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Secondary metabolite chemistry of the caribbean marine alga Sporochnus bolleanus: A basis for herbivore chemical defence

Activated chemical defenses in tropical versus temperate seaweeds

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 207:243-253 (2000) - doi:10.3354/meps207243 Activated chemical defenses in tropical versus temperate seaweeds Giancarlo L. Cetrulo1,*, Mark E. Hay2,** 1University of North Carolina at Chapel Hill, Institute of Marine Sciences, 3431 Arendell St., Morehead City, North Carolina 28557, USA 2School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332-0230, USA *Present address: SEA Laboratory, 1021 N. Harbor Dr., Redondo Beach, California 90266, USA **Corresponding author. E-mail: mark.hay@biology.gatech.edu ABSTRACT: Chemical defenses that are rapidly activated in response to injury have been reported in numerous species of vascular plants, but activated chemical defense has been demonstrated for only 1 genus of seaweed. To investigate the frequency of potential activated chemical defenses in seaweeds and to determine if there are geographic differences in the frequency of theses, we conducted urchin and fish feeding assays using chemical extracts from 42 species of temperate or tropical seaweeds that were damaged immediately before extraction in organic solvents (= the potentially activated extract) or placed in organic solvents before they were damaged (= the non-activated extract). Seven species exhibited changes in palatability consistent with activated defenses while 4 species became more, rather than less, palatable if they were damaged 30 s before extraction. Frequency of activation did not vary geographically. Seventeen percent of tropical species (4 of 24) and 17% of temperate species (3 of 18) exhibited changes in palatability that were consistent with activation of chemical defenses. Thin-layer chromatography of lipid-soluble extracts indicated that damaging the thallus prior to extraction caused noticeable chemical changes in 70% of the species evaluated. Investigations of algal chemical defenses thus need to consider the effects of injury during herbivore attacks and the effects of extraction methodology on the types of, and concentrations of, metabolites discovered in seaweeds. KEY WORDS: Activated chemical defenses · Fish · Marine · Plant-herbivore interaction · Sea urchin · Seaweed Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 207. Online publication date: November 22, 2000 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2000 Inter-Research.

Effects of epibiosis on consumer-prey interactions

Herbivore Resistance to Seaweed Chemical Defense: The Roles of Mobility and Predation Risk

Numerous small sedentary herbivores (mesograzers such as amphipods, small crabs, and gastropods) are resistant to seaweed secondary metabolites that deter larger, more mobile herbivorous fishes. In addition, specialist mesograzers experience reduced predation from fishes when living on seaweeds that produce these compounds. In this study we tested the hypothesis that generalist, as opposed to specialist, mesograzers can also benefit from reduced predation when they occupy chemically defended plants. Secondly, we assessed the hypothesis that low herbivore mobility, unconfounded by herbivore size or specialized feeding, selects for tolerance of seaweed chemical defenses, by comparing responses to the chemically defended brown seaweed Dictyota menstrualis of three sympatric, generalist amphipods that differ in mobility (Ampithoe longimana, Ampithoe valida, and Gammarus mucronatus). Response to Dictyota's chemical defenses varied as much among these three amphipods as among the phylogenetically distant fishes and mesograzers studied previously and supported the hypothesis that less mobile herbivores should be most tolerant of plant chemical defenses. In laboratory experiments, A. longimana moved little, preferentially consumed Dictyota over other seaweeds, and was unaffected by all Dictyota secondary metabolites tested. In contrast, G. mucronatus was active, it did not feed on Dictyota, and two of three Dictyota secondary metabolites deterred its grazing. Distribution of amphipods in the field suggested that these feeding patterns affected amphipod risk of predation. A. longimana reached its highest abundance on Dictyota, which is unpalatable to omnivorous fish predators, during the season when fish are most abundant. At the same time, the highly active G. mucronatus decreased to near extinction. Like G. mucronatus, A. valida was detterred by two Dictyota secondary metabolites, did not eat Dictyota, and disappeared when fishes were abundant. Experiments confirmed that A. longimana was less vulnerable to fish predation when occupying a chemically defended seaweed than when occupying a palatable seaweed. This decreased predation resulted primarily from a decreased frequency of encounter with predators when amphipods were on chemically defended plants. When we experimentally equalized encounter rates between omnivorous pinfish (Lagodon rhomboides) and the seaweed Dictyota menstrualis and Ulva curvata (unpalatable and palatable, respectively, to pinfish) in the laboratory, amphipods occupying these two plants were eaten at similar rates. In contrast, when live amphipods were affixed to Ulva and Dictyota and deployed in the field, amphipods survived only on Dictyota. Heavy fish grazing on Ulva in the latter experiment suggests that poor survival of amphipods on Ulva may have resulted from greater detection and/or incidental ingestion of amphipods on this plant, due to frequent visitation by fishes. Infrequent visitation of Dictyota by foraging fish also may explain A. longimana's persistence through the summer on this chemically defended seaweed while the two Ulva—associated amphipods declined precipitously. These results (1) confirm that association with chemically defended plants can reduce predation on generalist, as well as specialist, herbivores and (2) suggest that preferential feeding on chemically defended plants is most likely for sedentary mesograzers because low mobility enhances the ability to exploit chemically defended seaweeds as refuges from fish predation.

