Sexual life stages and temperature dependent morphological changes allow cryptic occurrence of the Florida red tide dinoflagellate Karenia brevis

• Karenia brevis sexual life-cycle is heterothallic with isogamous gametes.
• The late zygote resembles the “possible cyst” found earlier but does not form a true resting cyst.
• Temperature induced morphological changes and differences between life cycle stages allow cryptic occurrence.

Karenia brevis, the Florida red tide dinoflagellate, has been studied extensively, but very little attention has been paid to its sexual life cycle. We found that the life cycle of K. brevis is heterothallic, most probably not resting cyst-producing, but with life stages of different morphology. The isogamous gametes were slightly smaller than vegetative cells and not as broad and flat. The late zygote was yellow-brown in appearance with a thicker wall and more rounded shape lacking carina. Pellicle cysts of these zygotes closely resembled the few earlier descriptions of “possible cysts” of the species. In addition, temperature-dependent, morphological changes and pellicle-cyst formation were observed. Cells placed in the cold (15 °C) formed spherical, thin-walled pellicle cysts that germinated into cells that were round in cross-section and longer than wide – so morphologically different from vegetative cells that they would not be correctly identified if encountered in field samples. Cells grown at 25 °C were wider and flatter than cells grown at 20 °C. Cells warmed from cold conditions became flat and wide within hours, returning to the typical shape. Also the morphological differences between sexual life stages were large enough to allow misidentification and cryptic occurrence of K. brevis. The cell shape of K. brevis was not fixed, but could vary from very wide and flat to elongate with rounded cross-section in the same culture of clonal cells and in the same cells within a short time (hours).In addition to the culture studies, sediment samples from a Karenia “hot spot” area were concentrated, and the dinoflagellate cyst fraction was investigated for resting cysts. Cysts were not found, and Karenia cells did not germinate from slurry cultures of the concentrated cyst fraction.