Polymorphism in the desmid Staurastrum pentacerum (Wolle) G. M. Smith

Abstract

Morphological studies, with emphasis on apical radiation, were carried out on phytoplankton samples from several Muskoka District (Ontario, Canada) lakes which contained desmids identifiable as Staurastrum pentacerum (Wolle) G. M. Smith var. pentacerum (5-radiate), S. pentacerum var. hexacerum Irénée-Marie (6-radiate), S. pentacerum var. tetracerum (Wolle) G. M. Smith (4-radiate) and Staurastrum sublongipes G. M. Smith (3-radiate). These taxa could not be distinguished on any morphological character except apical radiation. Cells which had differing patterns of radiation on the two semicells were observed in almost all populations. These dichotypical (“Janus”) forms indicate that apical radiation is a variable character. Thus these four taxa belong to a single highly polymorphic species, S. pentacerum.