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Author: Hay, Mark E. × End date: 1996 × Start date: 1995 × Has files: Yes × Type: Text ×

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Induction of seaweed chemical defenses by amphipod grazing

1996-12 , Cronin, Greg , Hay, Mark E.

Grazing by the generalist amphipod Ampithoe longimana induced increased concentrations of defensive secondary metabolites in the brown alga Dictyota menstrualis and made the seaweed less susceptible to further attack by the amphipod. Although A. longimana preferentially consumes D. menstrualis, its feeding rates can be reduced significantly by high concentrations of diterpenoid dictyols produced by the alga. In 1991, D. menstrualis from sites with high numbers of A. longimana had higher levels of grazing scars, higher concentrations of dictyols, and were less palatable to A. longimana than plants from sites with few amphipods. Among—site differences in palatability to amphipods did not correlate with plant differences in protein, nitrogen, or carbon content. Within a site, plants that had apparent amphipod grazing scars were significantly less palatable to A. longimana than neighboring undamaged plants. Controlled field experiments manipulating A. longimana densities supported the hypothesis that feeding by this amphipod induced elevated chemical defenses in the alga. Compared to undamaged control plants, amphipod—damaged plants had 19—34% more of three diterpenoid secondary metabolites and were 50% less palatable to amphipods. Soluble protein and thallus toughness were unaffected by amphipod grazing and thus could not have caused the differences in palatability. High—pressure liquid chromatography evaluation of adventitious branches growing from blade margins at sites of amphipod grazing scars showed that these branches had significantly elevated levels of two diterpenoids relative to normal blade apices or middles. Thus, the amphipod—induced resistance to further attack occurs through an increase in chemical defenses, and these defenses are, to some extent, localized within the plant thallus. Among—site differences in amphipod densities, grazing scars, seaweed defensive chemistry, and plant palatability that we documented in 1991 varied considerably during 1992 and 1993, suggesting that these interrelationships may be complex. In 1992, A. longimana densities did not differ between sites, and there were no between—site differences in palatability or concentrations of deterrent secondary metabolites. In 1993, however, A. longimana densities did differ between sites, but between—site differences were less dramatic than in 1991. Some secondary metabolites were slightly, but significantly, increased at the site with higher densities of A. longimana, but this had no effect on A. longimana feeding. It has been long recognized that marine herbivores are active participants in seaweed—herbivore interactions and can greatly influence the structure of benthic algal communities. Our findings suggest that seaweeds are not passive participants in these interactions, but can actively alter their susceptibility to herbivores in ecological time. Induced responses to herbivory help explain both spatial (i.e., within—thallus, within—site, and among—site) and temporal variation in the chemical defenses of D. menstrualis.

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Defensive synergisms?: Reply to Pennings

1996-09 , Hay, Mark E.

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Responses of temperate reef fishes to alterations in algal structure and species composition

1996-04-25 , Levin, Phillip S. , Hay, Mark E.

Reef habitats in the South Atlantic Bight comprise only a small proportion of the available benthic habitat, but support enormous densities of fishes, invertebrates, and algae. Because these reefs are distant from shore and in deeper waters (usually >20 m), we know very little about the factors affecting the distribution of fishes or the structure of fish assemblages on these reefs. Our goal in this study was to determine how variance in macroalgal structure and species composition on reefs off the coast of North Carolina affects reef fishes. Manipulations of algal density and species composition in artificially created 1.5 m2 plots indicated that: (1) patches with Sargassum filipendula had greater numbers and higher species richness of reef fishes than patches without algae; (2) fishes responded to variability in Sargassum density with high fish abundance in patches with higher algal density, but this response varied among fish species; (3) greater numbers of individuals and species of fish used Sargassum patches compared to patches of 2 other brown algae common in this region (Zonaria and Dictyopteris); (4) fish densities were higher in patches with taller algae and greater percent cover of algae. Many of the fishes using experimental patches were small, young-of-the-year individuals. Our results indicate that variability in the structure of seaweed beds on reefs in the South Atlantic Bight can influence the distribution and abundance of reef fishes. Although the algal beds in the South Atlantic Bight are more diminutive and less extensive than kelp beds of cold temperate waters, they may be a critical habitat for juvenile reef fishes in this region, and the algal species dominating the beds will have a large effect on their value as juvenile habitat.

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Distribution, density, and sequestration of host chemical defenses by the specialist nudibranch Tritonia hamnerorum found at high densities on the sea fan Gorgonia ventalina

1995-03-23 , Cronin, Greg , Hay, Mark E. , Fenical, William , Lindquist, Niels Lyle

The dendronotid nudibranch Tritonia hamnerorum was observed on some reefs in the Florida Keys, USA, at very high densities during the summer of 1992. T. hamnerorum specializes on the sea fan Gorgonia ventalina and sequesters the furano-germacrene julieannafuran from its host; this compound effectively protects the nudibranch from consumption by the common predatory reef fish Thalassoma bifasciatum. T. hamnerorum densities were extremely high at some locations, with as many as 1700 nudibranchs found on a single G. ventalina colony. At high densities, nudibranch feeding killed large areas on some sea fan colonies by stripping all tissue from portions of the sea fan and allowing filamentous algae and other epibionts to colonize. The density of T. hamnerorum on G. ventalina varied greatly on scales of centimeters, meters and kilometers. High density patches of nudibranchs on individual sea fans were usually composed of equivalent-sized nudibranchs. These observations suggest that pelagic veligers have an incredible capability to find and settle synchronously on one portion of a sea fan or that the larvae or juveniles hatch from egg masses and develop without leaving the sea fan. This study adds to a growing number of marine examples suggesting that feeding specialization occurs primarily among small, sedentary consumers that deter or escape predators by associating with defended hosts.

