Examination of diatom type material: Nitzschia delicatissima Cleve,Thalassiosira minuscula Krasske,and Cyclotella nana Hustedt

Type material of Thalassiosira minuscula Krasske and material identified by the respective authors as Nitzschia delicatissima Cleve and Cyclotella nana Hustedt (= Thalassiosira pseudonana Hasle et Heimdal) have been examined in light and electron microscopes. It is suggested that the slide labelled “Nitzschia delicatissima Helder 7/5–97” is accepted as neotype of the species since the original material must be regarded as lost, and that Thalassiosira pseudonana is identical to the diatom isolated by Guillard as clone 3-H. Nitzschia actydrophila Hasle and Thalassiosira monoporocyclus Hasle are rejected as separate species, the first being conspecific with N. delicatissima and the second with T. minuscula. The question is raised whether more emphasis should be laid on iconotypes of small diatom species identifiable only in the electron microscope than on real individuals.