Effect of dissolved domoic acid on the grazing rate of krill

Some amino acids released from phytoplankton into the water are known to be feeding stimulants to zooplankton. However, domoic acid (DA), a neuroexcitatory amino acid released by 14 some species of the diatom genus Pseudo-nitzschia, has the potential to be a feeding deterrent due to its neurotoxicity. Euphausiids (krill) are important members of the zooplankton grazer community and key prey items for many high level carnivores in the world’s oceans. Although it is known that krill do consume toxic Pseudo-nitzschia spp. during blooms, there is currently no information concerning the effect of dissolved DA on the rate of feeding or the grazing behavior of krill. Therefore, we conducted experiments in which Euphausia pacifica were fed non-toxic P. pungens along with different added levels of either dissolved DA or glutamic acid (GA), an amino acid with a similar structure to DA. Krill grazing rates did not change significantly with added GA, but decreased significantly in the presence of DA. The rapid response of krill to dissolved DA suggests that either direct toxic action on the filtering apparatus or a chemoreceptor-like mechanism may suppress grazing in the krill. We suggest that these dissolved DA concentrations are relevant, given the DA concentrations found in Pseudo-nitzschia spp. populations in Monterey Bay and the feeding biology of krill. In either case, the suppression of feeding by dissolved DA suggests that high dissolved DA concentrations, should they occur in the field, may be a mechanism whereby blooms are perpetuated due to reduced losses to grazers such as krill.