Ecology of Deep Oceans: Hadal Trenches

Abstract The majority of the Earth is covered by ocean which itself averages greater than 3500 m deep. It is comprised mainly of the vast deep ocean plains, which span a depth range of 2000–6000 m. The maximum depth of the ocean however is nearly 11 000 m deep. These extremely deep areas are a result of tectonic plate convergence where an oceanic plate is subducted beneath a neighbouring continental plate. This subduction forms extraordinarily deep trenches which comprise what is known as the ‘hadal zone’. The hadal zone (6000–11 000 m) is therefore encompassed by clusters of disjunct and often extremely isolated deep trenches and troughs. The morphology of these habitats had provided a setting where environmental conditions (hydrostatic pressure and food supply) differ greatly from the majority of the deep‐sea and has resulted in high levels of species endemism at the greatest depths. Key Concepts: The deepest biozone in the oceans is the hadal zone (6000–11 000 m deep). The hadal zone is comprised mainly of deep trenches formed by tectonic convergence. The trench environment is characterised by high hydrostatic pressure, low temperatures, the absence of light and a limited food supply. Trench topography provides a unique setting with regard to food supply input because organic matter (food) is accumulated at the deepest trench axis. High levels of species endemism are observed in trenches as a result of their geographical isolation. Physiological adaptation to high pressure and low food supply is an essential prerequisite for survival in the trenches. Most metazoan taxa are represented in these deep trenches. Until recently, the technical challenges of studying the hadal zone were responsible for a dearth of information regarding the ecology at these depths. Research into the ecology of the hadal zone is in its infancy relative to shallower biozones.