Scalariella a new genus of monoraphid diatom (Bacillariophyta) with a bipolar distribution
Résumé
The identity of a small-celled diatom Naviculadicta pseudofallacia WITKOWSKI, METZELTIN et LANGE-BERTALOT, originally described from Bear Island, southernmost island of the Norwegian archipelago Svalbard, is reconsidered. Observations of marine samples from the Kerguelen archipelago (Southern Ocean, Indian Ocean sector) revealed that this species also occurs in the Subantarctic region. The original classification within Naviculadicta was erroneous since this species is a monoraphid taxon and belongs to the Achnanthales. Its morphological features justify creating a new genus Scalariella RIAUX-GOBIN, which is distinguished from other achnanthoid genera by a peculiar raphe system, the stria structure of the sternum valve (each stria composed of a depressed macroareola), and the presence of a lateral solid area in the raphe valve, splitting each stria into two areolae. Based on light and electron microscopy, N. pseudofallacia is renamed Scalariella pseudofallacia (WITKOWSKI, METZELTIN et LANGE-BERTALOT) RIAUX-GOBIN et WITKOWSKI comb. nov. The genus also includes a second and rare species, observed in the Kerguelen material, S. oblongella RIAUX-GOBIN, WITKOWSKI et RUPPEL, which is described and illustrated, but which needs complementary observations. The morphology of Scalariella is compared to that of some genera split from the genus Achnanthes BORY. The biogeography of Scalariella pseudofallacia, a marine taxon probably misidentified in the past due to its small size, is reconsidered with respect to its affinity for subpolar, cold water habitats, in both hemispheres.