LETHALITY OF THE HELIOZOON OXNERELLA-MARITIMA ON THE ROTIFER BRACHIONUS-ROTUNDIFORMIS

A heliozoon was isolated from a mass culture tank for rotifers when th e production of the rotifer became unstable. The green algae Chlamydom onas 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 hou rs' LC50 for the rotifer was about 10(4) individ./ml. The isolated hel iozoon is free-living and has a round-shape, 10-15 mu m phi cell body. It has one eccentric nucleus, but in the center of the cell body ther e is a centroplast which radiates axonemal microtubules. TEM observati ons 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 Oxne rella maritima. This is the first record of a heriozoon killing the ro tifer Brachionus rotundiformis.