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Hay,
Mark E.
Hay,
Mark E.
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ItemHost-plant specialization decreases predation on a marine amphipod: an herbivore in plant's clothing(Georgia Institute of Technology, 1990) Hay, Mark E. ; Duffy, J. Emmett ; Fenical, WilliamWe 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 Viva 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 VIva was treated with pachydictyol-A (the major secondary metabolite produced by Dictyota bartayresiz), 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.
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ItemChemical defense in the seaweed Dictyopteris delicatula: differential effects against reef fishes and amphipods(Georgia Institute of Technology, 1988-09-21) Hay, Mark E. ; Duffy, J. Emmett ; Fenical, William ; Gustafson, KirkMany seaweeds produce chemicals that deter feedlng by fishes and sea urchins. A growing body of evidence suggests that small, relatively immobile herbivores (mesograzers) such as amphpods, polychaetes, and ascoglossan gastropods are often unaffected by these compounds and may preferentially consume seaweeds that are chemically defended from fishes. We tested this hypothesis by examining the responses of reef fishes and amphipods to a mutture of 2 C,, hydrocarbons, &ctyopterenes A and B, produced by the Canbbean brown alga D~ctyopteris delicatula. This alga was intermediate in preference for reef fishes, and the dictyopterenes reduced fish grazing by a significant 40 %. In contrast, D. delicatula was highly preferred by a muted-species group of amphipods and the dlctyopterenes had no effect on their feeding Despite the tendency for mesograzers to selectively consume some seaweeds that are chemically deterrent to fishes, true specialization by these or other marine herbivores appears to be rare in companson with terrestnal systems. Plant-dwelling amphipods at our study site in the Grenadine Islands were found on, and consumed a variety of, macrophytes; they were not restrict~velys pecialized to D. delicatula. Many terrestnal insects are very specialized feeders, sequester toxins from theu food plants, and use these as duect defenses against predation. In contrast, sequestenng of seaweed toxlns by marine mesograzers appears to be relahvely rare. However, the indirect advantage of llving on seaweeds that are not eaten by fishes may be considerable. We hypothesize that mesograzers living on plants chemically defended from fishes wlll experience less predation than those living on plants preferred by fishes.
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ItemChemical defense against different marine herbivores: are amphipods insect equivalents?(Georgia Institute of Technology, 1987) Hay, Mark E. ; Duffy, J. Emmett ; Pfister, Catherine A. ; Fenical, WilliamThe 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 known to be relatively low preference foods for tropical fishes and urchins. We evaluated the effect of various concentrations of these compounds on feeding by the three 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 were offered pieces ofthe palatable seaweed Graci/aria tikvahiae coated with either (1) dictyol-E or pachydictyol-A dissolved in diethyl ether or (2) diethyl ether alone. 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 (P = .07). 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%. In feeding preference experiments with several seaweeds, Dictyota ranks low for fish and urchins but high for amphipods. This is consistent with the hypothesis that the secondary metabolites produced by Dictyota playa major role in determining its susceptibility to herbivores. The ability of amphipods to circumvent the chemical defenses of Dictyota, and the fact that the two species of algae most readily consumed by amphipods (Codium and Dictyota) were the two species least readily consumed by fish, suggest that predation and herbivory by fishes may be major factors selecting for amphipods that can live on, and eat, seaweeds that are unpalatable to fishes. Amphipods that fed on Dictyota did not appear to sequester the Dictyota metabolites; when exposed to fish predation, Dictyota-fed amphipods were eaten as readily as amphipods that had fed on an alga with no defensive chemistry. Tubicolous amphipods and other small marine herbivores that may spend significant portions of their lives on only a few plants may be under very different evolutionary constraints than the larger, more mobile herbivores that commonly move between many plants. Several characteristics of these smaller, less mobile, and much less studied, marine herbivores suggest that they may be ecologically similar to terrestrial insects and may play a large, but presently unappreciated, role in structuring marine plant communities.