Lethal effect of the dinoflagellate Heterocapsa circularisquama upon the tintinnid ciliate Favella taraikaensis

We examined the effect of the toxic dinoflagellate Heterocapsa circularisquama in bloom concentrations on the mortality process of the tintinnid ciliate Favella taraikaensis. The number of viable F. taraikaensis decreased during the first 7 h of the incubation experiment and disappeared within 24 h at a H. circularisquama concentration of greater than or equal to 6.4 x 10(3) cells ml-l. Mortality of the ciliates occurred more rapidly with increasing concentrations of H. circularisquama. Morphological changes of F. taraikaensis cells, characterized by swollen and/or spherical forms, were observed after 4 and 7 h of incubation. Microscopic observations showed that H. circularisquama cells frequently adhered to the adoral membranelles of F. taraikaensis. The mean adhesion time of H. circularisquama cells to the adoral membranelles of F. taraikaensis was significantly longer than cells of the non-toxic dinoflagellate Heterocapsa triquetra. During the adhesion process, contact of H. circularisquama cells with the cytoplasm around the oral plug of F. taraikaensis was observed. Filtrate of H. circularisquama culture solution had no effect on the activity of F. taraikaensis cells. These results suggest that H. circularisquama causes physiological problems for F. taraikaensis through cell contact, with higher algal concentrations increasing the frequency of cell contact between H. circularisquama and F. taraikaensis, which may enhance the noxious effect of H. circularisquama on F. taraikaensis.