Feeding and grazing impact of the newly described heterotrophic dinoflagellate Stoeckeria algicida on the harmful alga Heterosigma akashiwo

To investigate the roles of the newly described thecate heterotrophic dinoflagellate Stoeckeria algicida (GenBank accession number = AJ841809), which was abundant during the red tides dominated by the harmful raphidophyte Heterosigma akashiwo, we measured the abundances of H, akashiwo and S. algicida in Masan Bay, Korea, from May to July 2004. We also measured growth and ingestion rates of S. algicida when feeding on H. akashiwo in the laboratory. Grazing coefficients were calculated by combining field data on abundances of S. algicida and co-occurring H. akashiwo with laboratory data on ingestion rates obtained in the present study. The maximum abundances of H. akashiwo during 2 H. akashiwo red tides were 58 400 and 99 200 cells ml, while those of S. algicida were 1130 and 17 400 cells ml(-1), respectively. The time lags between the abundance peaks of H. akashiwo and S. algicida in these red tide periods were 1 to 2 d. S. algicida fed on H. akashiwo using a peduncle after anchoring the prey by a tow filament. Specific growth rates of S. algicida increased rapidly with increasing mean prey concentration before saturating at H. akashiwo concentrations of ca. 350 ng C ml(-1) (3500 cells ml(-1)). The maximum specific growth rate of S. algicida on H. akashiwo was 1.63 d(-1). The threshold prey concentration (where net growth = 0) was 1.9 ng C ml(-1) (19 cells ml(-1)). Maximum ingestion and clearance rates of S. algicida on H. akashiwo were 0.75 ng C grazer(-1) d(-1) (7.5 cells grazer(-1) d(-1)) and 3.7 mu l grazer(-1) h(-1), respectively. Calculated grazing coefficients for S. algicida on H. akashiwo were up to 0.142 min(-1) (i.e. 13% of H. akashiwo populations were removed by a S. algicida population in 1 min). The results of the present study suggest that S. algicida sometimes has a considerable grazing impact on populations of H. akashiwo.