Testudodinium magnum sp. nov. (Dinophyceae), a novel marine sand-dwelling dinoflagellate from subtropical Japan

摘要

A new marine benthic dinoflagellate, Testudodinium magnum sp. nov., was described from a sand sample collected on the seafloor at a depth of 35 m off Mageshima Island, Kagoshima, Japan. The dinoflagellate possessed the characteristic features of the genus Testudodinium, but was distinguished by its extremely large size and pebbled dorsal surface of the hyposome, caused by the presence of numerous nodules. The cells of this dinoflagellate possessed a dominant sessile and a rare motile form that were morphologically distinct from each other: The sessile form was larger and the small rounded episome with longitudinal furrow was completely embedded in the hyposome; the motile form was smaller and the episome was only partly embedded in the hyposome. The ultrastructural investigations revealed the presence of a large, circular, starch-sheathed pyrenoid whose matrix was traversed by randomly arranged thylakoid lamellae. The chloroplasts formed a network radiating outward from the pyrenoid. Uniquely, this dinoflagellate possessed internal props - structures that spanned the thickness of the cell and were fibrous in nature. The props were numerous and passed through organelles and cell contents. Phylogenetic analyses based on the small-subunit ribosomal DNA gene sequences placed this dinoflagellate firmly within the Testudodinium clade with high support. On the basis of the morphological features and phylogenetic analyses, we concluded that this dinoflagellate was a new species in the genus Testudodinium.