Induced resistance in intertidal macroalgae modifies feeding behaviour of herbivorous snails

Herbivory in terrestrial and marine systems can induce changes in plant chemistry affecting the foraging behaviour of herbivores. A model based on terrestrial plant-herbivore interactions predicts herbivory-induced changes in leaf chemistry to be manifested in (1) increased herbivore mobility, (2) increased feeding dispersal and (3) reduced tissue consumption by herbivores. This study is the first to demonstrate that herbivory-induced changes in the tissue chemistry of the brown seaweed Ascophyllum nodosum elicit the same response in the feeding behaviour of the gastropod Littorina obtusata as predicted for herbivorous insects, providing good evidence for the models validity across different ecosystems. The potential benefit of increased feeding dispersal to terrestrial plants as suggested by the model is the prevention of concentrated damage to apical tissues thereby preserving the plants ability to compete for light; A. nodosum does not conform to these predictions. Increased dispersal of feeding damage on A. nodosum away from primary frond tissues would reduce the likelihood of frond breakage implying a fitness benefit of induced resistance.     

Influence of light and nitrogen on the phlorotannin content of the brown seaweeds Ascophyllum nodosum and Fucus vesiculosus

Phlorotannins, C-based defence compounds in brown seaweeds, show a high degree of spatial and temporal variation within seaweed species. One important model explaining this variation is the Carbon Nutrient Balance Model (CNBM), which states that the relative supply of carbon and limiting nutrients will determine the level of defence compounds in plants. Nitrogen is often considered to be the limiting nutrient for marine macroalgal growth and the CNBM thus predicts that when the carbon:nitrogen ratio is high, photosynthetically fixed carbon will be allocated to production of phlorotannins. In the present study, we evaluated the effects of light (i.e. carbon) and nitrogen on the phlorotannin content of two intertidal brown seaweeds, Ascophyllum nodosum and Fucus vesiculosus. This was done in an observational field study, as well as in a manipulative experiment where plants from habitats with different light regimes were subjected to different nitrogen and light treatments, and their phlorotannin content was measured after 14 days. The results showed that there was a negative relationship between tissue nitrogen and phlorotannin content in natural populations of F. vesiculosus, but not in A. nodosum. In the short term, the phlorotannin content in both algal species was not affected by changes in nitrogen availability. Exposure to sunlight had a positive effect on the phlorotannin content in natural populations of both algal species but, in the manipulative experiment, only F. vesiculosus showed a rapid response to changes in light intensities. Plants subjected to sunlight contained higher phlorotannin content than shaded plants. In conclusion, the results imply that nitrogen availability explains some of the natural variation in the phlorotannin content of F. vesiculosus, but the light environment has greater importance than nitrogen availability in predicting the phlorotannin content of each species.     

High acclimation potential in floating Macrocystis pyrifera to abiotic conditions even under grazing pressure – a field study

The persistence of floating seaweeds, which depends on abiotic conditions but also herbivory, had previously been mostly tested in outdoor mesocosm experiments. In order to investigate if the obtained mesocosm results of high seaweed persistence under natural environmental conditions and under grazing pressure can be extrapolated to field situations, we conducted in situ experiments. During two summers (2007 and 2008), Macrocystis pyrifera was tethered (for 14 d) to lines in the presence and absence of the amphipod Peramphithoe femorata at three sites (Iquique, Coquimbo, Calfuco). We hypothesized that grazing damage and seaweed persistence vary among sites due to different abiotic factors. By incubating the sporophytes in mesh bags, we were either able to isolate (grazing) or exclude (control) amphipods. To test for a mesh bag artifact, a set of sporophytes was incubated without mesh bags (natural). Mesh bags used to exclude herbivores influenced sporophyte growth and physiological performance. The chlorophyll a (Chl a) content depended largely on grazers and grazed sporophytes grew less than natural and control sporophytes within the two summers. A decrease in Chl a content was found for the sites with the highest prevailing irradiances and temperatures, suggesting an efficient acclimation to these sea surface conditions. Our field‐based results of sporophyte acclimation ability even under grazing pressure widely align with previous mesocosm results. We conclude that M. pyrifera and other temperate floating seaweeds can function as long‐distance dispersal vectors even with hitchhiking mesoherbivores.     

