Abstract
A new genus and species of marine coccoid dinoflagellate from subtropical Japan, Halostylodinium arenarium Horiguchi et Yoshizawa‐Ebata, gen. et sp. nov., is described. The dominant stage of the dinoflagellate is a nonmotile ovoidal to spheroidal cell with a distinct stalk. The stalk consists of an upper thick tubule, a lower thin tubule, and a discoidal holdfast. The dinoflagellate possesses a yellowish‐brown chloroplast with multiple lobes radiating from a central pyrenoid. It reproduces by the formation of two motile cells, which swim for a short period and then transform directly into the stalked nonmotile cell. The stalk is produced during transformation from the apical stalk complex present in the apex of the motile cell. The apical stalk complex consists of a double‐folded apical pore plate and doughnut‐shaped holdfast‐building material. The ultrastructure of the apical stalk complex is compared with those of Bysmatrum arenicola and Stylodinium littorale. Halostylodinium arenarium possesses delicate thecal plates, and the thecal plate formula is Po, 5′, 2a, 7″, 7c, 6s, 5″′, 1p, 2″″. A phylogenetic study based on the 18S ribosomal RNA gene did not show any clear affinities between this organism and any species included in the analysis.
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