Coral-Seaweed-Grazer-Nutrient Interactions on Temperate Reefs

Why corals dominate tropical reefs but are rare or absent on temperate reefs is one of the more intriguing global-scale questions in marine ecology. Restriction of corals from temperate reefs has been suggested to be due to low temperature, competition with seaweeds, and synergistic interactions of physical and biological factors. However, most studies addressing these hypotheses have been non-experimental and conducted on tropical corals and reefs near the extremes of their distribution rather than in temperate habitats using corals that are physiologically suited to temperate environments. Some corals occur only in temperate regions, but their ecology is largely unstudied. In an attempt to understand how temperate corals function at latitudes where corals are relatively rare and where reefs tend to be dominated by seaweeds, we examined responses of the temperate coral Oculina arbuscula to competition with seaweeds on reefs in North Carolina (USA). We asked how competitive interactions were affected by levels of grazing, nutrients, or interactions of these factors. Sampling of 12 reef habitats ranging from 1 m deep, inshore sites to 25 m deep, offshore sites demonstrated a strong negative relationship between percentage cover of seaweeds and density of Oculina colonies. Seaweeds dominated well-lit habitats (shallow inshore reefs or deep offshore reefs with clear waters), while coral was significantly more abundant in low-light habitats where seaweeds were rare (deep inshore habitats or nearshore reefs with turbid waters). This general among-site pattern also occurred within an individual site as one moved from shallow, well-lit waters to deeper, darker waters. Corals transplanted into seaweed-dominated areas grew well if seaweed canopies were removed but grew poorly, or not at all, if canopies were left intact. Seaweeds also significantly inhibited recruitment of O. arbuscula. At a turbid inshore reef, recruitment was high at a depth of 6 m where seaweeds were rare, but very low at a depth of 1 m where seaweeds were abundant. Removal of seaweeds from shallow plots increased recruitment about 12-fold to levels that did not differ significantly from those at 6 m. In grazer-exclusion cages on offshore reefs, Oculina recruitment varied significantly between sites and years. Almost no recruitment was observed at a well-lit, plant-dominated site while recruitment was higher, to very high, at a turbid site with few seaweeds. At this turbid site, recruitment was facilitated by grazing but could not be related to grazer effects on seaweeds. Facilitation was apparently due to consumers removing barnacles, which dominated this low-light site if grazers were excluded. Cage exclusion of larger herbivores at two offshore sites (primarily fishes) and one inshore site (fishes and urchins) had no significant effect on coral growth. At the well-lit inshore site, herbivores had a large effect on the species composition of the seaweed community, but little effect on the total abundance of seaweeds. Large grazers caused palatable red seaweeds to be replaced by similar amounts of unpalatable brown seaweeds. Therefore, herbivory alone had little impact on total seaweed abundance and the levels of seaweed competition affecting co-occurring O. arbuscula. In contrast, when we conducted a factorial experiment manipulating both herbivory and nutrient levels, exclusion cages significantly reduced coral growth. In this experiment, nutrient addition had no effect on brown seaweeds but significantly increased the percentage cover of red seaweeds in exclusion cages. Nutrient addition also tended to suppress coral growth in herbivore-exclusion cages where red seaweeds were stimulated, but to increase coral growth in open cages where herbivores had removed the red seaweeds.

