Growth and reproduction of the parthenogenetic freshwater nematode,Plectus palustris, were studied at different controlled levels of food densities at 20° C. A bacteria-sloppy agar mixture was used as substrate and food medium. No growth or reproduction occurred at the lowest food density (8.107 bacterial cells ml-1). At 8.108 cells ml-1, the larval duration was 18.5 days, the instantaneous growth rate (g) of young larvae 0.2 d-1 and the daily fecundity rate during a prolonged period of constant egg production 12.6 eggs·d-1. At a food density of 8.109 cells ml-1, the corresponding values are 12.5 days, 0.4 d-1 and 37.7 eggs d-1.By including the data on respiration from a previous paper (Klekowski et al., 1979), the energetics of the species at different food densities can be discussed: production processes are apparently more dependent on food supply than respiration. However, prolongation of the larval phase in lower food densities greatly increases the cumulated respiratory costs per unit production. A second point is the ability to produce smaller-sized primiparous females in sub-optimal food which shortens the immature life period and serves to reduce the burden of cumulated metabolic costs for attaining sexual maturity.A comparison of the range of food densities used in the experiments with bacterial densities known from lake sediments of different trophic type suggests that food is likely to be the main factor governing the population dynamics of bacterivorous species under field situations.

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