Encystment and excystment under laboratory conditions of the nontoxic dinoflagellate Alexandrium fraterculus (Dinophyceae) isolated from the Seto Inland Sea, Japan

The sexuality (i.e. encystment and excystment) of the nontoxic dinoflagellate Alexandrium fraterculus, isolated from the Seto Inland Sea, Japan, was clarified for the first time under laboratory conditions. Sexual reproduction was by conjugation of isogametes, and plasmogamy was completed in 25-60 min after the conjugation started and produced a planozygote with one transverse and two longitudinal flagella, then a cyst was formed. Cysts were round or elliptical. The cyst size was 37.5-50.0 μm in diameter. The surface of cysts was smooth, and there was no paratabulation. Encystment through sexual reproduction was observed in two of 28 pairs, which included seven self-crossings, by use of seven nonaxenic clonal strains. No planozygote formation or encystment was found in any of the self-crossings, indicating that this species is heterothallic. Successful excystment was observed when the cysts, which had been preserved at 10°C in the dark for 3 mo, were incubated under light conditions at 25^QC, thus showing that this species has a similar life cycle as reported in A. tamarenselcatenellaltamiyavanichii.