Seaweed–herbivore interactions at a small scale: direct tests of feeding deterrence by filamentous algae

High growth rates and temporal or spatial opportunism are considered central to the success of filamentous algae, in particular for escaping or minimising the effects of herbivory. However, the role of chemical defences in filamentous algae has received far less attention. We investigated possible chemical feeding deterrence by filamentous red algae that have conspicuous cellular inclusions (Asparagopsis armata, Anotrichium tenue and Balliella amphiglanda) and 2 others without inclusions (Callithamnion korfense and Ulva sp.). The 3 algae with cellular inclusions were consumed at lower rates by a generalist amphipod, Hyale nigra, than the other 2 algae. To determine the potential role of chemical defences for A. armata, we conducted tests against herbivores using algae in which the production of halogenated metabolites was manipulated. This manipulation had no effect on carbon and nitrogen values of the algae, and allowed us to directly test the role of algal secondary metabolites in defence against herbivores without using artificial diets. Bromide (+) algae (with halogenated metabolites) deterred grazing by 2 mesograzers (Hyale nigra and juvenile abalone Haliotis rubra), which consumed up to 4 times more bromide (–) (metabolite-free) algae than bromide (+) algae. Juveniles of the sea hare Aplysia parvula were not deterred by the chemical defences in bromide (+) A. armata. In field assays, artificial diets containing a crude extract of A. armata were also active against herbivores. Although functional form models typically predict that tolerance—not resistance—should be the key defensive strategy for marine algae with simple architecture, this study demonstrates that resistance traits may also be important and more broadly utilised in filamentous species.