The ultrastructure of a symbiotic dinoflagellate Zooxanthella nutricula within the radiolarian Didymocyrtis tetrathalamus collected off the northwestern coast of Okinawa Island, Japan was examined in the symbiotic state and the cultured motile stage. In hospite, Z. nutricula had a coccoid morphology, and the typical dinoflagellate structures, i.e., cingulum, sulcus, and flagella were lost. The coccoid non-motile cells were surrounded by a perialgal envelope of the host radiolarian cytoplasm, and they possessed a dinokaryon with condensed chromosomes and mitochondria with tubular cristae. The chloroplast was located at the periphery of the cell and had one or two pyrenoids. In culture, however, the motile cells exhibited a typical cell shape of peridinioid dinoflagellates. The cell covering consisted of a plasma membrane and amphiesmal vesicles containing the thecal plates whose arrangement was congruent to the order Peridiniales. The chloroplast was enclosed by three membranes and the pyrenoid was the double-stalked type. Pusules constructed by tubules with invaginations were present, but eyespot and trichocysts were lacked. Symbiotic dinoflagellates, such as Zooxanthella, Symbiodinium, and Amphidinium, have historically been referred to using the general term ‘zooxanthella(e)' because of their similar appearances as yellow-brown spherical minute cells under light microscopy. However, the motile cells of Z. nutricula exhibited a typical peridinioid morphology and thecal plate arrangement that obviously differed from those of the other Symbiodiniaceae genera and Amphidinium. Considering the ultrastructural features and the currently available sequence database, it was suggested that Z. nutricula is a symbiotic dinoflagellate that lives specifically in holoplanktonic protist radiolarians.
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