The genus Orthoseira Thwaites: historical overview, current status and recommendations for the future

We provide a detailed history of the taxa included in the genera Orthoseira, Guarreraea and Phycocavernosa, their circumscription, taxa included in them, and current status of each taxon. The historical overview, covering over 160 years, includes the many ideas about the genus Orthoseira, from its original description by Thwaites, to other interpretations by Smith, Crawford, Spaulding and Kociolek, and Houk. Most recently, Orthoseira species have been assumed to possess the feature of carinoportulae, which are centrally-placed pores differentiated in size and structure from areolae. The challenges presented by the generitype of Orthoseira, O. americana are discussed, since its original illustration did not indicate the presence of carinoportulae and original material of this diatom is apparently lost. The following recommendations are made in light of the current situation: 1) That the genus name Orthoseira be reserved only for O. americana, the species described as Melosira americana by Kutzing from South America, since this species was never illustrated with carinoportulae. 2) Further revision of taxa previously assigned to Orthoseira, as well as taxa shown to possess carinoportulae but assigned to other genera, as well as descriptions of new taxa, are encouraged. When new species are proposed they could provisionally be placed in Ortho seira and later transferred to different genera if required. 3) Formal phylogenetic analysis with molecular and morphological data should be carried out. The phylogenetic relationships from these analyses should be the basis of the classification system for the group.