The distribution and classification of the problematic Fragilaria (Virescens v.) exigua Grun.Fragilaria exiguiformis (Grun.) Lange-Bertalot: a new species or a new genus?
The nomenclature of the taxon first recorded as Fragilaria (virescens) v. exigua by Grunow in Cleve & Moeller (1878) is examined with regard to the current botanical name, F. exiguiformis Lange-Bertalot. The distribution of this taxon in relation to pH is described from surface sediment samples collected from more than 200 soft-water lakes in both NW Europe and Antarctica. The optimum pH requirement is shown to be different in the two regions with a lower optimum (pH 5.7) for the European data set. One reason for this difference is the lack of moderately acid sites in the Antarctic data set. Another is that SEM investigation of F. exiguiformis from several localities shows that at least two taxa, characterized inter alia by presence or absence of spines, are present within this species. Lack of a rimoportula indicates that taxa in the F. exiguiformis complex should be transferred elsewhere. Also, the recognition of a non-spinose taxon within F. exiguiformis indicates that a new species description is required. Consequently and because the species complex cannot be incorporated satisfactorily into existing genera, a new genus, Stauroforma, and a new species, S. inermis are described. The species epithet is retained for the spinose taxon which becomes S. exiguiformis. Only S. inermis occurs in the Antarctic samples but both species frequently co-occur elsewhere. These two species have different ecological optima but since they cannot be easily separated by the light microscope using bright-field or phase contrast optics, their relative distributions and ecological preferences in NW European lakes remain uncertain.
Authors: Flower, R. J., Jones, V. J., Round, F. E.