Life-cycle stages of Dinophysis acuminata (Dinophyceae) in the Baltic Sea

Abstract

Despite many observations of different life-cycle stages of Dinophysis species, the complete life history of the genus is still unknown owing to the difficulties encountered in culturing these species. The seasonal distribution of D. acuminata was followed at two offshore stations in the brackish Baltic Sea by means of in situ observations of life-cycle stages of this species. In April 2000, D. acuminata was present in low numbers in typical vegetative form and size. Cell numbers increased sharply in early May, when small-sized morphotypes comprised 17% of the total population at one of the stations, and increased to >60% at its peak abundance on 5 June 2000. At this time, several cells of both similar and unequal size were in sexual conjugation (putative anisogamous or isogamous gametes). Recently divided vegetative cells, as well as ‘naked’ cells (possible early stages of a temporary cyst), were also found. Cell numbers decreased sharply in early July 2000 and small
cells were no longer found. This is the first observation of sexual reproduction of D. acuminata in the Baltic Sea. However, the environmental parameters favouring sexual reproduction remain uncertain.