Abstract
The prospect that the cyst might be a seed population in the bloom of Peridinium penardii in spring in Nagase Reservoir, Kochi Prefecture, the conditions regulating encystment of this species were examined by use of clonal population in a synthetic medium (MW2). Sexual re-production was induced by incubating late logarithmic phase cells in nitrogen and phosphorus deficient medium. Two gametes, which were small and naked in shape, were produced from thecate cells. The gamete was isogamous and homothallic. The planozygote did not retain red bodies and remained motile for 7-14 days. About 14 days after plasmogamy the zygote became normtile, the protoplast expanded and the wall thickened. The theca of planozygote split at the girdle region and the hypnozygote (cyst) was produced. The maximum frequency of en-cystment was 40% under 10, 000lx illumination at 15°C in N-free or N-and P-free medium. The frequencies of encystment in P-free medium and fresh medium containing N and P were about 5 and 0%, respectively. Optimum condition for encystment was 15°C and 5, 000 to 10, 000lx.