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Are tropical plants better defended? Palatability and defenses of temperate vs. tropical seaweeds

1996-12 , Bolser, Robin C. , Hay, Mark E.

Investigators working in both terrestrial and marine systems hypothesize that prey defenses are better developed in tropical than in temperate habitats because tropical consumers are more active and diverse. To investigate the hypothesis that chemical defenses are more potent in tropical than in temperate seaweeds, we conducted 38 feeding assays involving seven genera of algae. We offered tropical and temperate sea urchins the choice of a temperate alga from North Carolina or a closely related tropical alga from the Bahamas. Plants were freeze—dried, ground to a fine powder, embedded in agar, and presented in congeneric pairs (North Carolina species vs. Bahamian species), or same—species pairs, to the temperate urchin Arbacia punctulata and the tropical urchin Lytechinus variegatus. For the genera we tested (the red alga Amphiroa, the green alga Udotea, and the brown algae Dictyopteris, Dictyota, Lobophora, Padina, and Sargassum), we documented a general pattern in which the mean amount of North Carolina seaweeds eaten was approximately twice that of their Bahamian congeners. Dictyota was an exception to the overall pattern, in that mean consumption of temperate and tropical species was equivalent. In addition to showing a latitudinal difference in seaweed palatability, we found that some species exhibited significant, between—population variation in herbivore resistance on a local scale. The most notable example was Dictyota menstrualis from North Carolina. When collected from a shallow inshore site, this species was significantly more palatable than tropical species of Dictyota in four of six contrasts. When collected from a deeper offshore site, it was significantly less palatable than the tropical species in four of six contrasts. Thus, for this species, the between—population variance in palatability within a geographic region (North Carolina) bracketed the variance in palatability that we found among species and between regions. Feeding assays with both temperate and tropical urchins produced similar results, indicating that choices were made based on fundamental characteristics of the algae rather than on greater familiarity with local prey species. To see if chemical defenses could explain the differences in herbivore resistance that we noted using freeze—dried algae, we tested the effects of lipid—soluble and water—soluble extracts from each alga on urchin feeding by incorporating extracts into a standard palatable food. We also measured protein content and percent ash—free dry mass of the seaweeds in order to correlate urchin feeding preferences with these measures of food quality. Lipophilic chemical extracts explained urchin preferences in 9 of 15 (60%) North Carolina—Bahamas contrasts. Water—soluble extracts, protein content, and percent ash—free dry mass generally did not account for feeding patterns seen in the original assays. Our results support the general hypothesis that tropical seaweeds have stronger chemical defenses than temperate seaweeds. For some genera, however, smaller—scale local variability in herbivore resistance within a species can be as striking as overall latitudinal differences.

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Coral-seaweed-grazer-nutrient interactions on temperate reefs

1996-08 , Miller, Margaret W. , Hay, Mark E.

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. Thus, nutrients and herbivory may have acted synergistically to affect seaweeds, and hence corals, on this temperate reef. Different groups of algae (red vs. brown) experienced differential degrees of nutrient limitation and exerted differential competitive effects on corals at this site. Our findings show that competition with seaweeds plays a large role in excluding Oculina from well—lit temperate reefs, and support the hypothesis that seaweed competition may interact with latitudinal changes in physical parameters to limit coral recruitment, growth, and accumulation at high latitudes, thus suppressing the potential for reef development.

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Chemical defenses, protein content, and susceptibility to herbivory of diploid vs. hapliod stages of the isomorphic brown alga Dictyota ciliolata (Phaeophyta).

1996 , Cronin, Greg , Hay, Mark E.

Seaweeds with free-living diploid and haploid stages might express recessive traits during haploid stages, or exhibit other differences, that would allow those stages to differ in fitness under different environmental conditions. Heteromorphic seaweeds are well known to have variable ecological traits associated with their morphological differences, but ecological differences among isomorphic stages have rarely been investigated. The chemically defended brown alga Diclyola ciliolala has a life history with isomorphic alternation of generations, allowing us to assess how chemical traits and susceptibility to herbivory differ among stages. Herbivorous amphipods and sea urchins consumed similar amounts of diploid sporophytes and haploid female and male gametophytes. Concomitant with similar palatability, the different life stages had similar concentrations of soluble protein and levels of chemical defenses. Thus, in addition to morphological similarities, the life stages of D. cilio/(/ta appear to share these ecological similarities.

