ALBERT

Description: Herbivory can modify seaweed community structure by affecting variation in the abundance and distribution over spatial and temporal scales. To deter herbivores, seaweeds have developed several mechanisms, including the induction of chemical and morphological defenses, which may be influenced by environmental factors such as nutrient availability. In two sets of experiments 5 red, 4 brown, and 1 green seaweeds were tested for inducible defenses following exposure to direct consumption by an amphipod community. In addition, the effects of consumer presence and neighborhood grazing (indirect consumption) on the induction of defenses were examined in the red seaweed Gracilaria cearensis and the green seaweed Codium decorticatum under natural and additionally 100% elevated nutrient levels for C. decorticatum. In preliminary studies, suitable organisms for the induction experiments were determined in testing survival of potential tropical algae from the Brazilian littoral under laboratory conditions and performing multiple-choice feeding assays for four important Brazilian herbivore species in order to determine their preference rankings of these algae. The amphipod community dominated by Elasmopus brasiliensis was identified as the most suitable grazers. Induction of defense was evaluated after consumer exposure in choice-feeding experiments by offering live algae (LA, total defense) and reconstituted food (RF, chemical defense) of previously consumed (treated) and unconsumed algae (control). Nutrient enrichment did not affect induction of defense in C. decorticatum, yet unfertilized seaweeds were more palatable relatively to fertilized conspecifics. Different consumer activities did not affect palatability in G. cearensis, yet C. decorticatum seemed to induce resistance (LA) after reception of waterbome cues from consumed conspecifics. Unexpectedly C. decorticatum RF exhibited higher susceptibility to amphipods after consumer exposure in both experiments performed. While no significant decrease in palatability was observed in previously consumed algae compared to unconsumed controls after one week, two red seaweeds, Chondrophycus flagellifera (RF) and Pterocladiella capillacea (LA), exhibited significantly reduced palatability after consumer exposure. The brown seaweed Lobophora variegata showed significantly retarded chemical resistance after three weeks. This is the first report of inducible chemical defense in a red seaweed.