Abstract
Our understanding of the ecology of the hadal zone (> 6000 m depth) is based solely on subduction trenches, leaving other geomorphological features, such as fracture zones, troughs, and basins, understudied. To address this knowledge gap, the Wallaby-Zenith Fracture Zone, Indian Ocean (WZFZ; ~ 22°S, 102°E; maximum depth 6625 m measured during Expedition SO258) was studied using free-fall baited landers. We assessed the amphipod distribution and community assemblage of this non-subduction hadal feature and compared it to subduction hadal features. Eleven species were identified across the abyssal-hadal transition zone using a paired morphological and DNA barcoding approach. The community composition was found to change gradually from abyssal to hadal depths, which contrasts with the ecotone shift characteristic of subduction trenches. A large population of Bathycallisoma schellenbergi (Birstein & Vinogradov, 1958), a quintessential hadal amphipod, was present at the flat bottom of the WZFZ. Further, an mtDNA phylogeny resolved a degree of phylogeographic structure between the B. schellenbergi WZFZ population and four previously sampled Pacific Ocean subduction trench populations, indicating these features are not interconnected through ongoing gene flow. Combined, these data indicate that some amphipods have far broader distributions than previously understood, with some species present in both hadal subduction trenches and non-subduction fracture zones and basins interspersed across the abyssal plains. This initial exploration highlights that whilst non-subduction features are an overlooked minor fraction of the total hadal area, they are essential to our understanding of the ecological and evolutionary dynamics across the hadal zone.
Highlights
The hadal zone comprises 47 known, geographically disjunct, marine features that extend deeper than 6000 m below sea level
To begin to address this knowledge and sampling gap, we present the first biological investigation of a non-subduction hadal feature, the Wallaby-Zenith Fracture Zone (WZFZ) in the Indian Ocean and the abyssal base of the Afanasy Nikitin Seamount as a comparative Indian Ocean reference point
The WZFZ was the first non-subduction hadal feature to be studied allowing us to further elucidate the effect of total area, topography, and total depth on scavenging amphipod community structure. This initial description of a hadal fracture zone highlights that whilst non-subduction features only account for a minor fraction of the global hadal area, sampling them is important to gain a more comprehensive understanding of drivers of community ecology and distribution of species at hadal depths
Summary
The hadal zone comprises 47 known, geographically disjunct, marine features that extend deeper than 6000 m below sea level. The remaining features are troughs, fracture zones, trench faults, and other features that are not necessarily associated with plate convergence zones and can span across the abyssal plain interiors (~ 50,500 k m2; Jamieson 2015) Whilst these features all experience high hydrostatic pressure, low temperature, and limited food availability, the geomorphological characteristics, such as total depth, total area, geographic isolation, seismicity, geologic age, and topographic complexity, vary widely between each habitat (Stewart and Jamieson 2018). These deep marine regions host faunal communities with a high degree of endemism, comprised crustaceans, echinoderms, polychaetes, molluscs, foraminifera, cnidarians,. As scavenging amphipods can be readily and consistently recovered in large numbers via baited trap landers, they represent model taxa to study the ecological dynamics of the abyssal and hadal zones across the wide variation of geomorphic settings (Fujii et al 2013; Duffy et al 2016; Lacey et al 2016)
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