The hadal zone, which is composed primarily of ocean trenches, represents the deepest marine habitat on Earth (6,000-11,000m). All the trenches together occupy an area about the size of Australia. Much of our knowledge of hadal biology is derived from two sampling campaigns in the 1950s (the Danish Galathea and the Soviet Vitjaz expeditions). These exploratory campaigns culminated in an initial catalogue of hadal species but they did not strategically sample at comparable depths or with sufficient replication to permit intra- or inter-trench comparisons upon which to draw ecological conclusions regarding demography or spatial population dynamics. Far from being devoid of life as originally perceived (Forbes & Austen 1859), additional opportunistic observations have since stimulated the hypotheses that the hadal zone hosts a substantial diversity and abundance of fauna with a high degree of endemism (Wolff 1960; 1970). However, as a result of historical factors and severe technical challenges associated with the extremes of hydrostatic pressure and distance from the sea surface, hadal systems remain among the most poorly investigated habitats on Earth.
The hadal zone cannot be considered as simply a continuation of the deep-sea environment because when depths exceed 6,000 m the habitat splits into clusters of disjunct and isolated trenches. Furthermore, there is compelling evidence that food supply (in the form of particulate organic carbon; POC) accumulates along trench axes as a result of the characteristic V-shaped topography funneling resources downwards (Itou 2000; Otosaka & Noriki 2000; Danovaro et al. 2003; Itoh et al. 2011). The combination of extremely high hydrostatic pressure, accumulation of food along trench axes and geographical isolation is believed to have resulted in these habitats having a high degree of endemism, and an extraordinarily high abundance of certain taxa and yet exclusion of other faunal groups.
Areas of specific and often extreme topography such as seamounts, canyons and trench systems foster a diverse range of habitats for deep-sea life (Vetter & Dayton 1998; Shank 2004; Tyler et al. 2009; Shank 2010; Clark et al. 2010; DeLeo et al.2010). Geological, oceanographic, and biological interactions that can create and maintain “biological hotspots” in the ocean, which can ultimately control regional biodiversity, biogeography and the evolution of the deep-sea fauna (Weaver et al. 2004; Shank 2004). These same processes may however also serve to limit productivity, endemism and the diversification of populations among habitats. A number of hypotheses, encapsulated below, have been put forward to advance our ability to explain these observations in different regions of the oceans, including the influences of hydrographic boundaries, depth and species-specific life history differences, co-evolution and climate variation (Ruhl & Smith 2004; Jamieson et al. 2010).
A recent international meeting of leading trench scientists (Trench Connection, Tokyo, 2010) and a review of hadal environments (Jamieson et al. 2010) have highlighted the need to differentiate between the variables of hydrostatic pressure (and associated variables), food supply, and trench topography as drivers of community structure (including species diversity and endemism). Discerning between these key factors is currently impossible given the lack of data for any of them individually, but results and insights from a systematic co-located investigation of each will provide the currently needed foundation for understanding the formative processes that structure hadal ecosystems. It is therefore a high priority to pursue scientific inquiry in the deepest yet grossly under sampled environments on Earth (Webb et al. 2010). Given the fundamental scientific discovery and impact expected from basic systematic observations of trench environments, why haven’t these data already been collected?
The majority of faunal samples retrieved from hadal depths have been opportunistic, if not haphazardly collected with non-standardised, semi-quantitative gear such as grabs, trawls and traps, unable to document robustly the structure of benthic communities and provide major advances in hadal ecological studies. These few samples have provided a wealth of information about the basic biology of some trench organisms (e.g., Wolff 1960, 1970). This notwithstanding, the lack of deep-submergence technologies to conduct systematic seafloor imaging and sampling surveys has prevented the scientific community access to quantitative faunal distributions, compositions, and an absence of environmental and physiological data in space and time, which have hindered the application and development of essential ecological theory in this environment (Belyaev 1989).
The successful development of the fully functional Hybrid Remotely-Operated Vehicle Nereus (Fig. 1; Bowen et al. 2009) has now prompted scientists from around the world to come together to identify the foremost standing questions in hadal science. In short, the ecological and evolutionary significance of depth/pressure adaptations, food-supply/trophic relationships, and topographically and depth-driven isolation of trench fauna pose fundamental questions form the basis of our overarching hypotheses below. Through the Hadal Ecosystems Study (HADES) program, we will determine the composition and distribution of hadal species, the role of hadal pressures (piezolyte concentrations, enzyme function under pressure), food supply (distribution of POC with the abundance and biomass of trench organisms, metabolic rates/energetic demand), and depth/topography (spatial connectivity of populations) on impacting community structure.
HADES will investigate the major environmental drivers of trench ecology. Megafaunal composition and distribution will be examined as a function of depth and location by systematic high-definition imaging and sediment/faunal sampling transects from abyssal to full trench depths both along and perpendicular to the trench axis. Data from this sampling will enable us to explore the relationship between POC and benthic bacterial biomass, and faunal community structure. Population genetic approaches and the identification of evolutionarily independent lineages will assess the role of depth and topography in a trench and adjacent abyssal plain in promoting the formation of species and their populations. Physiological constraints will be investigated by examining in-situ respiration of selected fauna and tissue concentrations of such protein stabilizers as trimethylamine oxide (TMAO), and the structural adaptations of macromolecules. We feel these objectives represent an achievable and powerful combination of current technological capability, scientific understanding and theory, and the expertise of an international consortium of scientists. Given the historical knowledge of trenches around the world and ongoing research within our consortium, our objectives are best met when focussed on conducting them in the Kermadec Trench. The Kermadec Trench (SW Pacific) is a hadal environment that provided data for the earliest appreciations of trench ecosystems (Wolff 1960) and where some of the most recent trench research has been (and is being) undertaken (Blankenship et al. 2006, Blankenship & Levin 2007; Jamieson et al. 2009abc, 2011). Also key to enabling HADES-Kermadec is the immediate scientific leverage that comes from the “wealth” of historical data, and both active and anticipated research on the taxonomy, ecology, and evolution of fauna at seamounts, slopes, vents, and canyons in the vicinity of the Kermadec Trench. The regional biogeography is reasonably well known (CANZ 2008) and a 6-year program involving our NIWA co-PIs (AR, MC with TS) is underway to understand the faunal differences between deep habitats in the region. The Kermadec Trench has a gradient in depth both across its axis and along its length, and lies in close proximity to a variety of other habitat types in the Kermadec region (seamounts, ridges, slope, and abyssal plain). As such, extensive comparisons between these habitats and the trench and abyssal environments can be made, placing the hadal fauna in a direct ecological context for the first time. Our co-PIs (AJJ et al.) currently have two funded field programs that will take place in the Kermadec Trench in the next two years to investigate scavenging fauna between 7 and 10 km deep.