Host‐Plant Specialization Decreases Predation on a Marine Amphipod: An Herbivore in Plant's Clothing

We investigated the factors selecting for host—plant specialization and the roles that plant defensive chemistry may play in this process. We studied marine organisms because marine communities contain fewer specialized herbivores than terrestrial communities and, therefore, provide a simplified system for investigating certain aspects of host—plant specialization. Our study focused on the unusual domicile—building and feeding behavior of the amphipod Pseudamphithoides incurvaria, which is the only herbivorous marine amphipod known to specialize on a few closely related seaweeds. P. incurvaria lives in a portable, bivalved domicile that it constructs from the chemically defended brown alga Dictyota bartayresii. Chemical assays indicate that natural populations of Pseudamphithoides construct their domiciles from D. bartayresii even when this alga is rare compared to other Dictyota species and to other related genera in the family Dictyotaceae. In both choice and no—choice tests in the laboratory, Pseudamphithoides built domiciles from and selectively consumed species of Dictyota that produced dictyol—class diterpenes that deter feeding by reef fishes. Other brown seaweeds in the family Dictyotaceae, including a Dictyota species, that did not produce these fish—feeding deterrents were avoided by the amphipod. Amphipods removed from their domiciles were rapidly eaten when presented to predatory fish; amphipods in their normal domiciles were consistently rejected by fish. The defensive value of the domicile appeared to result from specific characteristics of the Dictyota from which it was built, since amphipods forced to build domiciles from the palatable green seaweed Ulva were rapidly eaten when these amphipods, in their domiciles, were exposed to predatory fish. Algal defensive chemistry directly cued domicile building. When the green alga Ulva was treated with pachydictyol—A (the major secondary metabolite produced by Dictyota bartayresii), domicile building by Pseudamphithoides increased in proportion to the concentration of pachydictyol—A. All data collected during this study are consistent with the hypothesis that predator escape and deterrence are primary factors selecting for host specialization by Pseudamphithoides incurvaria. Similar conclusions can be drawn for the limited number of other marine herbivores that are relatively specialized.