orange line. High nodule percent cover (>50% cover) was not observed during this study.