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M. LURLNG AND W. BEEKMAN. 1999. Grazer-induced defenses in Scenedesmus (Chlorococcales; Chlorophyceae): coenobium and spine formation. Phycologia 38: 368-376.
Three strains of the nonspiny Scenedesmus acutus Meyen, one strain of the nonspiny S. falcatus Chodat, and three strains of the spined Scenedesmus quadricauda (Turpin) Br6bisson were cultured in standard medium and in medium containing filtered water from a culture of the grazer Daphnia. Nonspiny strains were examined to detect grazer-induced colony formation. Spine length was measured in spined strains. Strains varied considerably in their response to chemicals from Daphnia. Three nonspiny strains showed rapid formation of four- and eight-celled coenobia in the presence of Daphnia water, whereas all nonspiny cultures remained unicellular in standard medium. By contrast, no grazer-induced colony formation occurred in spined strains. One spined strain formed eight-celled coenobia both in the absence and the presence of Daphnia medium. No evidence for grazer-induced spine formation was found. Growth rates were not affected by Daphnia water in all strains, but differed among strains. The phenomenon of grazer-induced coenobium formation seems widespread, but not universal within Scenedesmus.
INTRODUCTION
Morphological variability is widespread among phytoplankton (Trainor et aL 1971). In the pelagic zone, changing environmental conditions may favor one clone over another (Wood & Leatham 1992), or organisms belonging to the same clone may alter their morphology (West-Eberhard 1989). The latter phenomenon, phenotypic plasticity, is characteristic of the common green algal genus Scenedesmus (Trainor 1991). This genus is a representative of the 'colonial' Chlorococcales (Van den Hoek et al. 1995) and has been reported to contain more than 100 species (Uherkovich 1966; Hegewald 1982). Approximately 1330 taxa have been described (Hegewald & Silva 1988), but taking into account phenotypic plasticity in Scenedesmus, the number of species is most probably closer to 13 (Trainor & Egan 1990). Although a member of the 'colonial' Chlorococcales and possessing a typical colonial appearance in the field (e.g. Holtmann & Hegewald 1986; Krienitz 1987; Canter-Lund & Lund 1995), in culture Scenedesmus often fails to form colonies and remains unicellular (Fott 1968; Van den Hoek et al. 1995; Trainor 1998).
Many factors may influence the formation of unicellular or coenobial ecomorphs; among these, chemicals released from grazers such as Daphnia are of particular interest. After exposure to these chemicals, unicellular Scenedesmus...