| Summary: | Ostracods are small-sized crustaceans, which inhabit all aquatic ecosystems and, because they have a comprehensive fossil record, are important environmental and paleoenvironmental indicators. However several aspects of the ecology of modern species (the basis for the paleontological investigations) are still controversial. Previous authors have raised the hypothesis that benthic ostracods, because of their calcified carapaces, are unable to survive below the Carbonate Compensation Depth (CCD). Herein we test this hypothesis based on (1) ostracods newly collected from the Kuril-Kamchatka Trench at depths far below the CCD during the KuramBio II expedition; and (2) a compilation of all previously published records of (geologically) Recent deep-sea Ostracoda in regions deeper than 3500 m. The KuramBio II expedition provided hundreds of living, hadal ostracods from at least 30 species and 21 genera from thousands of meters deeper than the CCD in the Kuril-Kamchatka Trench region. Additionally, the KuramBio II expedition provided the deepest record (9307 m) of a living ostracod with calcified carapaces: specimens of the genus Krithe. Finally, the compilation of all published information on living ostracods show that a highly diverse assemblage both at high and low taxonomic levels (2 subclasses, 4 suborders, 25 families, 89 genera and at least 206 species) occur below 3500 m. Therefore, we conclude that contrary to previous beliefs, the new data from the Kuril-Kamchatka Trench and the compilation of the literature show that ostracods do live and are even sometimes abundant below the CCD.
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