Sex-specific reproductive trade-offs in the gregarious fucoid macroalga Ascophyllum nodosum

The existence of sex-specific reproductive trade-offs is well established in plants. They usually occur because females invest more resources into reproduction than males, and have to compensate by sacrificing growth or defence. Investigations into by-sex differences in resource allocation by seaweeds are comparatively scarce. A small number of authors report differences between the sexes in some red algae, but known by-sex differences in brown algae are largely confined to the reproductive structures. In this study, sex-specific reproductive trade-offs are investigated in the common temperate brown alga Ascophyllum nodosum from three distinct populations. Quantified investments into growth, defence, and reproduction of a large number of individuals (n = 720) selected across a full annual cycle are presented in parallel with feeding trials using a common gastropod herbivore and a comprehensive assessment of the biotic and abiotic stressors impacting on A. nodosum at three sites. These reveal that sex-specific reproductive trade-offs occur in two of the three populations, as females invest more into reproduction than males and are subsequently less chemically defended for the months post gamete release. Feeding trials confirm that this leaves females more vulnerable to grazing pressure during these months, although mortality and competitive ability appear unaffected in the field. Possible causes of the trade-offs made by females are discussed, and new avenues of investigation are identified which could reveal interesting parallels between seaweeds and higher plants.     

Increased chemical resistance explains low herbivore colonization of introduced seaweed

The success of introduced species is often attributed to release from co-evolved enemies in the new range and a subsequent decreased allocation to defense (EICA), but these hypotheses have rarely been evaluated for systems with low host-specificity of enemies. Here, we compare herbivore utilization of the brown seaweed, Fucus evanescens, and its coexisting competitors both in its native and new ranges, to test certain predictions derived from these hypotheses in a system dominated by generalist herbivores. While F. evanescens was shown to be a preferred host in its native range, invading populations supported a less diverse herbivore fauna and it was less preferred in laboratory choice experiments with important herbivores, when compared to co-occurring seaweeds. These results are consistent with the enemy release hypothesis, despite the fact that the herbivore communities in both regions were mainly composed of generalist species. However, in contrast to the prediction of EICA, analysis of anti-grazing compounds indicated a higher allocation to defense in introduced compared to native F. evanescens. The results suggest that the invader is subjected to less intense enemy control in the new range, but that this is due to an increased allocation to defense rather than release from specialized herbivores. This indicates that increased resistance to herbivory might be an important strategy for invasion success in systems dominated by generalist herbivores.     

Seaweed-herbivore-predator interactions: host-plant specialization reduces predation on small herbivores

Summary Because feeding specialization among marine herbivores is rare, marine communities provide a simplified system for identifying factors selecting for specialization. On Australia's Great Barrier Reef, we investigated interactions among the chemically-defended seaweed Chlorodesmis fastigiata, herbivores specialized on this alga, and potential predators of these herbivores. Chlorodesmis is a low preference food for reef fishes but appears to be the only food of the crab Caphyra rotundifrons and the ascoglossan gastropods Elysia sp. and Cyerce nigricans. The crab is found only in patches of Chlorodesmis, feeds solely on the alga, and selectively shelters in it in laboratory choice experiments. Crab grazing on the red seaweed Acanthophora spicifera was stimulated when this alga was coated with increasing concentrations of the cytotoxic diterpenoid chlorodesmin, the major secondary metabolite of Chlorodesmis. Crabs did not sequester Chlorodesmis metabolites but avoided predators by sheltering in the unpalatable alga. All crabs tethered on the reef without access to Chlorodesmis patches were rapidly eaten; those with access to Chlorodesmis patches were much less susceptible to predation. The cryptic ascoglossan Elysia sp. was found exclusively in patches of Chlorodesmis and sequestered metabolites from the alga. Living Elysia were unpalatable to the common wrasse Thalassoma lunare in laboratory assays, but the crude organic extract of Elysia did not significantly deter feeding by Thalassoma. Elysia sequestered chlorodesmin, which deterred feeding by reef fishes in field assays but was ineffective against Thalassoma in laboratory assays at 5% food dry mass. Unlike Elysia, the aposematically colored ascoglossan Cyerce nigricans sequestered Chlorodesmis metabolites in relatively small amounts, but produced larger amounts of unrelated polypropionate compounds. Cyerce were never attacked by fishes and the crude organic extract of this slug strongly deterred feeding by wrasses in laboratory assays. The dorid nudibranch Gymnodoris sp. was found only in Chlorodesmis patches and appeared to be a specialized predator on Elysia; it would not prey on Cyerce. Data from this and other recent investigations demonstrate that some small marine herbivores feed selectively or exclusively on seaweeds that are chemically defended from fishes. This association reduces predation on the herbivores and suggests that escape from and deterrence of predation may be a dominant factor selecting for specialization among these herbivores.     

