Encystment and excystment in the life cycle of a dinoflagellate Peridinium bipes f. occulatum (Lindem.) Lef., which was isolated from Lake Kizaki, were investigated by use of a clonal population. Sexual reproduction was induced in P. bipes inoculating exponentially growing cells into N-free Carefoot's medium. The sexual life history of P. bipes resembles that reported for P. willei and P. cinctum in many respects: isogampus and homothallic gamete; the gametes resemble small, naked, vegetative cells ; lateral fusion with a long living planozygotic stage in which the zygotes enlarge and become warty ; hypnozygote with 3 walls (exsospore, mesospore and endospore); hypnozygote characterized by one or more large red oil droplets. However, the difference of P. bipes from P. cinctum and P. willei is that the post-zygotic cell of P. bipes divided into one cell with a red oil droplet and one without it. From the encystment experiment of P. bipes, very low encystment was observed below 10℃. The temperature range which permitted encystment was 15 to 25℃. Although cyst formation was observed even in continuous dark, the frequency of encystment increased with the light intensity. Under continuous dark conditions, no excystment was observed at any temperature from 5 to 25℃. The cysts illuminated at 105 μE/㎡/S excysted at the frequency of 81 % after 13 days incubation at 15℃, and lower light intensities led to decreases in germination frequency. Excystment frequency of this species depended on light intensity. The experiments described here demonstrate that light is a critical factor in the germination of P. bipes cysts, affecting excystment frequency.Article信州大学理学部紀要 27(2): 87-104(1993)departmental bulletin pape
