STUDIES ON THE DIATOM GENUS NAVICULA BORY. VIII. VARIATION IN VALVE MORPHOLOGY IN RELATION TO THE GENERIC DIAGNOSIS BASED ON NAVICULA TRIPUNCTATA (O. F. MÜLLER) BORY

Abstract

Ultrastructural variation in valve morphology between members of Navicula “sensu stricto” is reviewed. Central and polar external raphe fissure paths, internal raphe features, the type and arrangement of the areolae, and girdle band morphology are documented and compared with these features in the generitype, Navicula tripunctata. Although the latter is the nomenclatural type, it is not required by the Botanical Code to be morphologically typical of the genus, although the generic circumscription must encompass it. Thus morphological variation across a genus should be expected to be wider than that shown by the generitype and taxonomic decisions should not be based on comparisons with the generitype only. The observed morphological variation within Navicula is discussed in relation to the recognition and definition of characters, particularly those used as the basis of some recent taxonomic revisions. It is argued that there is a continuum of variation in many morphological features within Navicula, and that, unless clear discontinuities in characters can be demonstrated, separation at the generic level is undesirable. The generic description is amended to reflect the observed range of morphology and structure.