Despite its often featureless appearance, the deep-ocean floor includes some of the most diverse habitats on Earth. However, the accurate assessment of global deep-sea diversity is impeded by a paucity of data on the geographical ranges of bottom-dwelling species, particularly at the genetic level. Here, we present molecular evidence for exceptionally wide distribution of benthic foraminifera, which constitute the major part of deep-sea meiofauna. Our analyses of nuclear ribosomal RNA genes revealed high genetic similarity between Arctic and Antarctic populations of three common deep-sea foraminiferal species (Epistominella exigua, Cibicides wuellerstorfi and Oridorsalis umbonatus), separated by distances of up to 17, 000 km. Our results contrast with the substantial level of cryptic diversity usually revealed by molecular studies, of shallow-water benthic and planktonic marine organisms. The very broad ranges of the deep-sea foraminifera that we examined support the hypothesis of global distribution of small eukaryotes and suggest that deep-sea biodiversity may be more modest at global scales than present estimates suggest.
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