Person:

Hay, Mark E.

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https://hdl.handle.net/1853/71334

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School of Biological Sciences

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Chemical 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, 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.
The ecology of rubble structures of the South Atlantic bight: a community profile

(Georgia Institute of Technology, 1988-09) Hay, Mark E. ; Sutherland, John P. ; Watzin, Mary C.
Chemical defense against different marine herbivores: are amphipods insect equivalents?

(Georgia Institute of Technology, 1987) Hay, Mark E. ; Duffy, J. Emmett ; Pfister, Catherine A. ; Fenical, William

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 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.
Chemical defense against diverse coral reef herbivores

(Georgia Institute of Technology, 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.
Associational plant defenses and the maintenance of species diversity: turning competitors into accomplices

(Georgia Institute of Technology, 1986-11) Hay, Mark E.
Seaweed susceptibility to herbivory: chemical and morphological correlates

(Georgia Institute of Technology, 1986-10-21) Paul, Valerie J. ; Hay, Mark E.

The susceptibility of 82 species of tropical seaweeds to grazing by herbivorous fishes was assessed on 8 different coral reefs in the Florida Keys, USA. Most species were simultaneously assayed for the presence or absence of unusual secondary metabolites and recorded as having either calcified or uncalcified thalli. Both production of secondary metabolites and of a calcified thallus were associated with low susceptibility to herbivory. However, the relative importance of calcification versus chemical deterrents cannot be assessed for the calcified species since almost all these also contained secondary metabolites. Eighty-five % of calcified species, but only 39 % of uncalcified species, produced secondary metabolites. Secondary metabolites were produced by 71 % of the species least susceptible to herbivory (<25 % eaten) but by only 20 % of the species most susceptible to herbivory (>75% eaten). Calcified thalli were produced by 50 % of the lowest preference species but by only 9 % of the highest preference species. Thus. several common reef seaweeds appear to resist herbivory by relying primarily on chemical deterrents (genera Dictyota, Dilophus, Stypopodium, Lobophora, Avrainvillea, and some Caulerpa species) but many appear to combine both chemical and morphological defenses (genera Penicillus, Halimeda, Rhipocephalus, Udotea, Amphiroa, and Galaxaura). We suggest that multiple defenses will be common among seaweeds on coral reefs since herbivore diversity is high and it is unlikely that any single defense will be effective against the many types of herbivores that encounter these plants. This may account, in part, for the diversity of secondary metabolites produced by some tropical seaweeds.
Functional morphology of intertidal seaweeds: adaptive significance of aggregate vs. solitary forms

(Georgia Institute of Technology, 1984-08-15) Taylor, Phillip R. ; Hay, Mark E.

Many intertidal seaweeds show a tremendous gradient of morphological form ranging from spatially separated thalli, to thalli that are aggregated into dense turfs. Aggregation of seaweeds into turfs decreases productivity per g organic weight due to crowding of thalli but increases resistance to desiccation. The intertidal distribution of the turf growth form is correlated with the intensity of desiccation stress. Also turfs transplanted into tide pools developed the non-turf morphology, while non-turfs transplanted to emergent substrate either developed the turf form or died. The turf growth form is energetically expensive; apparent productivity of turfs was 23 to 48 % less than that of individuals. Increasing light and nutrients available to turfs by separating the thalli increased apparent productivity by 36 to 113 %. We tested the hypothesis that these turfs minimize energetic costs of this aggregated growth form through the spatial partitioning of photosynthetic and respirative activity. The lower portions of turfs showed less apparent photosynthesis than the upper portions (reductions of 37 to 85 %); however, this spatial partitioning was also found in the individual forms (37 to 63 % reductions) of these relatively simple seaweeds. Spatial differentiation of respiration was similar in turfs and nonturfs. The seaweeds, examined are able to alter the extent of thalli compaction in accordance with varying levels of environmental stress. This phenotypic plasticity allows seaweeds to adopt morphological features that maximize fitness in a wide variety of habitats without being developmentally committed.
Coral Reef Ecology: Have We Been Putting All of Our Herbivores in One Basket?

(Georgia Institute of Technology, 1984-05-01) Hay, Mark E.
Patterns of fish and urchin grazing on Caribbean coral reefs: are previous results typical?

(Georgia Institute of Technology, 1984-04) Hay, Mark E.

Strips of the seagrass Thalassia testudinum were used in a field bioassay to assess herbivory on 11 coral reefs scattered throughout the Caribbean. Patterns of herbivory on overfished reefs in Haiti and the United States Virgin Islands were compared to patterns on seven less fished reefs (United States Virgin Islands, Panama, Honduras, Belize, and the Bahamas). On the overfished reefs, the rate of Thalassia removal increased significantly with depth, urchin densities were high, and urchin grazing was equal to, or greater than, fish grazing in shallow (<10 m deep) habitats. On reefs subject to little fishing pressure, the rate of Thalassia removal decreased with depth, urchin densities were low, and herbivorous fishes were responsible for almost all Thalassia removal. Previous studies assessing the importance of urchin grazing in the Caribbean have been conducted on overfished reefs where urchin densities were unusually high and the density of grazing fish unusually low. It is doubtful that the intensity of urchin—algal and urchin—coral interactions observed on these heavily fished reefs occurs on reefs unaffected by humans.
Between-habitat differences in herbivore impact on Caribbean coral reefs

(Georgia Institute of Technology, 1983-12) Hay, Mark E. ; Goertemiller, Tim

Transplanted sections of the seagrass Thalassia testudinum were used as a bioassay to assess between-habitat differences in herbivory on three Caribbean reefs. Consumption of Thalassia by herbivorous fishes on shallow (1-10 m) reef slopes was significantly higher than on deep (30-40 m) reef slopes or on shallow reef flats. Seaweeds typical of reef flat habitats were rapidly consumed when placed on shallow reef slopes. Seaweeds typical of either deep or shallow reef slopes were relatively resistant to herbivory and a high proportion of these species are known to contain secondary chemical compounds that appear to deter herbivorous fishes. Shallow reef flats provide seaweeds with a predictable spatial escape from major reef herbivores; algae characteristic of these habitats have evolved few, if any, characteristics that significantly reduce losses to herbivory.