Online ISSN : 2186-8565
Print ISSN : 0911-9310
ISSN-L : 0911-9310
Diatoms in upper Pliocene–lower Pleistocene sediments, subpolar North Atlantic: 4. Thalassiosira hexagona sp. nov.
Tatsuya Hayashi Masao Ohno
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2022 Volume 38 Pages 1-13


A new diatom species, Thalassiosira hexagona sp. nov., is described from lowermost Pleistocene sediments in the subpolar North Atlantic. The new species is morphologically characterized by linear striae, a ring of six evenly spaced subcentral fultoportulae, a single ring of marginal fultoportulae, spines fused with external tubes of the marginal fultoportulae, and a shallow mantle area with ribs. The new species is readily identified under scanning electron microscope observation because both the hexagonal ring of subcentral fultoportulae and the spines fused with external tubes of marginal fultoportulae are species-specific characteristics. However, the new species may have been misidentified as other centric diatom species with linear striae in previous studies because the species-specific characteristics are difficult to identify under light microscope observation. Among the known species, T. anguste-lineata is the morphologically most similar species to T. hexagona because both species exhibit linear striae and a ring of subcentral fultoportulae. However, all species with a ring of subcentral fultoportulae, except T. hexagona, display intraspecific changes in the valve face striation pattern and the number of subcentral fultoportulae. The changes in the shape of the ring of subcentral fultoportulae and the changes in the valve face striation may be related to each other. Thalassiosira hexagona also resembles species in the genus Planktoniella in the presence of two rimoportulae and mantle ribs. A recent molecular phylogenetic study suggested that T. anguste-lineata, the species most similar to T. hexagona, belongs to a clade that is sister to Planktoniella species. Thus, in future it may be necessary to transfer T. hexagona and T. anguste-lineata to another genus.

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© 2022 The Japanese Society of Diatomology
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