The hadal zone is the deepest marine ecosystem on Earth and is mostly made up of ocean trenches (6,000-11,000m). Together, the trenches cover an area about the size of Australia. We learned a lot about hadal biology from two sampling missions in the 1950s (the Danish Galathea and the Soviet Vitjaz expeditions). This initial catalogue of hadal species was the result of these exploratory campaigns, but they did not strategically sample at comparable depths or with enough replication to allow intra- or inter-trench comparisons from which to infer ecological conclusions about demography or spatial population dynamics.
Additional opportunistic observations have since sparked the theories that the hadal zone has a significant diversity and quantity of fauna with a high degree of endemism, refuting the original perception (Forbes & Austen 1859) that it was lifeless (Wolff 1960; 1970). Hadal systems remain among the least studied environments on Earth, however, as a result of historical reasons and difficult technical obstacles related to the extremes of hydrostatic pressure and distance from the sea surface.
Because the habitat breaks into groups of isolated and disjunct trenches at depths more than 6,000 m, the hadal zone cannot be viewed as merely a continuation of the deep-sea ecosystem. Additionally, strong evidence suggests that food supply (in the form of particulate organic carbon, or POC) accumulates along the trench axis as a result of the topography’s distinctive V-shape funnelling resources downward (Itou 2000; Otosaka & Noriki 2000; Danovaro et al. 2003; Itoh et al. 2011).
These environments are thought to exhibit a high degree of endemism, an abnormally high abundance of some taxa, and the exclusion of other faunal groups due to a combination of geographical isolation, extremely high hydrostatic pressure, and food buildup along the trench axis.
Areas with unique and frequently harsh topography, such as seamounts, canyons, and trench systems, support a variety of deep-sea living habitats (Vetter & Dayton 1998; Shank 2004; Tyler et al. 2009; Shank 2010; Clark et al. 2010; DeLeo et al.2010). Regional biodiversity, biogeography, and the development of the deep-sea fauna can all be controlled by geological, oceanographic, and biological interactions that can establish and maintain “biological hotspots” in the ocean (Weaver et al. 2004; Shank 2004).
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However, these same processes may also work to restrict population diversity among habitats, productivity, and endemism. To improve our understanding of how to explain these facts in various oceanic locations, a number of ideas have been put out. These include the effects of hydrographic boundaries, depth, species-specific life history variations, co-evolution, and climatic fluctuation (Ruhl & Smith 2004; Jamieson et al. 2010).
The need to distinguish between hydrostatic pressure (and associated variables), food supply and trench topography as drivers of community structure was recently brought to light by an international gathering of top trench scientists (Trench Connection, Tokyo, 2010) and a review of hadal environments (Jamieson et al. 2010). (including species diversity and endemism).
The lack of data for any of these key factors individually makes it impossible to distinguish between them at this time, but the findings and insights from a systematic co-located investigation of each will give us the foundation we currently lack for understanding the forming processes that shape hadal ecosystems.
Therefore, it is of utmost importance to conduct scientific research in the planet’s deepest habitats, which have sadly received far too few samples (Webb et al. 2010). Why haven’t these data been gathered given the potential for important scientific discovery and effect from simple systematic studies of trench environments?
The majority of faunal samples recovered from hadal depths have been opportunistic, if not randomly collected with non-standardized, semi-quantitative gear like grabs, trawls, and traps, unable to provide significant advancements in hadal ecological studies and robustly document the structure of benthic communities. These few samples have yielded a plethora of knowledge regarding some trench creatures’ basic biology (e.g., Wolff 1960, 1970).
Despite this, the scientific community has been unable to access quantitative faunal distributions and compositions, and there is a lack of environmental and physiological data in space and time, which has made it difficult to apply and advance fundamental ecological theory in this environment (Belyaev 1989).
Since the creation of the fully functional Hybrid Remotely-Operated Vehicle Nereus (HROVN), scientists from all over the world have gathered to discuss the most pressing open problems in the field of hadal science. Our main ideas, which are based on fundamental questions about the ecological and evolutionary implications of food-supply/trophic interactions, topographic and depth-driven isolation of trench fauna, and depth/pressure adaptations, are summarised as follows.
We will ascertain the composition and distribution of hadal species through the Hadal Ecosystems Study (HADES) programme, as well as the effects 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 community structure.
HADES will look into the key environmental factors influencing trench ecology. By using systematic high-definition imaging and sediment/faunal sample transects from abyssal to full trench depths both along and perpendicular to the trench axis, we will evaluate the composition and distribution of the megafauna as a function of depth and location. We will be able to investigate the connection between POC, benthic bacterial biomass, and faunal community structure using the data from this survey.
The formation of species and their populations will be evaluated in relation to the depth and topography of a trench and the nearby abyssal plain using population genetic methods and the identification of evolutionarily independent lineages. Examining in-situ respiration of specific species, tissue concentrations of protein stabilisers such trimethylamine oxide (TMAO), and macromolecule structural adaptations will be used to study physiological restrictions.
These goals, in our opinion, reflect an effective and attainable synthesis of existing technological capabilities, knowledge and theory from the scientific community, and the experience of a global group of scientists. Our goals can be best achieved by concentrating on carrying them out in the Kermadec Trench given the historical knowledge of trenches throughout the world and ongoing study within our consortium.
The Kermadec Trench (SW Pacific) is a hadal environment where some of the most recent trench research has been (and is being) conducted. It gave data for the initial assessments of trench ecosystems (Wolff 1960). (Blankenship et al. 2006, Blankenship & Levin 2007; Jamieson et al. 2009abc, 2011).
The immediate scientific leverage provided by the “wealth” of historical data, as well as ongoing and upcoming studies on the taxonomy, ecology, and evolution of fauna at seamounts, slopes, vents, and canyons close to the Kermadec Trench, are also essential to the success of HADES-Kermadec. A six-year programme led by our NIWA co-PIs (AR, MC with TS) is underway to investigate the faunal distinctions across the region’s deep habitats.
The regional biogeography is pretty well established (CANZ 2008). The Kermadec region contains a range of diverse habitat types that are near the Kermadec Trench, which has a gradient in depth along its length and across its axis (seamounts, ridges, slope, and abyssal plain).
As a result, numerous comparisons between these habitats and the trench and abyssal environments are possible, putting the hadal fauna for the first time in a direct ecological context. In the next two years, our co-PIs (AJJ et al.) will conduct two sponsored field projects in the Kermadec Trench to study scavenging fauna between 7 and 10 km deep.
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