Specific Features of Mixed Culture of Invertebrates Maintained under Laboratory Conditions

Competition among invertebrates was studied in mixed culture (the ciliates Paramecium caudatum and Paramecium bursaria and the rotifer Brachionis plicatilis) under laboratory conditions. The invertebrates were fed on various food sources: 1) Chlorella biomass and 2) biomasses of bacteria and yeast. The P.bursaria – Chlorella condutrix endosymbiosis was used a natural model of a simple biotic cycle. P.caudatum was found to be a poor competitor: even when the initial percentage of P.caudatum cells was large and there was no food limitation, this ciliate had been expelled by the end of the experiment. The main reason for the low competitive ability of P.caudatum in the studied system was likely the inhibiting effect of invertebrates’ metabolites. The outcome of the competition between the ciliate P. bursaria and the rotifer B. plicatilis was different depending upon the food source: 1) Chlorella or 2) bacteria and yeast. When fed on Chlorella, P. bursaria was a better competitor than B. plicatilis. With bacteria and yeast used as food, the opposite outcome of competition was observed: the competitive ability of B. plicatilis was better than that of P. bursaria. It was assumed that due to turbidity of the culture solution in experiments with bacteria and yeast used as food, the zoochlorella photosynthesis rate decreased and, hence, the P. bursaria – zoochlorella symbiosis became less competitive. Analysis of experimental results of P. bursaria and P. caudatum single species cultures in the light and in the dark, with bacteria used as a food source, confirmed that assumption. The results of the experiments confirmed the beneficial role of symbiotic relationships of the ciliate P. bursaria and zoochlorella. The number of symbiotic units in the light reached 220 units/ml, while in the dark it was much smaller. The P. caudatum number remained practically the same in the light and in the dark.