GEOGRAPHIC AND GENETIC VARIATION IN FEEDING PREFERENCE FOR CHEMICALLY DEFENDED SEAWEEDS

Genetic variation in tolerance for seaweed chemical defenses among populations of marine herbivores is poorly described, either because few marine studies have addressed the issue or because the feeding preferences of populations of marine herbivores rarely differentiate. We address this using the polyphagous amphipod Ampithoe longimana (Crustacea), which commonly consumes, and lives on, terpene-rich brown seaweeds in the tropical genus Dictyota. Previous work demonstrated that in areas where the amphipod and seaweeds overlap geographically, these chemically deterrent seaweeds provide the amphipod with spatial refuges from predation by omnivorous fishes that avoid consuming Dictyota. However, the amphipod's distribution extends northward of the seaweed's distribution, making this benefit of association unavailable to more northerly populations of amphipods. On average, populations sympatric with Dictyota have stronger feeding preference for Dictyota species and greater fitness when raised on Dictyota than do populations that are outside Dictyota's geographic endpoint. These results are consistent with the hypothesis that in areas where the amphipods co-exist with Dictyota, selection favors amphipods that tolerate Dictyota's chemical defenses and thereby access its enemy-free space. Amphipods allopatric with Dictyota are unable to tolerate Dictyota's chemical defenses, either because of selection, drift, or some mix of both forces. A trade-off between preference for Dictyota and for the chemically distinct seaweed Hypnea musciformis is suggested by the finding that populations with high preference for Dictyota tended to have lower preference for Hypnea. However, this population-level pattern appears to be the result of independent evolution and not a genetic trade-off: an analysis of full-sib families within a single population detected heritable variation in preferences for Dictyota and suggested this for Hypnea (although the latter was not statistically significant), yet no family-level covariation was detected. Phylogeographic analysis of mitocondrial DNA and nuclear sequences indicates a strong historical break between populations sympatric with Dictyota and populations more than 500 km beyond Dictyota's geographic endpoint. The historical separation of these most northerly populations could have allowed a random accumulation of alleles to effectively degrade tolerance for Dictyota, although selection may also be responsible for the degradation.

Associational Plant Defenses and the Maintenance of Species Diversity: Turning Competitors Into Accomplices

The palatable plants investigated in this study gained significant protection from herbivores by associating with abundant competitors that were less susceptible to herbivory. When herbivores were excluded, palatable species associated with unpalatable ones grew at only 14%-19% of the rate of palatable plants separated from unpalatable ones. When herbivores were present, however, palatable species appeared to depend completely on unpalatable competitors to provide microsites of reduced herbivory to prevent grazers from causing local extinction of the preferred species. For the species studied here, the cost of being associated with a larger, unpalatable competitor was much less than the cost of increased consumption in the absence of that competitor. Under these conditions, one competitor can have a strong positive effect on another. Associational defenses can provide an unappreciated mechanism for maintaining species richness within communities that are dominated by one or a few major species. In this community, increases in the abundance of common late-successional competitors led to increases, not decreases, in the abundance and number of other species and to retention, not exclusion, of early-successional species.

Appendix C. A table showing further purification of deterrent crude extracts for all species for which we had adequate material.

A table showing further purification of deterrent crude extracts for all species for which we had adequate material.

Appendix B. Results of regression analyses testing for relationships between mean effect size and experimental duration.

Results of regression analyses testing for relationships between mean effect size and experimental duration.

Biodiversity has a positive but saturating effect on imperiled coral reefs

Coral diversity positively affects corals, but these benefits have limits and are threatened by ongoing species loss.

Faculty Opinions recommendation of Observations of biologically generated turbulence in a coastal inlet.

