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

Abstract

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 Kützing 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.

Keywords:

• Cyclotella
• landmarks
• morphometrics
• morphospecies

Acknowledgements

Thanks to Koenraad Muylaert and Koen Sabbe for providing the sample from the Schelde. This paper was made possible by B. Beszteri's attendance at the Amsterdam Morphometrics Workshop in 2002, helpful suggestions from Edward Theriot, and the Stony Brook Morphometrics web site (http://life.bio.sunysb.edu/morph/). We would also like to thank Friedel Hinz and Ute Bock for their help with the electron microscopy, Uwe John and Alberto Garcia Sáez for discussions, and Eileen Cox, Marina Montresor, Richard Crawford and two anonymous reviewers for valuable comments on different versions of the manuscript, which helped to improve it greatly. This work was in part supported by the project ‘Algaterra’ of the German Federal Ministry of Education and Research (project ID 01LC0026, http://www.algaterra.org) and by grant no. FKFP-0154/2000 of the Hungarian Ministry of Education.