Media release

From: Springer Nature

Depth zones divide shelled and soft-bodied deep-sea creatures

A distinct transition zone, more than 4 km below the water’s surface, that divides deep sea organisms by body type is reported in a study in Nature Ecology & Evolution. Animals with shells were found to be abundant above the transition boundary, whereas soft-bodied animals dominated in the deep abyss.

The abyssal zone of the deep sea is the most extensive habitat on Earth, covering over 60% of the Earth’s surface, but also the least explored. It occurs between 3 and 6 km below the surface, where sunlight cannot penetrate, temperatures range between 0.5 and 3 °C, and organisms must be adapted to cope with extreme pressures. Although it is thought that there are fewer species in the deep ocean than in shallower ecosystems, it is unclear how biodiversity changes within the abyssal zone.

Erik Simon-Lledó and colleagues analyse data on the occurrence of more than 50,000 megafauna (animals larger than 10 mm) living near the seabed, collated from images taken from 12 deep-sea expeditions within the Clarion–Clipperton Zone — an area in the Pacific Ocean. The authors identify two distinct zones of deep-sea fauna: shallow abyssal communities at depths between 3.8 and 4.3km that are dominated by soft corals, brittle stars (a close relative of starfish) and shelled molluscs; and deep abyssal communities at depths between 4.8 and 5.3 km depth that are dominated by anemones, glass sponges and sea cucumbers. The intermediate transition zone between these zones consisted of a mixture of the two communities. Biodiversity was maintained across these boundaries, instead of decreasing with depth as is typically expected in the deep ocean. The authors suggest that this strong zonation in the abyss is likely to be driven by the carbonate compensation depth: the point at which calcium carbonate in seawater becomes undersaturated, and conditions for the formation of animal shells become unfavourable.

The authors argue that climate change and ocean acidification may alter this zonation, which, together with the effects of deep-sea mining, makes this a vulnerable ecosystem.