Differential interactions between the bloom-forming dinoflagellates Karenia bicuneiformis and Karenia selliformis and heterotrophic dinoflagellates

Many species of the dinoflagellate genus <i>Karenia</i> produce neurotoxins and often cause harmful algal blooms. Heterotrophic dinoflagellates are major grazers of bloom-forming dinoflagellates. Therefore, to understand the population dynamics of <i>Karenia</i> species, it is necessary to investigate the interactions between <i>Karenia</i> species and their potential heterotrophic dinoflagellate predators. We examined the interactions between the bloom-forming dinoflagellates <i>Karenia bicuneiformis</i> and <i>Karenia selliformis</i> and eight common heterotrophic dinoflagellates. <i>Gyrodinium dominans</i>, <i>Gyrodinium moestrupii</i>, <i>Oxyrrhis marina</i>, <i>Oblea rotunda</i>, and <i>Protoperidinium pellucidum</i> fed on <i>K. bicuneiformis</i>, whereas <i>Gyrodiniellum shiwhaense</i>, <i>Pfiesteria piscicida</i>, and <i>Noctiluca scintillans</i> did not. Furthermore, <i>K. bicuneiformis</i> supported the positive growth of <i>G. dominans</i>, <i>G. moestrupii</i>, <i>O. marina</i>, and <i>P. pellucidum</i>, but <i>K. bicuneiformis</i> did not support the growth of <i>O. rotunda</i>. With increasing prey concentration, the growth and ingestion rates of<i> P. pellucidum</i> on <i>K. bicuneiformis</i> increased and then became saturated. Maximum growth and ingestion rates of <i>P. pellucidum</i> on <i>K. bicuneiformis</i> were 0.19 d<sup>-1</sup> and 0.86 ng C predator<sup>-1</sup> d<sup>-1</sup> (1.26 cells predator<sup>-1</sup> d<sup>-1</sup>), respectively. However, all eight heterotrophic dinoflagellates tested were lysed by <i>K. selliformis</i>. At a <i>K. selliformis</i> concentration of 100 cells mL<sup>-1</sup> within 48 h, the survival of <i>G. dominans</i> and <i>G. moestrupii</i> was only 0 and 13%, respectively. Therefore, <i>K. bicuneiformis</i>can be prey for the heterotrophic dinoflagellates, whereas <i>K. selliformis</i> kills them. These differential interactions may have resulted in different ecological niches for these two Karenia species.