Chemotaxis and chemical defenses in seaweed susceptibility to herbivory.

Recent studies have show that small marine herbivores with limited mobility (mesograzers) often feed on macroalgae chemically defended against fishes or sea-urchins. In order to verify the involved mechanisms of chemotaxis or chemical defense into this process in Brazilian littoral, two species of brown alga Dictyota menstrualis and Dictyota mertensii were studied against the limited mobility herbivores, the amphipod Parhyale hawaiensis and the crab Pachygrapsus transversus. These two species were studied in order to verify the action of their crude extracts in the defense and chemotaxis processes related to limited mobility of these herbivores. Feeding preference assays revealed that P. hawaiensis do not eaten these Dictyota species. P. transversus do not eaten D. menstrualis either, but consumed large amounts of D. mertensii. Chemical deterrent assays showed that extracts of these species act as feeding deterrent to both species of herbivores. In addition, chemotaxis assays demonstrated that both herbivores are significantly negative chemotactic probably due to the presence of complementary metabolites into artificial foods. Considering that both Dictyota species exhibit active extracts against these small herbivores, we suppose that the non-occurrence of these herbivore species in close relationship with the seaweeds D. menstrualis and D. mertensii may explain the defense action of both extracts related to these mesograzers.