Fragilariopsis (Bacillariophyceae) of the Northern Hemisphere - morphology, taxonomy, phylogeny and distribution, with a description of F. pacifica sp. nov.

In polar areas, Fragilariopsis are among the most abundant phytoplankton organisms in plankton and ice. Four Fragilariopsis species, F. oceanica, E reginae-jahniae, F. atlantica and F. pacifica sp. nov., recorded from and apparently restricted to the Northern Hemisphere have been explored with regard to morphology, distribution and phylogeny. Morphologically the examined species are pair-wise very similar and easily confused. Hence, the numerous records of F. oceanica from sea ice and plankton in Arctic waters most likely include F reginae-jahniae. Fragilariopsis atlantica and the sibling species F. pacifica were found in the plankton, not in ice. All four species exhibit a pronounced valve variation related to the diminution of the apical length through vegetative reproduction (stadial variation). In F. oceanica and F. reginae-jahniae this is expressed by a variation in valve outline from being more linear to almost elliptical and in F. atlantica and F. pacifica by heteropolarity of the greater specimens. Fragilariopsis oceanica and F. reginae-jahniae differed in cell length and width, valve and girdle structures as did E atlantica and F. pacifica. In the phylogenetic tree of Fragilariopsis, a biogeographical subdivision appeared with the Antarctic Fragilariopsis taxa forming one clade, the bipolar taxa a second chide and the Arctic taxa were found in a third clade and basal in the tree. The phylogenetic analyses support the morphological data and show F. oceanica and E reginae-jahniae to be sister taxa. The morphologically defined E oceanica were found to be paraphyletic, representing two cryptic species.