Do brominated natural products defend marine worms from consumers? Some do, most don’t

Worms and other marine invertebrates living in soft sediments commonly produce brominated natural products that have been hypothesized to function as defenses against consumers, but this hypothesis has not been tested directly. When 16 species of worms from a Georgia mud flat were fed to two sympatric fishes ( Fundulus heteroclitus , Leiostomus xanthurus ) and a crab ( Callinectes similis ), 15 species (94×;) were palatable to all three predators. Only the hemichordate Saccoglossus kowalevskii was unpalatable to both fishes, but even it was readily consumed by the crab. Bioassay‐guided chemical investigations demonstrated that Saccoglossus kowalevskii was rejected by fishes because it contained 2,3,4‐tribromopyrrole at 0.2×; of worm dry mass. This is the first direct test of brominated worm metabolites as defenses against sympatric consumers. The deterrence of 2,3,4‐tribromopyrrole in S. kowalevskii may explain why densities of this worm increase 40‐fold during seasons when predation is high and densities of palatable worms decline sharply. To more broadly examine the effects of brominated metabolites on worm palatability, we collected from North Carolina, Georgia, and Florida 14 species reported to produce brominated metabolites. These species were fed to sympatric consumers to assess palatability and also analyzed for brominated metabolites by gas chromatography mass spectrometry (GC/MS). Nine of the fourteen species contained brominated metabolites, but only two were unpalatable. In a final test, five additional brominated metabolites produced by marine worms were added to palatable foods at natural concentrations and at up to 20× natural concentrations. None deterred feeding at natural concentrations, one was deterrent at 5−15× natural concentration, and four had no effect at even 20× natural concentration. Thus, while one worm was defended by a brominated compound, most worms containing brominated metabolites were palatable, and brominated natural products seldom functioned as chemical defenses against consumers.

Chemical Defense of the Eastern Newt (Notophthalmus viridescens): Variation in Efficiency against Different Consumers and in Different Habitats

Amphibian secondary metabolites are well known chemically, but their ecological functions are poorly understood--even for well-studied species. For example, the eastern newt (Notophthalmus viridescens) is a well known secretor of tetrodotoxin (TTX), with this compound hypothesized to facilitate this salamander's coexistence with a variety of aquatic consumers across the eastern United States. However, this assumption of chemical defense is primarily based on observational data with low replication against only a few predator types. Therefore, we tested the hypothesis that N. viridescens is chemically defended against co-occurring fishes, invertebrates, and amphibian generalist predators and that this defense confers high survivorship when newts are transplanted into both fish-containing and fishless habitats. We found that adult eastern newts were unpalatable to predatory fishes (Micropterus salmoides, Lepomis macrochirus) and a crayfish (Procambarus clarkii), but were readily consumed by bullfrogs (Lithobates catesbeianus). The eggs and neonate larvae were also unpalatable to fish (L. macrochirus). Bioassay-guided fractionation confirmed that deterrence is chemical and that ecologically relevant concentrations of TTX would deter feeding. Despite predatory fishes rejecting eastern newts in laboratory assays, field experiments demonstrated that tethered newts suffered high rates of predation in fish-containing ponds. We suggest that this may be due to predation by amphibians (frogs) and reptiles (turtles) that co-occur with fishes rather than from fishes directly. Fishes suppress invertebrate consumers that prey on bullfrog larvae, leading to higher bullfrog densities in fish containing ponds and thus considerable consumption of newts due to bullfrog tolerance of newt chemical defenses. Amphibian chemical defenses, and consumer responses to them, may be more complex and indirect than previously appreciated.

Effects of nutrients versus herbivores on reef algae: A new method for manipulating nutrients on coral reefs

There has been much discussion and some controversy regarding the role of nutrient enrichment versus other factors, such as altered rates of herbivory, in the degradation of coral reef ecosystems. The resolution of this controversy has been hampered by the lack of manipulative field studies testing the effects of ecologically relevant levels of nutrient enrichment on coral reef communities. We present a new method for adding ecologically relevant levels of nutrients to experimental substrates on coral reefs. The method elevates nutrients for sustained periods of time (>41 d without replenishment of nutrients) and allows for testing interactions of nutrients and altered levels of herbivory. Results from an offshore reef in Key Largo, Florida, show strong effects of excluding large herbivorous fishes, negligible effects of nutrient enrichment–;or effects that are opposite of predictions, and no interaction between nutrient levels and herbivory in affecting algal abundance. Patterns observed for this reef did not confirm predictions of previously proposed models that frondose macroalgal or crustose algal abundance would be enhanced with nutrient enrichment or that dominance of filamentous turfs would be greater in unenriched conditions. In contrast to previous predictions, the abundance of larger macroalgae at this site was not increased by elevating nutrients above predicted threshold response levels of 1.0 µM for total inorganic nitrogen or 0.10 µM for soluble reactive phosphate. Also conflicting with some models, filamentous, nitrogen‐fixing cyanobacteria were enhanced, rather than suppressed, by nutrient enrichment.

