Abstract
It has been observed that the ciliate Euplotes grows significantly in mass cultures of the rotifer Brachionus plicatilis when the cultures deteriorate. To reveal the growth factors of Euplotes sp., we carried out culture experiments with individually inoculated Euplotes sp. by using five different microbes preferred by the rotifer for food, and observed the intracellular contents of the ciliate by microscopy. Euplotes sp. showed high growth rates (μmax; 1.0∼2.0) on the food microbes including the inactive microalgae, Nannochloropsis oculata. However, active cells of N. oculata did not promote the growth of Euplotes sp., but significantly suppressed it compared with the control experiment of seawater alone. Furthermore, the active alga was never observed within the cells of Euplotes sp. These findings suggest that Euplotes sp. competes for dietal microbes preferred by B. plicatilis except for active N. oculata, and that active N. oculata is an effective food for preventing the proliferation of contaminated Euplotes sp. in a stable culture of the rotifer.
