Interspecific Relations between Marine Rotifer Brachionus rotundiformis and Zooplankton Species Contaminating in the Rotifer Mass Culture Tank

We investigated the interaction between marine rotifer Brachionus rotundiformis (formerly called S-type B. plicatilis) and zooplankton species which contaminate in the rotifer mass culture tanks. These species include Brachionus plicatilis (formerly called L-type B. plicatilis), Diaphanosoma celebensis (cladoceran), Tigriopus japonicus (copepod) and Euplotes sp. (protozoan). We compared the population growth of 40ml single-species and two-species of mixed cultures fed on phytoplankton Nannochloropsis oculata at 8×105 cells/ml (food limitation level). We observed three manners of interspecific relations that include competition, commensalism and amensalism. Predations or mechanical damages were not observed between tested species. The population growth of B. rotundiformis, B. plicatilis, and D. celebensis in mixed-species cultures were significantly suppressed when compared with those in single-species cultures. This indicates that the competition for the limited amount of food suppressed mutual population growth. Thepopulation growth of B. rotundiformis was not affected by the co-existence of T. japonicus, however. T. japonicus grew better when cultured with B. rotundiformis. Contrary, the existence of B. rotundiformis did not affect the growth of Euplotes sp, but Euplotes sp. population interferes the growth of B. rotundiformis. These suggest the possibility of bacterial intervention in the interspecific relation of T. japonicus and Euplotes sp. with B. rotundiformis.