Competing processes shape variable deep seafloor diversity between hemispheres

Abstract The deep sea is the last ecosystem for which patterns of biodiversity are poorly defined. The occurrence, strength, shape and drivers of distributional trends in species richness throughout the deep sea are broadly inconclusive and are poorly explored at a global scale1,2, but this environment faces accelerating pressures with a changing climate and as technological developments open it to human exploitation3,4. Historically, surveys of the deep ocean have been spatially limited and despite recent efforts, biological collections have been taken from 250,000 species-level coordinate records for the entire global, bathymetric and taxonomic extent of the benthic marine class Asteroidea. For the first time, we have produced a complete comparison of seafloor diversity between hemispheres and across major oceanic biogeographic realms. We reveal highly variable patterns of biodiversity by depth zone and substantial hemispheric divergence, explained in part by geographic, environmental, and taxonomic factors. Global shallow diversity peaks in tropical waters, closely matching seafloor temperature variation. Contrastingly, peaks of species richness shift towards temperate latitudes throughout bathyal and abyssal zones, and are better characterised by a combination of temperature and nutrient flux, with the relative importance of each factor differing between hemispheres and depth zones. Uniquely, we show deep-water biodiversity is restricted above a thermal threshold, with the coldest waters unable to support high species richness regardless of other factors. We also show for the first time that diversity differs enormously between hemispheres and Atlantic and Indo-West Pacific realms and find temperature, carbon flux and geographic complexity as main drivers for these differences. We therefore present a rare and valuable spatial description of biodiversity across the global oceans and describe patterns unlike those previously reported for shallow-water or terrestrial environments.