Herbivore Grazing Increases Polyphenolic Defenses in the Intertidal Brown Alga Fucus Distichus

Although predator-induced defenses have been reported for several species of terrestrial vascular plants, they have not been previously described in aquatic or nonvascular plants. In this study, field manipulations were used to demonstrate the presence of inducible chemical defense production in the intertidal brown alga Fucus distichus. When experimentally damaged, Fucus increased its concentrations of polyphenolic compounds by @?20% over uninjured control plants within 2 wk. These increases occurred in the area where the plant was injured and within adjacent undamaged branches. The increase in concentrations of polyphenolic compounds in clipped plants in these experiments corresponded well with differences in phenolic levels in naturally grazed and ungrazed algae. Herbivorous snails (Littorina sitkana) showed a preference for clipped plants immediately after they were wounded. However, over a 2-wk period the snails shifted their preference towards the uninjured control plants, corresponding with the increase in polyphenolic levels within the experimentally damaged plants. L. sitkana spent less time feeding on clipped plants, and these plants lost @?50% less tissue (by surface area) to grazers than did uninjured algae. The presence of induced defenses in algae causes plant quality to vary spatially and temporally. This may result in variation in intra- and interspecific food preferences of herbivores, and ultimately may affect benthic algal community structure.