Energy export drives unique global patterns of deep-sea biodiversity

The deep ocean is both the largest and least explored ecosystem on Earth, and a uniquely energy-poor environment. While originally presumed lifeless due to its great depth and lack of sunlight, it has since revealed surprising... [ view full abstract ]

The deep ocean is both the largest and least explored ecosystem on Earth, and a uniquely energy-poor environment. While originally presumed lifeless due to its great depth and lack of sunlight, it has since revealed surprising diversity. However, global patterns of seafloor biodiversity, as well as their drivers and origins, are poorly understood due to the vast scale, extreme conditions, and limited accessibility of this ecosystem. Here we analyse a global database of 165,044 distribution records of Ophiuroidea (brittle and basket stars), a dominant component of seafloor fauna, to show that both global patterns and environmental predictors of deep-water (2000-6500 m) species richness fundamentally differ from those found on coastal (0-20 m), continental shelf (20-200 m), and upper slope waters (200-2000 m). Continental shelf to lower limit of upper slope water richness peaked consistently in the tropical Indo-West Pacific and Caribbean (0-30° latitude), and was well explained by global variation in ambient water temperature. In contrast, deep-sea species showed maximum diversity at higher latitudes (30-50°), concentrated in areas of high carbon export flux and close to continental margins. These findings support the species-energy hypothesis to explain gradients in seafloor species richness, but demonstrate that while kinetic (thermal) energy predicts shallow-water assemblages, chemical energy (export productivity) and proximity to slope habitats drives deep-sea diversity. Our results show that deep-sea ecosystems show a unique biodiversity pattern that conforms to neither shallow-water nor terrestrial paradigms, with significant implications for conservation planning in light of mounting impacts of deep-sea mining, fishing, and other industrial uses.