Taxonomic revision of Coscinodiscus variabilis (Bacillariophyta): analysis of type material, lectotypification and transfer to the genus Actinocyclus

Abstract

The centric diatom Coscinodiscus variabilis, described by Joaquín Frenguelli for the coastal waters of Mar del Plata City (Buenos Aires, Argentina), was examined to review and elucidate its taxonomic position. The type material contained in the Frenguelli Collection was studied with light microscopy. The species has a particular and distinctive stellate pattern of hyaline rays on the valve face that helps in its initial identification and exhibits a remarkable morphological variation. The presence of a distinctive pseudonodulus, radial sectoring and circular valves support its transfer to the genus Actinocyclus as Actinocyclus ferrarioae nom. nov. The valve morphology of A. ferrarioae is described and compared to its original description and closely related taxa. We have estimated the rimoportulae density as a morphometric character for comparing morphologically similar Actinocyclus taxa. Since Frenguelli did not formally designate a holotype in the original description, we selected a lectotype and provide an expanded description of the species.