REVISION OF THE GENUS FRAGILARIA

Abstract

A large number of species usually placed in Fragilaria have been examined, mainly by scanning electron microscopy. As a result Fragilaria is more narrowly circumscribed, with F.capucina as the type species. Six species are definitely placed in Fragilaria and others noted for further investigation. Five genera are separated from Fragilaria; these are Staurosira (to encompass the ‘construens’ group), Staurosirella (type species S.lapponica = F.lapponica), Pseudostaurosira (type species S. brevistriata = F. brevistriata), Punctastriata (type species P. linearis) and Neofragilaria (type species N.virescens = F.virescens). Some species other than the type have also been transferred to the new genera and others indicated as requiring further study. The characters used for their distinction are discussed, and include valve striation, linking spines, apical pore fields, rimoportulae, girdle and plastids.