Chemical defense against different marine herbivores: are amphipods insect equivalents?

Abstract

The structurally similar diterpenoid alcohols pachydictyol-A and dictyol-E are produced by the brown seaweed Dictyota dichotoma. This seaweed and several related species that also produce these compounds are relatively low preference foods for tropical fishes and urchins. The effect of various concentrations of these compounds were evaluated on feeding by the 3 common types of herbivores that co-occur with Dictyota in coastal North Carolina, fish (Diplodus holbrooki), sea urchins (Arbacia punctulata) and a mixed species group of gammarid amphipods. Dictyol-E significantly reduced consumption by fish and urchins at concentrations of 0.5 and 1.0% of algal dry mass, but had no effect on amphipod grazing. Pachydictyol-A significantly reduced fish grazing at the relatively high concentrations of 1.0 and 1.3% of plant dry mass; at 0.5% it tended to decrease grazing, but the effect was not significant. Pachydictyol-A had no effect on urchin grazing and significantly increased amphipod grazing. When Pachydictyol-A was fed to fish as 1.0% of food dry mass, their growth rate was reduced by a significant 48%. -from Authors