Lethality of the Heliozoon Oxnerella maritima on the Rotifer Brachionus rotundiformis

Abstract

A heliozoon was isolated from a mass culture tank for rotifers when the production of the rotifer became unstable. The green algae Chlamydomonas sp. was a more suitable food for the propagation of the heliozoon compared to Tetraselmis tetrathele, Nannochloropsis oculata and baker's yeast. When the heliozoon was placed with the rotifer, the rotifer gradually ceased to move, sank to the bottom, and finally died. 24 hours' LC50 for the rotifer was about 104 individ./ml. The isolated heliozoon is free-living and has a round-shape, 10-15 μmø cell body. It has one eccentric nucleus, but in the center of the cell body there is a centroplast which radiates axonemal microtubules. TEM observations revealed that each axoneme is composed of six hexagonally arranged microtubules. Further, no coating structure was observed on the cell surface. From these characteristics, we identified the species as Oxnerella maritima. This is the first record of a heliozoon killing the rotifer Brachionus rotundiformis.