Rapid bioassays to evaluate the plant growth promoting activity of <Emphasis Type="Italic">Ascophyllum nodosum</Emphasis> (L.) Le Jol. using a model plant, <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> (L.) Heynh

Ascophyllum nodosum extract products are used commercially in the form of liquid concentrate and soluble powder. These formulations are manufactured from seaweeds that are harvested from natural habitats with inherent environmental variability. The seaweeds by themselves are at different stages of their development life-cycle. Owing to these differences, there could be variability in chemical composition that could in turn affect product consistency and performance. Here, we have tested the applicability of using Arabidopsis thaliana as a model to study the activity of two different extracts from A. nodosum. Three different bioassays: Arabidopsis root-tip elongation bioassay, Arabidopsis liquid growth bioassay and greenhouse growth bioassay were evaluated as growth assays. Our results indicate that both extracts promoted root and shoot growth in comparison to controls. Further, using Arabidopsis plants with a DR5:GUS reporter gene construct, we provide evidence that components of the commercial A. nodosum extracts modulates the concentration and localisation of auxins which could account, at least in part, for the enhanced plant growth. The results suggest that A. thaliana could be used effectively as a rapid means to test the bioactivity of seaweed extracts and fractions.     

Induction of Attachment of the Mussel Perna perna by Natural Products from the Brown Seaweed Stypopodium zonale

Marine invertebrates settle, attach, and/or metamorphose in response to signals from several sources, including seaweeds. In response to the aquaculture challenge of producing constant numbers of juveniles from cultured species, natural inducers have been screened for their ability to improve those processes. However, few chemical inducers of attachment of invertebrates have been identified, and even less of these were secondary metabolites. The goal of this work was to isolate the natural products responsible for induction activity using bioassay-guided fractionation of the organic extract of the brown seaweed Stypopodium zonale and the attachment of juveniles of the common brown mussel, Perna perna, as a model. The meroditerpene epitaondiol, identified by comparison of spectral data with the literature, promoted as much as 4.7 times more mussel attachment compared to controls at the natural concentration found in this alga (0.041% of the crude extract or 0.012% of algal dry weight). This is the first report showing that a seaweed produces terpenoid compounds as cues for invertebrate attachment, and future studies evaluating this action on settlement of mussels in the field are expected to improve aquaculture technology by increasing mussel spat production.     

Floating seaweed and the influences of temperature, grazing and clump size on raft longevity — A microcosm study

Laboratory experiments were conducted to evaluate the longevity, and consequently also the rafting capacity of the brown seaweeds Fucus vesiculosus and Ascophyllum nodosum. The seaweed degradation process and the activity of the grazer Idotea baltica were strongly influenced by temperature: only at 5 °C, the seaweed growth exceeded the weight loss. At higher temperatures, seaweed fragments sank quickly (within 100 days at temperatures higher than 15 °C). This process was significantly accelerated in the presence of I. baltica, resulting in a decrease of raft longevity of 60-70%. At a constant temperature of 15 °C and in the absence of grazers, fragments of A. nodosum floated longer (mean 45 weeks) than fragments of F. vesiculosus (mean 15 weeks). The results indicate that floating seaweeds have the potential to stay afloat for a long time, but that their longevity is temperature-dependent and can be strongly reduced by grazing activity of associated herbivores.     

