Taxonomy and ultrastructure of five naviculoid diatoms (class Bacillariophyceae) from the Rocky Mountains of Colorado (USA), with the description of a new genus and four new species
Kociolek, J. Patrick Thomas
The taxonomy and valve ultrastructure of five naviculoid diatoms from Colorado are considered. A population of Placoneis navicularis (syn. Navicula semen sensu Donkin) shows the size diminution series of this rarely reported taxon. Scanning electron microscopy of both the internal and external valve morphology suggests this species is quite different from both Placoneis and Navicula sensu stricto taxa and other known naviculoid genera. Based on these differences, a new genus, Rexlowea, is proposed to accommodate this taxon. In addition, the size diminution series and valve ultrastructure of four new species of Placoneis are considered. Each species is described and compared with other known members of the genus. Similarities and differences between these new species and other previously documented taxa are discussed. The range of morphological diversity currently assigned to Placoneis suggests that it is either a much larger genus than originally circumscribed or that it is becoming a place to assign species whose phylogenetic relationships may be distant.