Prompt induction of chemical defenses in the red seaweed Laurencia dendroidea: The role of herbivory and epibiosis

Induced chemical defense is still - uncertain process in seaweed, especially in tropical areas. Attention has been given to changes on phlorotannin contents due herbivore pressure, while knowledge considering - other secondary metabolites or other environmental factors as inducers is limited. We used laboratory experiments to assess induced chemical defense in the tropical red seaweed Laurencia dendroidea in response to simulated herbivory, presence of epibionts (microalga and bacterium), and bacterial infestation, addressing the following questions: 1) does inducible chemical defense by simulated herbivory, as well as microalgal and bacterial epibiosis, occur?; 2) are chemical defenses induced by bacterial infestation after simulated herbivory?; 3) is there a trade-off between inducible defense and seaweed growth?; and 4) how do variations in chemical defense affect seaweed susceptibility to specialist and generalist consumers? Inducible defense was measured as changes in sesquiterpene elatol levels, and growth was taken to infer whether induced chemical defense incurred metabolic costs. Specimens of L. dendroidea under simulated herbivory produced higher concentrations of elatol up to 2 days after treatment, followed by relaxation after 7 days, while epibiosis (by microalga and bacterium) and bacterial infestation did not induce a defense response. Simulated herbivory resulted in reduced L. dendroidea growth, reinforcing that induced chemical defense is a cost-saving strategy as stated by the Optimal Defense Theory (ODT), -in addition there was a tendency of epibiosis for biomass decreasing. The defensive effect varied according to elatol concentration and consumer identity, since aplysiids tolerated 10-fold natural concentrations (NC) whereas sea urchins were inhibited by NC. Our results highlight that inducible chemical defenses may also be a more rapid process than previously evidenced in seaweeds. Higher levels of elatol promoted decreased consumption, demonstrating that seaweed susceptibility is dose-dependent and induction is an important ecological strategy depending on environmental conditions.