Do metal-rich plants deter herbivores? A field test of the defence hypothesis

Some plant species growing on metalliferous soils are able to accumulate heavy metals in their shoots up to very high concentrations, but the selective advantage of this behaviour is still unknown. The most popular hypothesis, that metals protect plants against herbivores, has been tested several times in laboratory conditions, with contradictory results. We carried out the first large-scale test of the defence hypothesis in eight natural populations of the model Zn hyperaccumulator Thlaspi caerulescens J. and C. Presl (Brassicaceae). In two climatic regions (temperate, Belgium–Luxembourg, and Mediterranean, southern France), we worked in metalliferous and in normal, uncontaminated environments, with plants spanning a wide range of Zn concentrations. We also examined the importance of glucosinolates (main secondary metabolites of Brassicaceae) as antiherbivore defences. When exposed to natural herbivore populations, T. caerulescens suffered lower herbivory pressures in metal-enriched soils than in normal soils, both in Belgium–Luxembourg and in southern France. The trapping of gastropods shows an overall lower population density in metalliferous compared to normal environments, which suggests that herbivory pressure from gastropods is lower on metalliferous soils. In addition, foliar concentration of glucosinolates was constitutively lower in all populations from metal-enriched soils, suggesting that these have evolved towards lower investment in organic defences in response to lower herbivory pressure. The Zn concentration of plants had a protective role only for Belgian metallicolous plants when transplanted in normal soils of Luxembourg. These results do not support the hypothesis that Zn plays a key role in the protection of T. caerulescens against enemies. In contrast, glucosinolates appear to be directly involved in the defence of this hyperaccumulator against herbivores.     

WATER-BORNE CHEMICAL CUES AS ELICITORS OF ALGAL DEFENSES

Toth, G. & Pavia, H. Göteborg University, Tjärnö Marine Biological Laboratory, SE-452 96 Strömstad, Sweden It is well established that water-borne chemical cues from predators or predator-wounded conspecifics can induce defensive changes of aquatic prey animals, but few examples of such inducible defenses have been reported for aquatic algae. We have found that water-borne cues from actively feeding gastropods (Littorina obtusata) can induce increased concentrations of phlorotannins in the intertidal brown seaweed Ascophyllum nodosum. Elicited A. nodosum plants are also less susceptible to further grazing bygastropods compared to undamaged plants. Since seaweeds have poorly developed internal transport systems and may not be able to elicit systemic induced chemical defenses through conveyance of internal signals, induction through water-borne cues ensures that seaweeds can anticipate future periwinkle attacks without receiving direct damage by herbivores. We have also found that water-borne cues from a parasite (Parvilucifera infectans), can serve as signals to induce morphological defence in a toxic dinoflagellate (Alexandrium ostenfeldii). The dinoflagellates formed temporary cysts when exposed to filtrates from cultures with infected conspecifics. The effect of the filtrate disappeared after 8 h, indicating that the chemical cues are short-lived. Furthermore, the cysts were resistant to parasite infections for several weeks after exposure to direct parasite contact, indicating that they are an effective defense. We suggest that induction of defenses in marine algae through water-borne cues may be a common phenomenon and that more examples will be revealed as this field of science progresses.     

Temporal dynamics of inducible anti‐herbivory defenses in the brown seaweed Ascophyllum nodosum (Phaeophyceae)1

Anti‐herbivory defenses support persistence of seaweeds. Little is known, however, about temporal dynamics in the induction of grazer‐deterrent seaweed traits. In two induction experiments, consumption rates of the periwinkle Littorina obtusata (L.) on the brown seaweed Ascophyllum nodosum (L.) Le Jolis were measured in 3‐d intervals. Changes in palatability of directly grazed A. nodosum were tested every 3 d with feeding assays using fresh and reconstituted seaweed pieces. Likewise, assays with fresh A. nodosum assessed changes in seaweed palatability in response to water‐borne cues from nearby grazed conspecifics. Consumption rates of L. obtusata varied significantly during the 27‐d induction phase of each experiment. Direct grazing by L. obtusata lowered palatability of fresh and reconstituted A. nodosum pieces to conspecific grazers after 15 d as well as after 6 and 12 d, respectively. After 12, 18, and 24 d, fresh A. nodosum located downstream of L. obtusata‐grazed conspecifics was significantly less palatable than A. nodosum located downstream of ungrazed conspecifics. Changes in L. obtusata consumption rates and A. nodosum palatability during both induction experiments suggest temporal variation of grazer‐deterrent responses, which may complicate experimental detection of inducible anti‐herbivory defenses.     

