Influence of Acartia cf. bifilosa (Copepoda) on morphology and toxicity of Nodularia spumigena (Cyanophyceae)

Abstract

Nodularia spumigena was exposed directly and indirectly (grazer cages) to increasing densities of Acartia cf. bifilosa to investigate if the presence of copepods influenced the morphology and/or the toxicity of the cyanobacterium. Monocultures with only N. spumigena and mixed cultures, containing N. spumigena and the non-toxic Dunaliella tertiolecta, were included in each experiment. Following 6 days of incubation, the morphology and toxicity in grazer treatments were compared with grazer-free controls. We observed no effects of A. cf. bifilosa on either morphology or toxicity of N. spumigena. The lack of grazer induced nodularin production and morphological alterations suggest that these two potential defense strategies either has evolved as constitutive defenses or never evolved as grazer defenses. The mortality of copepods was higher in the monoculture than in the mixed culture treatments. Gut content observations indicated a low level of grazing in monoculture treatments and a higher level of grazing in mixed culture treatments. This higher level of grazing most likely occurred on the alternative food D. tertiolecta. Given the indications of low grazing and the concentrations of dissolved nodularin observed, we postulate that the higher mortality was not related to toxic effects, but to starvation. This in turn may have resulted from bad taste, production of unknown grazer deterrents or morphological constraints; although the size of the filaments would not have imposed an absolute limit for ingestion by A. cf. bifilosa. The higher copepod mortality observed on monocultures of N. spumigena may contribute to the success and maintenance of N. spumigena blooms.