The Scenedesmus (Chlorophyceae) unicell: its importance to the species concept in the genus

Abstract

Unicell production in 21 strains of Scenedesmus was investigated in our medium 7, Bristol's medium, and soil extract to gather data to be used in species determinations. All strains produced unicells. Three types of unicells were observed. Six strains consistently produced type A unicells. These were ovate or broadly ovoid in shape (4.0–15.3 × 2.2–9.5 μm) with four polar spines. One set of two spines was at right angles to the set at the opposing pole. Associated spine-bearing coenobia produced longitudinal ridges on the walls of the cells. Ten strains were identified as S. armatus (Chod.) Chod. sensu Egan & Trainor. Twenty-four taxa are placed in synonymy with S. armatus. Type B unicells (4.6–12.7 × 2.3–9.5 μm) possessed six or more spines that were not restricted to the poles of the cells. There were two type B subgroups. A specific coenobial form could not be associated consistently with either type B, or type C unicells. Type C unicells (4.6–12.1 × 2.9–8.1 μm) possessed five or six spines restricted to the polar regions of the cells. There were three type C subgroups. The unicells of two strains of S. communis Hegew. produced only small form coenobia distinct from typical S. communis-type coenobia. Key words: cell density, development, Scenedesmus, species, spine-pattern, unicell.