The ocean is divided into multiple depth zones; epipelagic (0-200m) mesopelagic (200-1000m), bathyal (1000-4000m), abyssal (4000-6000m) and the hadal (>6000m) zone.

As well as representing the deepest 45 percent of the ocean’s bathymetric range, the hadal zone is interesting due to its unique topographical features. Where most of the sea is large, open water masses, the hadal zone is comprised of 37 distinct ‘V-shaped’ trenches and depressions on the seafloor. These spatially disjunct trenches are formed at subduction zones along tectonic plate boundaries, and so the majority are found around the Pacific Rim. As well as sporting huge geographical distances between them, each trench varies in primary productivity, temperature, and maximum depth, ranging from 6,669 meters in the Middle American Trench to 10,989 meters in the Mariana Trench.

Like the rest of the deep sea, the hadal zone is an extreme environment characterized by the lack of natural light, low food availability and high levels of hydrostatic pressure increasing one atmospheres every 10 meters. These conditions make for a challenging environment for life to thrive in. A combination of these environmental factors with trench topography and geographical isolation are likely to have driven patterns of adaptation and ultimately speciation in the hadal zone.

To truly understand how life has evolved to survive in this challenging environment we need to fully explore our deep oceans. As it stands the hadal zone remains the most underexplored and misunderstood habitat on Earth.

This article was featured in the Deep Sea issue [January/February 2019] of ECO. To continue reading, click here.

Words by Heather Ritchie, University of Aberdeen