Conventional and geometric morphometric studies of valve ultrastructural variation in two closely related Cyclotella species (Bacillariophyta)

Conventional and landmark-based geometric morphometric approaches were used to clarify the taxonomic identity of a centric diatom morph. This morph, found in cultures isolated from the Geeste estuary ( northern Germany), showed an intermediate valve morphology between that of typical specimens of Cyclotella meneghiniana Kutzing and of Cyclotella scaldensis Muylaert & Sabbe. Its internal valve ultrastructure was compared to that of cells from ( 1) cultures of C. meneghiniana isolated from the same field samples and grown in the same conditions, ( 2) field material from the samples from which the cultures were isolated, and ( 3) a sample from the type locality of C. scaldensis. The morphometric analyses were used to determine ( 1) whether morphological variation of these morphs was continuous, or whether there were distinct morphological groups, and ( 2) how effective the alternative morphometric approaches were in answering this question. Both approaches proved informative and their results complemented each other, supporting the conclusion that three distinct size-reduction series were present in the samples investigated. Since the different morphs occurred sympatrically, we suggest that they probably belong to three reproductively isolated species.