Interactions among the toxic dinoflagellate Amphidinium carterae, the heterotrophic dinoflagellate Oxyrrhis marina, and the calanoid copepods Acartiaspp.

To investigate the interactions among a toxic dinoflagellate Amphidinium ca rterae, the heterotrophic dinoflagellate Oxyrrhis marina, and the calanoid copepods. Acartia spp, (A. omorii and A. hongi), we measured toxicity of A, carterae, the growth and ingestion rates of O. marina on A. carterae, the ingestion rate of Acartia spp. on A. carterae, the ingestion rate of Acarti a spp, on O. marina fed a non-toxic strain of Prorocentrum minimum, and the ingestion rate of Acartia spp. on O. marina, the latter originally satiate d with A. carterae and then starved, as a function of elapsed starvation ti me. The toxicity of A. carterae was 1 MU/1.3 x 10(8) cells when measured us ing the mouse bioassay, O. marina grew well on A. carterae. When the data w ere fitted to the Michaelis-Menten equation, maximum specific growth rate ( mu (max)) and threshold prey concentration of O. marina on A. carterae were 1.17 d(-1) and 1.3 ng C ml(-1) (13 cells ml-1), respectively. Maximum inge stion and clearance rates of O. marina were 2.8 ng C grazer(-1) d(-1) (28 c ells grazer(-1) d(-1)) and 2.4 mul grazer(-1) h(-1), respectively. Grazing by Acartia spp. on A. carterae was undetectable. The ingestion rate of Acar tia spp. on O. marina was very low (maximum = 749 Oxyrrhis predator(-1) d(- 1)) at Day 0 (O. marina starved for 0 to 1 d after satiation with A, carter ae), but increased with increasing elapsed starvation time. The maximum ing estion rate was 4710 Oxyrrhis predator(-1) d(-1) at Day 11 (O. marina starv ed for 11 to 12 d). The average ingestion rates of Acartia spp, on O. marin a fed P. minimum were not significantly higher than on O. marina fed A. car terae at Day 11 at similar mean prey concentrations, but much higher than t hose fed A. carterae at Day 0. This evidence suggests that O. marina can re duce its mortality rate due to the predation by Acartia spp, if it is satia ted with A. carterae, and the grazing of A. carterae by O. marina can somet imes transfer the carbon of A. carterae to Acartia spp., which cannot feed on A. carterae.