The implications of polymorphism for the systematics of Scenedesmus

Abstract

Systematic treatment of the species of Scenedesmus has always been difficult because of the extensive polymorphism. Unicells, associated with all spine bearing species, are a developmental stage found in the early log phase of growth. We compare data from four clones of S. armatus, and present simultaneously the changing ecomorph population. In a batch culture, working with a low initial cell density, unicells form first; spiny colonies and finally colonies with short spines and extensive ridges of wall material are seen in the later stages of growth. The protologue (original description) of S. armatus did not represent the full range of ecomorphs, only select late log phase and stationary phase colonies. The unicell is demonstrated to be one of the more important morphs produced; it will aid in determining species limits. The shortcomings of numerous publications in the last two decades, with an absence of, or incomplete data on individual ecomorphs, now become clear. Both the importance of the various developmental stages and the need for broader descriptions of species are discussed.