Life history and ecology of Goniodoma pseudogoniaulax (Pyrrhophyta) in a rockpool

The dinoflagellate, Goniodoma pseudogoniaulax, sometimes blooms in rockpools around the high tide level along the Pacific coast of Japan during spring and summer. Besides a motile pelagic form, this species has two nonmotile benthic stages (the resting cyst and temporary cyst). The resting cyst is dormant in winter, and possibly functions as seed for blooms in the subsequent warmer seasons. The temporary cyst is easily formed in culture and readily transformed into a motile stage. The temporary cyst often divides into two or four asexual products. Fusion of the smaller motile cells, presumed to be gametes, was observed. Diel observations of this organism in a rockpool showed that motile cells increased in number while the number of temporary cysts and the ratio of divided temporary cysts to divided and undivided ones decreased in the daytime; the variations in number and ratio were reversed at night. The number of the resting cysts remained unchanged day and night during the study period. It follows that motile cells lose their motility and settle to the bottom of a rockpool in the evening and change into the temporary cyst. The protoplast of some temporary cysts divide transforming it into a motile cell the next morning. The population of this species is thought to be maintained through the asexual reproduction of temporary cysts.