Copper Contamination Impairs Herbivore Initiation of Seaweed Inducible Defenses and Decreases Their Effectiveness

Seaweed-herbivore interactions are often mediated by environmental conditions, yet the roles of emerging anthropogenic stressors on these interactions are poorly understood. For example, chemical contaminants have unknown consequences on seaweed inducible resistance and herbivore response to these defenses despite known deleterious effects of contaminants on animal inducible defenses. Here, we investigated the effect of copper contamination on the interactions between a snail herbivore and a brown seaweed that displays inducible resistance to grazing. We examined seaweed inducible resistance and its effectiveness for organisms exposed to copper at two time points, either during induction or after herbivores had already induced seaweed defenses. Under ambient conditions, non-grazed tissues were more palatable than grazed tissues. However, copper additions negated the preference for non-grazed tissues regardless of the timing of copper exposure, suggesting that copper decreased both how herbivores initiated these inducible defenses and their subsequent effectiveness. Copper decreased stimulation of defenses, at least in part, by suppressing snail grazing pressure—the cue that turns inducible defenses on. Copper decreased effectiveness of defenses by preventing snails from preferentially consuming non-grazed seaweed. Thus, contaminants can potentially stress communities by changing seaweed-herbivore interactions mediated via inducible defenses. Given the ubiquity of seaweed inducible resistance and their potential influence on herbivores, we hypothesize that copper contamination may change the impact of these resistant traits on herbivores.     

Geographic variation in feeding preference of a generalist herbivore: the importance of seaweed chemical defenses

The ecological impacts of generalist herbivores depend on feeding preferences, which can vary across and within herbivore species. Among mesoherbivores, geographic variation in host use can occur because host plants have a more restricted geographic distribution than does the herbivore, or there is local evolution in host preference, or both. We tested the role of local evolution using the marine amphipod Ampithoe longimana by rearing multiple amphipod populations from three regions (subtropical Florida, warm-temperate North Carolina and cold-temperate New England) and assaying their feeding preferences toward ten seaweeds that occur in some but not all regions. Six of the ten seaweeds produce anti-herbivore secondary metabolites, and we detected geographic variation in feeding preference toward five (Dictyota menstrualis, Dictyota ciliolata, Fucus distichus, Chondrus crispus and Padina gymnospora, but not Caulerpa sertularioides). Amphipod populations that co-occur with a chemically-rich seaweed tended to have stronger feeding preferences for that seaweed, relative to populations that do not co-occur with the seaweed. A direct test indicated that geographic variation in feeding preference toward one seaweed (D. ciliolata) is mediated by feeding tolerance for lipophilic secondary metabolites. Among the four seaweeds that produce no known secondary metabolites (Acanthophora, Ectocarpus, Gracilaria and Hincksia/Feldmannia spp.), we detected no geographic variation in feeding preference. Thus, populations are more likely to evolve greater feeding preferences for local hosts when those hosts produce secondary metabolites. Microevolution of feeding behaviors of generalist marine consumers likely depends on the availability and identity of local hosts and the strength of their chemical defenses.     

Liquid seaweed extracts identified using <Superscript>1</Superscript>H NMR profiles

Aqueous extracts of Ascophyllum nodosum and several other brown seaweeds are manufactured commercially and widely distributed for use on agricultural crops. The increasingly regulated international trade in such products requires that they be standardized and defined to a degree not previously required. We examined commercially available extracts using quantitative ¹H NMR and principal components analysis (PCA) techniques. Extracts manufactured over a 4-year period using the same process exhibited characteristic profiles that, on PCA, clustered as a discrete group distinct from the other commercial products examined. In addition to recognizing extracts made from different seaweeds, analysis of the ¹H spectra in the 0.35–4.70 ppm region allowed us to distinguish amongst extracts produced from the same algal species by different manufacturers. This result established that the process used to make an extract is an important variable in defining its composition. A comparison of the ¹H NMR integrals for the regions 1.0–3.0 ppm and 3.0–4.38 ppm revealed small but significant changes in the A. nodosum spectra that we attribute to seasonal variation in gross composition of the harvested seaweed. Such changes are reflected in the PCA scores plots and contribute to the scatter observed within the data point cluster observed for Acadian soluble extracts when all data are pooled. Quantitative analysis using ¹H NMR (qNMR) with a certified external standard (caffeine) showed a linear relationship with extract concentration over at least an order of magnitude (2.5–33 mg/mL; R ² > 0.97) for both spectral regions integrated. We conclude that qNMR can be used to profile (or “fingerprint”) commercial seaweed extracts and to quantify the amount of extract present relative to a suitably chosen standard. Issued as NRCC no. 42,652.     

