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

Abstract

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.

Variation of amphipod assemblage along the Sargassum stenophyllum (Phaeophyta, Fucales) thallus

“…In contrast, the other herbivorous family, Ampithoidae, did not show any difference regarding its distribution along the Sargassum thallus. Such results concur with the general pattern for herbivorous epifauna along a vertical gradient on macroalgae observed by Hirst (2007), which suggested it may be related to the generalist feeding habit of marine mesoherbivores (Poore et al, 2008), although there are some exceptions (Hay et al, 1990;Poore and Steinberg 1999). Furthermore, we point out that the species of Ampithoidae often build domiciles on the algae using algal pieces and other materials (Conlan, 1982).…”

Why does herbivore sex matter? Sexual differences in utilization of Fucus vesiculosus by the isopod Idotea baltica

“…In small-sized marine herbivores habitat and food are closely tied. Their specialization to feeding on chemically well-defended species of algae, which at the same time offer protection against fish predation, has been attributed to selection from upper trophic levels (Hay et al 1989, 1990, Duffy and Hay 1991.…”

Defensive responses and payability of specialist herbivores: predation on NE Pacific ascoglossan gastropods

“…In many terrestrial communities, specialist herbivores are abundant and diverse (Root 1973, Bernays 1989. In contrast, marine herbivores are primarily generalist feeders with relatively few specialists (Lubchenco & Gaines 1981, Hay et al 1989, 1990a. Most of the described species of known marine specialist herbivores are ascoglossan sea slugs (Gastropoda: Opisthobranchia) -small suctorial herbivores.…”

“…Bernays & Graham 1988, Hay et al 1989, 1990a though evidence 'Present address: Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365, USA is mixed (Bernays 1989, Stamp 1992. There are at least 4 gaps in our understanding of whether predation selects for narrow host-plant range and diet in marine herbivores.…”