Interactions of plant stress and herbivory: intraspecific variation in the susceptibility of a palatable versus an unpalatable seaweed to sea urchin grazing

Susceptibility to Herbivores Depends on Recent History of both the Plant and Animal

Physical stress to seaweeds and hunger stress of herbivores can influence the outcome of chemically mediated seaweed-herbivore interactions. The unpalatable brown seaweed Dictyota ciliolata produces the diterpenoid secondary metabolites pachydictyol A, dictyol B acetate, and dictyodial. At natural concentrations, pachydictyol A deterred the sea urchin Arbacia punctulata but did not inhibit feeding by the pinfish Lagodon rhomboides or the amphipod Ampithoe longimana until concentrations were 2.5-5 times natural levels. Dictyol B acetate deterred the urchin, amphipod, and pinfish at, or far below, natural concentrations. Dictyodial was too unstable to assay directly, but indirect experiments suggested that natural concentrations deterred the urchin, but not the pinfish or amphipod. Mild desiccation of D. ciliolata reduced concentrations of the different secondary metabolites by 7-38% and plants became 2.6-3.4 times more susceptible to urchin and amphipod grazing. The combined concentrations of pachydictyol A and dictyol B acetate found in undesiccated Dictyota ciliolata deterred feeding by urchins, but this deterrent effect was lost at concentrations found in the desiccated plants. Desiccated and undesiccated plants did not differ in nutritive value (as measured by protein and total N content) or toughness. Thus, desiccated plants became more palatable because chemical defenses were lost, not because nutritive value was increased. The stress of near-surface ultraviolet radiation also caused significant physiological changes in Dictyota ciliolata. UV-exposed blades bleached, senesced, and grew 84% less than blades protected from UV radiation. Tissue loss and minimal growth of UV-stressed plants constrained our sample sizes, but the limited assays that could be run suggested that UV stress may lower chemical defenses and increase plant susceptibility to herbivores. Because many previous investigations of herbivore feeding patterns used animals that had been starved for days before an assay, we tested the effects of this commonly used procedure on feeding discrimination. Recently fed urchins always avoided food containing natural concentrations of pachydictyol A during separate feeding trials performed on each of four consecutive days. In contrast, urchins deprived of food for 3 d before this assay did not avoid the treated food on days 1 and 2 of feeding trials, but did avoid it on days 3 and 4 after their hunger was reduced by feeding during days 1 and 2. If we had used only starved urchins (a common procedure in previous investigations), we could have concluded, with apparent justification, that urchins were unaffected by pachydictyol A (if the assays were run for only 1-2 d) or that they needed 2 d of exposure to the compound in order to learn to avoid it. Both of these conclusions would have been incorrect.

Marine Chemical Ecology: Chemical Signals and Cues Structure Marine Populations, Communities, and Ecosystems

Chemical cues constitute much of the language of life in the sea. Our understanding of biotic interactions and their effects on marine ecosystems will advance more rapidly if this language is studied and understood. Here, I review how chemical cues regulate critical aspects of the behavior of marine organisms from bacteria to phytoplankton to benthic invertebrates and water column fishes. These chemically mediated interactions strongly affect population structure, community organization, and ecosystem function. Chemical cues determine foraging strategies, feeding choices, commensal associations, selection of mates and habitats, competitive interactions, and transfer of energy and nutrients within and among ecosystems. In numerous cases, the indirect effects of chemical signals on behavior have as much or more effect on community structure and function as the direct effects of consumers and pathogens. Chemical cues are critical for understanding marine systems, but their omnipresence and impact are inadequately recognized.