Abstract
Background and aims – The diatom genus Simonsenia has been considered for some time a minor taxon, limited in its distribution to fresh and slightly brackish waters. Recently, knowledge of its diversity and geographic distribution has been enhanced with new species described from brackish-marine waters of the southern Iberian Peninsula and from inland freshwaters of South China, and here we report novel Simonsenia from fully marine waters.
 Methods – New isolates of Simonsenia species were obtained from marine waters, the littoral zone of the Korean Yellow Sea coast and the Gulf of Mexico in Corpus Christi (Texas), and documented in LM, SEM and with DNA sequence data (plastid-encoded rbcL and psbC). Phylogenetic trees of raphid diatoms were constructed to assess the relationships of the new species and of the genus as a whole.
 Key results and conclusions – Two novel species of Simonsenia (S. eileencoxiae and S. paucistriata) are described and a further putative taxon is characterized morphologically. The molecular phylogeny of the new Simonsenia species and previously sequenced species supports both the monophyly of the genus and its place within the Bacillariaceae. The Simonsenia clade clusters with clades composed of Cylindrotheca, Denticula and some Nitzschia spp. (including N. amphibia, N. frustulum, N. inconspicua). Hence Simonsenia is firmly positioned within the Bacillariaceae by molecular phylogenies, confirming its position within this group based on the possession of a canal raphe and its ultrastructure, and rejecting its classification within the Surirellaceae. Morphological data from the new Simonsenia species is typical for the genus, with a “simonsenioid” canal raphe type supported over the valve face with fenestral braces, alar canals connecting the canal raphe with the cell lumen, and the presence of fenestrae between the alar canals externally. Our results indicate unequivocally that the biogeography and the biodiversity of Simonsenia remain highly underestimated.
Highlights
Simonsenia Lange-Bert. and its generitype species (S. delognei (Grunow in Van Heurck) Lange-Bert.) were distinguished from the Nitzschiaceae (= Bacillariaceae) on the basis of electron microscopy (SEM and TEM) observations of alar canals connecting the raphe canal with the cell lumen (Lange-Bertalot 1979)
Two new species of Simonsenia were isolated into clonal culture, whereas the third taxon was only observed in scanning electron microscopy (SEM)
Whereas the species isolated from Padori beach has a strongly eccentric raphe canal as in previously described Simonsenia species, in the Texas strain and in the Korean specimens from Shinan-Gun the raphe canal is only weakly displaced from the valve centre
Summary
Simonsenia Lange-Bert. and its generitype species (S. delognei (Grunow in Van Heurck) Lange-Bert.) were distinguished from the Nitzschiaceae (= Bacillariaceae) on the basis of electron microscopy (SEM and TEM) observations of alar canals connecting the raphe canal with the cell lumen (Lange-Bertalot 1979). Despite the morphological similarity of the alar canals and fenestrae, Simonsenia has a canal raphe clearly elevated and positioned at the valve margin, with the cell lumen closed by distinct fibulae (Lange-Bertalot 1979, Witkowski et al 2014, 2015, You et al 2016) – characters that are absent in Surirella (Krammer & Lange-Bertalot, 1988, Round et al 1990, Ruck & Kociolek 2004). While Simonsenia is strongly nested within Bacillariaceae based on the molecular phylogeny, it is morphologically distinct from its phylogenetic sister taxa
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