2015 Volume 31 Pages 1-11
A new diatom species, Cyclostephanos ramosus sp. nov., is described from Pleistocene sediments of the Paleo-Kathmandu Lake, Nepal. Cyclostephanos ramosus is a polymorphic species characterized by three morphological variations, which are related to valve silicification, valve size and stratigraphic distribution. Heavily-silicified valves exhibit an annulus and network-like structures in the valve center and narrow secondary costae in the mantle area. Other valves have a variable number of valve face areolae, mantle areolae, costae, and mantle fultoportulae, and show differences in the shape of the mantle costae. The number of costae and mantle fultoportulae also shows variations related to stratigraphic distribution, which occur on a time scale of a few tens of thousands of years. On the basis of morphological characteristic features, C. ramosus is most similar to Cyclostephanos malawiensis, but differs in its domed cribra on the mantle, much deeper mantle and larger distance between spines and mantle fultoportulae. The three morphological variations have different implications. Valves showing variation in silicification reflect an ecomorphological response to available silica, while valves with size-related variations show morphological modification brought about by vegetative cell divisions. The stratigraphic variations are highly suggestive because they indicate that the number and shape of costae, which have been traditionally used to delineate Cyclostephanos species, can vary intraspecifically on long time scales. These facts suggest the careful examination of intraspecific morphological variations on various time scales is required for the taxonomy of Cyclostephanos.
