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Hay, Mark E.

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Author: Gustafson, Kirk × End date: 1989 × Start date: 1980 ×

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Chemical defense in the seaweed Dictyopteris delicatula: differential effects against reef fishes and amphipods

1988-09-21 , Hay, Mark E. , Duffy, J. Emmett , Fenical, William , Gustafson, Kirk

Many 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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Chemical defense against diverse coral reef herbivores

1987 , Hay, Mark E. , Fenical, William , Gustafson, Kirk

Five secondary metabolites from tropical marine algae and one related compound from an herbivorous sea-hare (Aplysidae) were coated, at approximately natural concentrations, onto the palatable seagrass Thalassia testudinum and placed on coral reefs where they could be eaten by the diverse group of herbivorous fishes that occur there. Laboratory feeding assays with the herbivorous sea urchin Diadema antillarum were also conducted. When compared to appropriate controls, the following terpenoid compounds significantly reduced the amount of Thalassia eaten by both Diadema and reef fishes: stypotriol, from the brown seaweed Stypopodium zonale; pachydictyol-A, which is produced by several genera of tropical (Dictyota and Dilophus) and warm-temperate (Paehydiet yon and Glossophora) brown seaweeds; elatol, from the tropical red alga Laureneia obtusa; and isolaurinterol, which is produced by several tropical and warm-temperate species of Laureneia, Under very mild acid conditions, isolaurinterol is converted to a structurally similar compound, aplysin, found in high concentrations in sea-hares that feed on isolaurinterol-containing Laureneia species. Aplysin did not deter feeding by either type of herbivore. Cymopol, a terpenoid bromohydroquinone from the green alga Cymopolia barbata, significantly reduced feeding by reef fishes but significantly stimulated feeding by Diadema, Pharmacological and crude bioactivity tests suggest that several of these compounds function as generalized toxins. However, these generalized laboratory assays are not necessarily good predictors of how compounds will affect feeding by herbivores. For example, pachydictyol-A and stypotriol were equally effective at deterring fishes and Diadema, even though pachydictyol-A shows almost no bioactivity in laboratory assays while stypotriol and its oxidation product, stypoldione, are very bioactive. Herbivory on coral reefs is more intense than in any other habitat studied and the diversity of herbivore types is high. It appears that this intense grazing has provided strong selection for seaweeds that synthesize unique secondary metabolites that significantly reduce the consumption of plants exposed to attack by a diverse group of reef herbivores.

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