It was investigated whether (1) the number of cilia of the dorsal kineties 3 and 4 and (2) the cell length are species-specific characters which can be used to distinguish the sibling species S. mytilus and S. lemnae. The number of cilia of the dorsal kineties 3 and 4 is a relatively constant, reliable species-specific character in all investigated strains of both species and rather independent from the origin and the nutritional condition of the cells. The cell length is also a reliable species character, if strains of both species from Germany are compared (under identical nutritional conditions). However, all S. mytilus strains from China, Australia and Peru are significantly smaller forming one (or some) "small" subpopulations or subspecies, compared with a "big" subpopulation from Germany. The small ones cannot always be distinguished by size alone from S. lemnae cells. Thus the cell size in S. mytilus is not in all geographic regions a species character, but can be used to characterize subspecies. © 2008 Elsevier GmbH. All rights reserved.
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