Fragilariopsis
From Frustule
| GENUS Fragilariopsis | ||
| Hustedt (1919) | ||
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General Remarks
Notes from Hasal (1993):
Fragilariopsis Hustedt in Schmidt (1913) and Pseudonitzschia H. Peragallo in H. & M. Peragallo (1900) [Pseudo-nitzschia] are recognized as separate genera. The features distinguishing the two genera include differences in colony formation, valve symmetry and valve areolae occlusions.
Note that synonymies for living species are given in and give the reference for: Hasle, G.R. 1993 Nomenclatural notes on marine planctic diatoms. The family Bacillariaceae. Beiheft zur Nova Hedwegia, 106: 315-321
Notes from Hasle and Syvertsen 1996:
Fragilariopsis has a canal raphe which justifies the placement in the family Bacillariaceae. The raphe system as seen with EM is simple in construction compared to that of N. sigmoidea and several other Nitzschia species (Mann, 1986). The raphe is not raised above the valve surface, the external canal wall is not poroid, and the fibulae are small and not extending across the valve (Hasle, 1965a, 1968c, 1972c; Mann, 1978). In further contrast to the subgenus Nitzschia, Fragilariopsis has no conopea (flaps of silica extending out from near the raphe), most species are narrow, not sigmoid in girdle view, and the valve striae mostly have two rows of poroids.
The little information available from EM observations on the Fragilariopsis girdle demonstrates a certain unconformity that may to some extent be related to the silicification of the cell wall. The lightly silicified F. oceanica has several distinctly striated intercalary bands, whereas those of the more heavily silicified F. cylindrus and F. curta, and especially F. kerguelensis, have one or perhaps two bands with one row of perforations (Hasle, 1965a, 1972c; Medlin & Sims, 1993).
All Fragilariopsis species examined by Hasle (1965a) are present in polar waters and are dealt with in the handbook "Polar Marine Diatoms" (Medlin & Priddle, 1990) in which a key to species is constructed (Hasle & Medlin, 1990b), as was also done by Hasle (1965a). In this chapter we include only the species more commonly recorded from plankton. For the examination of samples collected from or near sea ice Hasle & Medlin (1990b) should be consulted.
Generic characters: Cells rectangular in girdle view. Cells in ribbons united by the entire or the greater part of the valve surface. Raphe strongly eccentric. Raphe not raised above the general level of the valve. Approximately equal numbers of interstriae and fibulae. Fibulae often more distinct than interstriae (LM).
Central larger interspace lacking in most species. Valve face more or less flattened and not undulated. Valves narrowly elliptical to lanceolate to broadly elliptical or subcircular, or linear to sublinear. Apical axis often heteropolar. Valve poles usually bluntly rounded. Striae parallel except near poles. Striae with two rows of poroids, seldom one or more than two. Chloroplasts-two plates lying along the girdle, one on either side of the median transapical plane. Resting spores rare (one species). Characters showing differences between species: Valve outline. Polarity of apical axis.
Notes from Gersonde and Barcena 1998:
Round et al. (1990) proposed to reinstate the genus Fragilariopsis, established by Husted (1913) separating taxa belonging to Fragilariopsis from the genus Nitzschia in order to provide greater taxonomic resolution within a complex and provide greater taxonomic resolution within a complex and broad group of taxa. Hasle (1993) grave an emended diagonsis of the genus Fragilariopsis, which can be summarized for the treatment of fossil diatoms as follows, considering comments of Medin and Sims (1993) and Round et al. (1990): The valve outline of Fragilariopsis taxa ranges from linear-lanceolate to elliptical with isopolar or heteropolar apices. The valve face is generally flat and well separated from the steep mantle. The striae that are well developed on the inner valve. Fragilariopsis has a canal rapeh system located in strictly eccentric position at the junction of valve face and mantle, not raised about the general level of the valve. The raphe lacks terminal fissures but may be separated by a central nodule. The number of fibular is approximately the same as the number of interstriae.
Considering this generic description we follow the strategy proposed by Round et al. (1990) and propose to transfer taxa belonging to the genus Nitzchia, that are important Neogene and Quaternary biostratigraphic markers in the Southern Ocean, to the genus Fragilariopsis.
Useful References
- F. Hustedt, 1913. A. Schmidt Atlas Diat., T. 299, figs 9-14
