Food, Age, and the Dynamics of a Laboratory Population of Rotifers

Two clones of the rotifer Euchlanis dilatata were derived from a single parent by parthenogenesis so that in succeeding generations the individuals of one clone (the young orthoclone) were always derived from young parents and those of the other (the old orthoclone) from old parents. These clones were fed four different food concentrations (1.6, 4.9, 16.4, and 49.2 mg/ml) of three different algal species (Chlamydomonas reinhardti, Euglena gracilis, and E. geniculata). Rates of increase, birth, death, survivorship, net reproduction, and generation time were measured for all combinations of these variables. Growth rates of individual animals depend on food concentration and not age or food species. Although population growth rates in mass culture are related to both food species and food concentration, the ultimate density of the population depends only on food concentration. Individuals of the young orthoclone have higher rates of population increase than those of the old orthoclone. When Euchlanis fees upon Chlamydomonas the rates of increase are higher than when Euglena gracilis serves as food. E. gracilis, in turn, leads to higher rates than E. genicultata. The rate of population increase is directly related to the density of food for concentrations up to 16.4 mg/ml, but a further increase in food concentration does not result in a corresponding increase in the rate of population growth. These different rates of increase result more from corresponding differences in net reproduction (number of eggs laid by the average female in her lifetime) than from the schedule of egg laying of survivorship. The different rates of population growth can not be explained by differences in the rate of food intake and must result from either differences in assimilation or chemical differences in the foods themselves.