The type species of the diatom genus Chaetoceros

Chaetoceros is one of the most abundant and diverse diatom genera in the phytoplankton of the world's oceans. The genus was erected by Ehrenberg in 1844 for two species from the Southern Ocean: C. dichaeta and C. tetrachaeta. According to his description, the former possesses two setae per valve and the latter possesses four. Chaetoceros tetrachaeta, or any species with four setae per valve, has never been reported again. In spite of that, Boyer, in 1927, designated it as the lectotype of the genus. In order to comprehend what Ehrenberg meant with C. dichaeta and C. tetrachaeta we re-examined the type material preserved on mica slides and compared specimens therein with the original descriptions, the drawings published by Ehrenberg in 1872 and notes accompanying the slides. Since no types have ever been designated, we here lectotypify specimens for the two species, guided by Ehrenberg's coding system explaining their location on the slides. Of the two species, C. dichaeta is well-studied and commonly reported. Its lectotype links unambiguously with those of specimens for which ultrastructural and molecular information has been published. Regarding C. tetrachaeta, our observations confirmed that each valve possesses just two setae, which are robust and thick, typical for the section Chaetoceros. A possible synonym, C. adelianus, occurs in the same region and exhibits the same overall morphology.