Feeding preferences of a generalist salt-marsh crab: Relative importance of multiple plant traits

Few studies have evaluated the relative importance of multiple plant traits to herbivore diet choice, especially with an experimental approach. Moreover, although circumstantial evidence points to plant toughness and silica content as important determinents of diet choice, few studies have experimentally demonstrated that these factors actually deter feeding by herbivores. We examined feeding preferences of a generalist saltmarsh crab, Armases cinereum, for all the common angiosperms in its habitat. We took an experimental approach to evaluating the importance of toughness, secondary chemistry, silica, salt, and protein in determining feeding preferences. Consumption of plants by Armases in two experiments was correlated with decreasing toughness. Consumption was more equitable when plants were ground up and reconstituted in agar discs, with reduced differences in toughness. In four pairwise choice tests, Armases always preferred the softer plant of the pair, but in three of four cases exhibited no preference when the same plants were presented in reconstituted discs. Several plant extracts significantly stimulated or deterred feeding by Armases, but these effects were not consistent with and could not be used to predict overall preferences for fresh plants. Salt in artificial diets stimulated feeding by Armases. Silica had no effect on feeding in two experiments and stimulated feeding in a third. Armases was mildly stimulated to feed by high levels of protein in artificial diets, but these levels were above those found in plants, and the protein content of plants did not correlate with feeding preferences. Our results point towards plant toughness as most important in determining feeding choices of Armases. Since Armases is omnivorous, it may escape dietary constraints that affect feeding choices of pure herbivores. The dogma that silica defends plants against herbivory needs to be re-examined with additional experimental studies.