Megafaunal Community Structure From the Abyssal to Hadal Zone in the Yap Trench

Dongsheng Zhang,Peng Xu,Shiquan Lin,Jichao Yang,Chunsheng Wang,Dong Sun,Yadong Zhou,Thomas Linley,Xiaogu Wang,Chengcheng Shen,Yueyun Wang,Ruiyan Zhang,Bo Lu

doi:10.3389/fmars.2021.617820

Abstract

Hadal trenches remain one of the unexplored ocean ecosystems due to the challenges of sampling at great depths. It is still unclear how a faunal community changes from the abyssal to the hadal zone, and which environmental variables are the key impacting factors. In this study, nine dives of the Human Occupied Vehicle (HOV) “JIAOLONG” were conducted from abyssal to hadal depths (4,435–6,796 m) in the Yap Trench on the southeastern boundary of the Philippine Sea Plate in the western Pacific, divided into 48,200 m video transects, to describe the megafaunal communities and reveal their relationship with environmental factors. A total of 1,171 megafauna organisms was recorded, 80 morphospecies (msps) from 8 phyla were identified based on the video data, most of which were reported for the first time in the Yap Trench. Arthropoda was the most abundant phylum and Echinodermata was the most diverse phylum of the megafaunal community. The faunal abundance increased with depth, whereas the Shannon diversity index decreased with depth. Cluster analysis suggested seven assemblages, with five abyssal groups, one mixed group, and one hadal dominant group. Although megafaunal communities changed gradually from abyssal zone to hadal zone, both PERMANOVA and PERMDISP analyses revealed that the communities are significantly different between abyssal zone and hadal zone, indicating 6,000 m as the boundary between the two depth zones. Depth, substrate, slope, and latitude were identified as four important environmental factors with significant influence on megafaunal community structure. This study proposed a transition pattern from the abyssal to hadal zone in the Yap Trench, highlighted the importance of habitat heterogeneity in structuring megafaunal community in a hadal trench.

Highlights

The deepest areas of the oceans (6,000–11,000 m) are restricted to the hadal trenches and referred to as the hadal zone (Wolff, 1960)
The Hadal Province 3 (HD3) province is located in the oligotrophic tropical northwest Pacific, where the particulate organic carbon (POC) flux sinking to the deep-sea floor is extremely low (Lutz et al, 2007)
Published data indicates that abundance of megafauna in the Yap Trench is obviously lower than in other hadal trenches (Belyaev, 1989), and is probably limited by the low food availability compared to other trenches

Summary

Introduction

The deepest areas of the oceans (6,000–11,000 m) are restricted to the hadal trenches and referred to as the hadal zone (Wolff, 1960). Hadal trenches represent 1–2% of the global benthic area of the ocean, but constitute the deepest 45% of the vertical depth, and remain one of the least understood habitats on earth (Jamieson et al, 2010; Jamieson, 2011). The hadal fauna is distinguished from that in the neighboring abyssal zone in several respects, including: (a) the dominance of amphipods, polychaetes, bivalves, isopods, actinians, gastropods, echiurids, and holothurians; (b) lower representation of non-holothuroids and sipunculids; (c) the insignificance of cnidarians, bryozoans, cumaceans, fishes, and lack of decapods; (d) mass occurrence of holothuroids at medium and maximum hadal depths; (e) a very uniform relative frequency at different levels in the hadal zone of different animal groups; (f) a great similarity in frequency and composition of the fauna among trenches (Belyaev, 1966; Wolff, 1970)

Methods

Results

Discussion

Conclusion