Climate change and disease: bleaching of a chemically defended seaweed

Disease is emerging as an important impact of global climate change, due to the effects of environmental change on host organisms and their pathogens. Climate‐mediated disease can have severe consequences in natural systems, particularly when ecosystem engineers, such as habitat‐formers or top predators are affected, as any impacts can cascade throughout entire food webs. In temperate marine ecosystems, seaweeds are the dominant habitat‐formers on rocky reefs. We investigated a putative bleaching disease affecting Delisea pulchra, a chemically defended seaweed that occurs within a global warming ‘hot‐spot’ and assessed how patterns of this phenomenon were influenced by ocean temperature, solar radiation, algal chemical defences and microbial pathogens. Warmer waters were consistently and positively correlated with higher frequencies of bleaching in seaweed populations, but patterns of bleaching were not consistently influenced by light levels. Bleached thalli had low levels of antibacterial chemical defences relative to healthy conspecifics and this was observed across entire thalli of partially bleached algae. Microbial communities associated with bleached algae were distinct from those on the surfaces of healthy seaweeds. Direct testing of the importance of algal chemical defences, done here for the first time in the field, demonstrated that they protected the seaweed from bleaching. Treatment of algal thalli with antibiotics reduced the severity of bleaching in experimental algae, especially at high water temperatures. These results indicate that bleaching in D. pulchra is the result of temperature‐mediated bacterial infections and highlight the potential for warming to influence disease dynamics by stressing hosts. Understanding the complex ways in which global change may affect important organisms such as habitat‐forming seaweeds, is essential for the management and conservation of natural resources.     

Comment on ‘Island biogeography: patterns of marine shallow‐water organisms’ by Hachich et al., Journal of Biogeography (2015)

In a recent article, Hachich et al. (2015, Journal of Biogeography, 42 , 1871–1882) studied the large‐scale biogeographical patterns of the species–area, species–island age and species–isolation relationships associated with marine shallow‐water groups (reef fish, gastropods and seaweeds) from 11 Atlantic archipelagos. We here express our concerns regarding the data accuracy used to compute the different models that tested the null hypothesis of species richness being independent of the selected variables. In our commentary, we focus mainly on the use of out‐of‐date checklists of gastropod and seaweed species from different archipelagos, but we also point out inaccuracies in some island age estimates and explain our disagreement with the use of the 200 m depth limit for the shallow‐water gastropods and seaweeds.     

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

Summary Interactions among environmental stresses, plant defensive characteristics, and plant nutrient status may significantly affect an alga's susceptibility to herbivores. Following desiccation, the palatable seaweed Gracilaria tikvahiae was less susceptible to grazing by the sea urchin Arbacia punctulata while the unpalatable alga Padina gymnospora became more susceptible. Increased grazing on desiccated Padina appeared to result from a loss of chemical defenses following desiccation. Palatable plants treated with organic extracts from desiccated Padina plants were consumed at more than twice the rate of plants treated with extracts from undesiccated plants. Increased susceptibility of Padina did not correlate with changes in protein content of the alga; reduced grazing on desiccated Gracilaria was associated with a decrease in protein content. When Padina was grazed by Arbacia or mechanically damaged to mimic urchin grazing, its susceptibility to Arbacia decreased within 1 to 5 days. These results demonstrate that history of physical or biological stress may affect a plant's susceptibility to herbivory. We hypothesize that urchins cue primarily on attractiveness features (e.g. nutrient content) of highpreference algae and deterrent features (e.g. chemical defenses) of low-preference algae. Stresses may therefore increase, decrease, or not affect a plant's susceptibility to herbivory depending upon the primary feeding cues used by the herbivore, the defensive mechanisms used by the plant, and the way these are altered by various environmental stresses.