Life History of a Marine Dinoflagellate Pyrophacus steinii (Schiller) Wall et Dale

Abstract

The life history of a marine dinoflagellate Pyrophacus steinii (Schiller)Wall et Dale was investigated using clonal cultures isolated from Tokyo Bay. The asexual reproduction is binary fission of the eleutheroschisis type. All processes of asexual and sexual cell division occur inside the theca of the mother cells. Sexual reproduction is anisogamous and heterothallic.Male gametes differ from female gametes and vegetative cells in size, shape and plate tabulation. The female gametes can not be differentiated from the vegetative cells. Cell fusion between the male and female gametes occurs within a few hours to several days after inoculation of the male gametes into a culture of a non-male clone. Zygotes are similar to the vegetative cells in shape except possessing two longitudinal flagella. One to two days after plasmogamy, the zygotes become non-motile, and their protoplasts contract. Within 1 to 3 days, following the condensation of protoplast, they transform to hypnozygotes (resting cysts). The specific features of the life cycle can be summerized as: i) the asexual reproduction is eleutheroschisis, ii) sexual fusion occurs after inoculation of the small round thecate cells, which are equivalent to male gametes, into a culture of a non-male clone, iii) male gametes are distinctive from vegetative cells in their smaller size, round shape, fewer plate number and pale chloroplasts, iv) female gametes can not be differentiated from the vegetative cells, v) the sexual life cycle is anisogamous and heterothallic.