Inducible chemical resistance to herbivory in the brown seaweed Ascophyllum nodosum

Physical damage caused by herbivores or artificial clipping is known to ind uce responses in secondary chemistry as well as increased resistance to fur ther grazing in a large number of terrestrial. vascular plants, but this ha s only rarely been described for marine algae. In the present study, it was found that a few weeks of grazing by the gastropod Littorina obtusata can induce substantially increased concentrations of phlorotannins in the inter tidal brown seaweed Ascophyllum nodosum. In contrast, grazing by the isopod Idotea granulosa and simulations of herbivory through momentary and contin uous clipping caused no significant changes in phlorotannin levels. This in dicates a high degree of specificity in the elicitation of chemical respons es to physical damage, something which previously has been shown for terres trial vascular plants but not for marine algae. Ascophyllum plants that had been grazed by L. obtusata were also less susceptible than undamaged plant s to further grazing by gastropods, but no such induced resistance was foun d in the experiments with I. granulosa. Feeding experiments with undamaged Ascophyllum plants and artificial food containing different levels of phlor otannins provided further support for the sensitivity of the feeding behavi or of L. obtusata, and the insensitivity of I. granulosa, to intraspecific variation in the phlorotannin content of Ascophyllum. Since L. obtusata is specialized to live and feed on a few fucoid species, including A. nodosum, the results imply that phlorotannins have an important mediating role in t he interactions between these macroalgae and L. obtusata. The experimentall y induced increase of phlorotannins was consistent with results from phloro tannin analyses of Ascophyllum individuals from natural populations, where plants that had been heavily grazed by L. obtusata contained significantly higher levels of phlorotannins compared to undamaged plants. A held survey of the distribution and abundance of L. obtusata revealed that the density of the gastropod is highly variable at the same spatial scale as the phloro tannin content of Ascophyllum in the study area. These results suggest that grazing by L. obtusata can be an important factor in explaining natural ph enotypic variation in the phlorotannin content of Ascophyllum. Together with the results from a few other studies on the interactions betw een meso-herbivores and marine algae, the results of this study support the previously proposed hypothesis that it is feeding by relatively small, les s mobile herbivores that is most likely to cue for induced production of de fense chemicals in seaweeds. More studies on such interactions may reveal t hat the apparent rarity of inducible chemical defenses in seaweeds is misle ading.