Marine tardigrades in the Antarctic and sub-Antarctic regions are poorly understood. During the 59th Japan Antarctic Research Expedition, a sediment sample was collected from Lützow-Holm Bay, East Antarctica. The sediment sample yielded three species of marine tardigrades belonging to the family Styraconyxidae Kristensen & Renaud-Mornant, 1983: Angursa antarctica Villora-Moreno, 1998, Styraconyx cf. qivitoq Kristensen & Higgins, 1984, and S. takeshii sp. nov. The new species resembles S. nanoqsunguak Kristensen & Higgins, 1984 by the overall morphology, but the new species has thick seminal receptacle ducts that do not terminate in typical, swelled vesicles, in contrast to S. nanoqsunguak’s narrow seminal receptacle ducts that terminate in small vesicles. The presence of the undivided mouth papilla and the larger body further differentiates the new species from S. nanoqsunguak. We also report a young adult female with a gonopore, but lacking seminal receptacles in the new species. As well as morphological information, we provided the sequences of the new species’ nuclear 28S rRNA and mitochondrial cytochrome c oxidase subunit I fragments. Using the former molecular data, phylogenetic analyses of Styraconyxidae were conducted and polyphyly of Styraconyx was suggested. The biogeography of marine tardigrades in the Antarctic and sub-Antarctic regions are also discussed.
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