Deep-sea-floor diversity in Asteroidea is shaped by competing processes across different latitudes and oceans

The occurrence, shape and drivers of global distributional trends in species richness throughout the deep sea are poorly explored. Here we present a spatial description of the global, bathymetric and taxonomic extent of the benthic marine class Asteroidea using a compiled dataset of ~200,000 species-level occurrence records. We used these data to produce comparisons of sea-floor richness between hemispheres and oceans. We show that species richness is significantly correlated with temperature and nutrient flux despite markedly different distributional patterns across oceans and latitudes that suggest further influence from a combination of additional geographic, taxonomic and environmental factors. The relative importance of temperature and nutrient levels also varies greatly with depth. Species richness peaks in the shallow-water tropics, closely matching sea-floor temperature variation, but at bathyal and abyssal depths it is higher at temperate latitudes, where nutrient flux levels are of greater importance. We show that richness in the deep benthos is restricted below ~1.5 °C, with this strong thermal threshold consistent among oceans irrespective of other factors.