Effects of Feeding Pressure and Low-light Stress on the Performance of Antiherbivore Defenses in the Two Red Algae Gracilaria edulis and Gracilaria salicornia from the Malay Peninsular

The objective of this study was to test for the inducibility of an anti-feeding defense in two red macroalgal from the coast of the Malay Peninsular as a response to grazing pressure. Whether the defense capacity in these seaweeds is generally impaired by low-light stress was also investigated. Aquaria with blackened walls connected to a flow-through water system were used as Experimental Units (EUs). Different light regimes were established using layers of black gauze material and clear perspex lids, mounted on top of the EU. The gauze allowed the stepwise reduction of ambient light. Six different light levels with eight replicates each were realised. The growth rates of algal exposed to different light levels were calculated using a linear regression analysis. Using outdoor mesocosm facilities, three fully-factorial experiments were realised during which low-light stress on different algae-grazer combinations was induced: 1) the red algae Gracilaria edulis with the sea hare Aplysia sp., 2) G. edulis with the abalone Haliotis diversicolor, and 3) Gracilaria salicornia with H. diversicolor. For each combination, 48 aquaria were used, while in a control group of another 48 tanks, algae were kept in the absence of herbivores. Following low- light and feeding-stress induction, herbivore consumption on algal material was assessed in feeding assays with living thalli fragments. Negative effects of light reduction on algal growth rates was observed in both experiments with G. edulis and in one trial with G. salicornia, while general effects of low-light stress on seaweed palatability were absent in our study. By contrast, for some herbivore-algal combinations, significant differences in consumption were detected on previously- grazed and ungrazed algal material, that indicates the presence of an inducible defense. G. edulis and G. salicornia produced defenses when exposed to grazing by the gastropod Aplysia sp. and H. diversicolor, respectively. In contrast, G. edulis showed no response to H. diversicolor. Our findings show that inducible defenses generally exist in the two investigated Gracilaria species, but vary with the identity of herbivores. Surprisingly, algal palatability was not affected throughout all low- light regimes, contrasting the widespread notion that defensive traits in seaweeds are dependent on metabolic energy. Thus, it appears that low-light stress effects play only a minor role on the performance of antiherbivore defenses in these two red algae.