Patterns of biodiversity in the deep ocean are fundamentally different from those that govern species richness in shallower waters or on land, according to a paper published online this week in Nature. The study analyses the global distribution of thousands of species of brittle and basket stars - close relatives of the starfish - and provides a baseline for conservation efforts across the sea floor, which faces increasing pressures from deep-sea fishing and mining.

Deep-sea environments represent the largest and least explored ecosystem on Earth. Despite being home to a huge variety of life, species diversity at these depths is poorly understood and a global assessment of this diversity has not been reported, until now.

Skipton Woolley and colleagues present the distribution of 2,099 species of brittle and basket stars using data from more than a thousand research expeditions. They compare biodiversity patterns across three different ocean depths: the continental shelf (20-200 m), upper continental slope (200-2,000 m) and deep-sea (2,000-6,500 m). The authors find that deep-sea species richness peaks at higher latitudes than in the continental shelf and upper continental slope. Although diversity at shallower depths is well explained by temperature variation, deep-sea diversity is not correlated with temperature, but instead with high levels of chemical energy export (which reflects food availability). They also find that proximity to the continental margin, where a continent meets the ocean floor, is a significant predictor of diversity in the deep sea.