Krill: Victim or vector? Examining the place and role of krill in domoic acid poisoning events in Monterey Bay, California

Recently the diatom Pseudo-nitzschia, which produces the toxin domoic acid (DA), a neurotoxic amino acid, has been shown to be responsible for deaths of higher organisms in the marine food chain. Euphausiids (krill) are important zooplankton grazers on diatoms as well as ubiquitous prey for many consumers. Thus, krill are potential vectors for DA transfer from the toxic diatom Pseudo-nitzschia to higher trophic levels in the Monterey Bay food web. An understanding of the quantitative trophic interactions and body burden of DA in krill is required to predict whether krill can effectively vector the toxin. The goal of this thesis is therefore to examine the place and role of a large herbivorous zooplankton group, krill, in domoic acid poisoning events that are caused by Pseudo-nitzschia. This thesis presents the results from toxin analysis and gut content examination of krill collected from Monterey Bay, CA, throughout the year 2000. The results also include the feeding rates of local euphausiids on toxic Pseudo-nitzschia multiseries and gut passage time of the food, and the feeding rates of Euphausia pacifica on non-toxic P. pungens with varying levels of added dissolved DA. Results demonstrate that krill contained detectable DA in their body and obtained the toxic Pseudo-nitzschia as part of their diet when the diatom was abundant in the water. The quantity of cells consumed and the feeding pattern over time is dependent on the species of euphausiid and the cellular content of toxin in the food, respectively. While krill did not change its feeding rate on non-toxic P. pungens when glutamic acid was present in the solution, the krill filtration and ingestion rates on non-toxic P. pungens decreased dramatically throughout the experiment in the presence of domoic acid. The decrease in krill feeding rates therefore appears to be induced uniquely by toxic domoic acid. These findings support very strongly the hypothesis that DA can be transferred through krill to higher trophic levels in the marine food web.