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

Abstract

To investigate the interactions among a toxic dinoflagellate Amphidinium carterae, 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 Acartia 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 satiated with A. carterae and then starved, as a function of elapsed starvation time. The toxicity of A. carterae was 1 MU/1.3 × 108 cells when measured using the mouse bioassay. O. marina grew well on A. carterae. When the data were fitted to the Michaelis-Menten equation, maximum specific growth rate (μmax) 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 ingestion and clearance rates of O. marina were 2.8 ng C grazer-1 d-1 (28 cells grazer-1 d-1) and 2.4 μl grazer-1 h-1, respectively. Grazing by Acartia spp. on A. carterae was undetectable. The ingestion rate of Acartia 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. carterae), but increased with increasing elapsed starvation time. The maximum ingestion rate was 4710 Oxyrrhis predator-1 d-1 at Day 11 (O. marina starved for 11 to 12 d). The average ingestion rates of Acartia spp. on O. marina fed P. minimum were not significantly higher than on O. marina fed A. carterae at Day 11 at similar mean prey concentrations, but much higher than those fed A. carterae at Day 0. This evidence suggests that O. marina can reduce its mortality rate due to the predation by Acartia spp. if it is satiated with A. carterae, and the grazing of A. carterae by O. marina can sometimes transfer the carbon of A. carterae to Acartia spp., which cannot feed on A. carterae.