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Palatability and chemical defenses of marine invertebrate larvae

1996-11 , Lindquist, Niels Lyle , Hay, Mark E.

Risk of larval mortality is a critical component of models and debates concerning the ecology and evolution of the differing reproductive characteristics exhibited by marine invertebrates. In these discussions, predation often is assumed to be a major source of larval mortality. Despite limited empirical support, most marine larvae are thought to be palatable and broadly susceptible to generalist predators. Previous studies of larval—planktivore interactions have focused primarily on larvae that typically feed, grow, and develop for weeks to months in the plankton. Such planktotrophic species commonly produce large numbers of small larvae that disperse over vast distances. In contrast, the nonfeeding lecithotrophic larvae from sessile invertebrates that brood are often large and conspicuous, lack morphological defenses, and have limited dispersal distances because they typically are competent to settle minutes to hours after spawning. Interactions between lecithotrophic larvae and consumers are not well studied. This has limited the ability of previous authors to test broad generalities about marine larvae. We show that brooded larvae of Caribbean sponges (11 species) and gorgonians (three species) as well as brooded larvae of temperate hydroids (two species) and a bryozoan are unpalatable to co—occurring fishes. In contrast, brooded larvae of temperate ascidians (three species), a temperate sponge, and Caribbean hard corals (three species) are readily consumed by fishes, as are larvae from four of six species of synchronous broadcast—spawning gorgonians from the Florida Keys. Frequencies of survivorship for larvae attacked and rejected by fishes were high and statistically indistinguishable from frequencies for unattacked control larvae. Frequency of metamorphosis (when it occurred) of rejected larvae never differed significantly from that of unattacked control larvae. Assays testing for larval vs. adult chemical defenses for five species with distasteful larvae showed that larvae of all five species were chemically distasteful to fishes, whereas only three of five adult extracts deterred fish feeding. A comparison of larval palatability among chemically rich taxa showed that brooded larvae were significantly more likely to be unpalatable (86% of the species tested) than larvae of broadcasters (33%), and that palatable larvae were rarely released during the day (23%) while unpalatable larvae usually were (89%). Additionally, the frequency of bright coloration was high (60%) for unpalatable larvae and low (0%) for palatable larvae, suggesting that unpalatable larvae often may be aposematically colored. Results of this broad survey cast doubt on the widely accepted notion that virtually all marine larvae are suitable prey for most generalized planktivores. Among species that do not chemically or physically protect larvae against fishes, selection appears to favor the release of larvae at night, or the production of smaller more numerous offspring that grow and develop at sea as a way of escaping consumer—rich benthic habitats. Because distasteful larvae are not similarly constrained, distasteful species should exhibit reproductive and larval characteristics selected more by the fitness—related consequences of larval development mode and dispersal distance than by the necessity of avoiding benthic predators. Production of large larvae and retention of offspring in parental habitats that have proved to be suitable for growth and reproduction have both been proposed as advantageous, but these advantages often were assumed to be offset by losses due to increased larval apparency to fishes. This assumed trade—off is not mandatory because larvae can be defended chemically. Distasteful larvae tend to be conspicuous, localized dispersers that can co—occur with benthic fishes, and yet not be consumed.

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Susceptibility to herbivores depends on recent history of both the plant and animal

1996-07 , Cronin, Greg , Hay, Mark E.

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.

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Can small rare prey be chemically defended? the case for marine larvae

1995-06 , Lindquist, Niels Lyle , Hay, Mark E.

Marine larvae are consumed by a wide variety of generalist fishes and particle—feeding invertebrates, but larvae of any particular species probably constitute a small and variable of the diet for these consumers. Because virtually all consumers can ingest small quantities of noxious compounds with minimal detrimental effects, it is uncertain that toxic chemicals in larvae could be consumed in quantities sufficient to select for predator recognition and avoidance. Despite this, chemically defended larvae do occur. We show that, at low doses, secondary metabolites (the didemnins) from adults and larvae of the Caribbean tunicate Trididemnum solidum induced vomiting in fish, resulting in rapid learned aversion of the didemnin—defended food. The particle—feeding anemone Aiptasia pallida did not learn to avoid the chemically defended food. When anemones ingested the chemical equivalent of 15 larvae/d, representing <2% of the mass of their total daily diet, the didemnins in the "larvae" significantly reduced: (1) growth of adults by 82%, (2) combined growth of adults and daughter clones by 76%, (3) production of daughter clones by 44%, and (4) average mass of individual daughter clones by 41%. At higher water temperatures, anemones cloned more rapidly, but the negative effects of consuming didemnins also occurred more rapidly. Significant differences in the number of daughter clones produced between treatment and control anemones occurred after only 4 d at seawater temperatures of 27°—29°C vs. 32 d at seawater temperatures of 18°—21°C. Thus consumption of even very small quantities of secondary metabolites can decrease consumer fitness substantially and select for predators that recognize and avoid chemically defended larvae, as do many consumers that co—occur with Trididemnum solidum larvae. This is the first rigorous demonstration that consumption of marine secondary metabolites can decrease consumer fitness when ingested at ecologically realistic doses.

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