Morphology and ecology of Cyclostephanos delicatus (Genkal) Casper Scheffler (Bacillariophyceae) in comparison with C. tholiformis Stoermer, Hakansson Theriot.
Dreßler, Mirko Hübener
Material derived from several mass developments of Cyclostephanos delicatus (Genkal) Casper & Scheffler in younger sediments of shallow, eutrophic Lake Vielbeck (Northern Germany) was used for morphological and ecological investigations. The mass occurrence of C. delicatus at a sediment depth of 40 cm is considered to have started approx. 70 years ago, and it became the dominant species of all diatoms recorded between a sediment depth of 30 cm and the top of the core. C. delicatus from these samples was compared with a very similar taxon, Cyclostephanos tholiformis Stoermer, Håkansson & Theriot. The examinations included light and scanning electron microscopy of both, C. tholiformis (Isotypes from the type locality Lazy Lagoon, North America) and C. delicatus (Lake Vielbeck, Northern Germany) as well as the evaluation of the ecological data of Lake Vielbeck. The latter is similar to reported data from other localities of both, C. delicatus and C. tholiformis, which prefer well-mixed shallow waters with high nutrient levels and summer temperatures, often connected with high organic load.The extensive material of C. delicatus obtained from sediments of Lake Vielbeck showed great morphological variability which mainly concerning cell diameter, degree of silicification, appearance and number of spines, mode and size of areolae and number of cowlings around the mantle area fultoportulae. In this regard, the original descriptions of C. delicatus and C. tholiformis proved not to be complete. Both, this fact and much confusion in the related literature, complicated the differentiation between these two species. As a result of our investigations it must be concluded that the morphological variability of C. delicatus is much greater than assumed in the hitherto existing literature. Furthermore, C. tholiformis lies within the range of morphological variability of C. delicatus and it must be formally expressed as a synonym of C. delicatus.A second important result of our investigation is the fact that the number of cowlings surrounding inside tubes of mantle area fultoportulae is not always constant within a species, because we found some valves of C. delicatus with two and some with three cowlings around the inside tubes of mantle area fultoportulae. It must therefore be stressed that the number of cowlings is not always a reliable criterion for species differentiation within the Thalassiosiraceae.