Properties of diurnal vertical migration by the harmful red-tide dinoflagellate Karenia selliformis in relation to salinity conditions

Many harmful red-tide flagellates accumulate at a certain depth amid diurnal vertical migration (DVM). Here, to identify the DVM properties of Karenia selliformis , a dinoflagellate species that causes huge harmful red tides, we examined DVM under salinity gradients and growth after rapid salinity changes in the laboratory. Seawater with a salinity of 33 psu was placed as the bottom layer and water with one of three different salinities (17, 25, or 33 psu) was placed as the upper layer in a cylindrical aquarium, and time-course variations in the dinoflagellate's vertical distribution were monitored as well as cell morphology and photosynthetic parameters. We found that K. selliformis displayed DVM only when the upper layer salinity was 33 psu. The ascent period was longer and the descent was less synchronized than in other red-tide flagellates, indicating that this dinoflagellate has a strong surface accumulation habit. When the upper layer salinity was lower, K. selliformis accumulated at the halocline (18 psu) or surface (25 psu) throughout the day. Morphology and a photosynthetic parameter of cells accumulated at 18 psu were different and dividing cells were few in comparison with the other salinity conditions. Our growth experiment revealed that K. selliformis could not acclimatize to salinities below 15 psu, and cells that had acclimatized to ≥20 psu quickly acclimatized and grew even when transferred to 33 psu. These results suggest that K. selliformis can ascend to the lowest salinity layer where it can survive, but its ability to descend decreases. • The harmful alga Karenia selliformis displays diurnal vertical migration. • Its ascent period is longer than that of other red-tide flagellates. • Its synchrony of descent is weaker than observed in some red-tide flagellates. • Low salinity blocks the ascent and weakens downward migration of K. selliformis . • Karenia selliformis is less tolerant of low salinity than other red-tide flagellates.