Variations in reproductive form of the Yashima Rotifer, Brachionus plicatilis between 1964 and 1967

Generally, rotifer reproduce mictically in natural sources of water. However, in 1964,the marine rotifer Brachionus plicatilis, collected from Yashima Bay, Kagawa Prefecture, Japan was cultured by feeding it with Nannochloropsis oculata in outdoor tanks at the Yashima Station, Seto Inland-Sea Farming Association. In the late autumn and early winter of 1964-65,many eggs of B.plicatilis were observed at the bottoms of the tanks. They remained in the egg state for the winter ; that is, they have remained in the amictic state year-around since 1966. Since then, B.plicatilis have been mass-cultured as live food organisms for feeding fish larvae in hatcheries. The reasons for the change in the reproductive form of B.plicatilis from mictic female to amictic female at the Station are described in this paper. The change was initiated by devising a culture medium of Nannochloropsis oculata (former common name ;"Marine Chlorella"). The mass-culture techniques of N.oculata were created by micro-succession methods at the Yashima Station. The culture medium consisted of ammonium sulfate (100 g/t), super-phosphate lime (15 g/t), urea (5 g/t), and Clewat-32 (commercial reagent as minerals with EDTA)(5 g/t). The N.oculata bloomed in that medium as climax organisms in early spring of 1964. The natural zooplankton collected by a net from Yashima Bay were inoculated into a tank culturing N.oculata. The B.plicatilis were propagated as climax organisms in the tank, 4 to 5 weeks later. Thereafter, the B.plicatilis were re-cultured by a tank transfer method. The cultures were regularly regenerated when the B.plicatilis had consumed the N.oculata. All of the culture experiments were repeated for 2-3 years outdoors without winter heating using 200 to 500 L tanks. These experiments were significant trial to improve the strain from mictic female to amictic female. In February 1967,the surfaces of the culture tanks were covered by ice. However, B.plicatils were still living in the water. They became amictic female strains by repetition of the regular culture process.記事区分：原