Stephanopyxis species (Bacillariophyceae) from shelf and slope waters of the Argentinean Sea: Ultrastructure and distribution

Abstract

Stephanopyxis is a cosmopolitan planktonic marine diatom genus. It comprises a large number of fossil species but only four living species: S. nipponica, S. orbicularis, S. palmeriana and S. turris. In this study, the morphology and occurrence of Stephanopyxis species were studied by means of light and scanning electron microscopy from the inner shelf to slope waters of the Argentinean Sea. Two species, S. nipponica and S. turris, were found. In both species the ultrastructure of the valve showed two different morphological forms. One form had true poroid aereolae, with an external foramen and an internal cribral velum while the second one had a network of hexagonal compartments with an external opening but lacking an internal cribral velum. The structure of the linking rimoportulae and the presence of acceptant process in the vegetative cells of S. nipponica distinguished this species from the other living Stephanopyxis species. We consider that the orientation of the rimoportulae located on the mantle margin as well as the presence of scattered rimoportulae on the valve surface in the resting spores are not useful taxonomic characters to differentiate between the vegetative and resistance cells of S. nipponica. Stephanopyxis turris' main morphometric and ultrastructural features coincided with the diagnosis of this species. However, some lightly silicified specimens presented a different type of areolae in the valvar mantle, not previously described. We give also new information on cingulum structure for S. turris, which is high with a narrow valvocopula and numerous segmented girdle bands, similar to other species of the genus. Finally, we provide data on the S. nipponica and S. turris occurrence, cell abundance and environmental data in the Argentinean Sea, including remarks on their distribution in southern South America.