Chemical defense of common antarctic shallow-water nudibranchTritoniella belli eliot (Mollusca: Tritonidae) and its prey,Clavularia frankliniana rouel (Cnidaria: Octocorallia)

Extracts of the dorid nudibranchTritoniella belli and stoloniferan coralClavularia frankliniana were chromatographed and analyzed by¹H NMR and thin-layer chromatography. Three glycerol ethers were detected inT. belli, primarily 1-O-hexadecyl glycerol (chimyl alcohol). Chimyl alcohol was also detected after gradient flash chromatography and reverse-phase HPLC purification in the tissues ofC. frankliniana. The common omnivorous predatory Antarctic sea starOdontaster validus, a likely predator of benthic invertebrates, showed feeding deterrence to small cubes ofT. belli mantle tissue placed on the tube feet along the ambulacral feeding groove, while always extruding the cardiac stomach when presented with cubes of shrimp tissue of similar size. Filter-paper disks soaked in an aqueous shrimp solution and then dried were found to elicit a broad range of feeding behaviors inO. validus, including movement of the shrimp disk to the mouth, extrusion of the cardiac stomach, and the assumption of a humped feeding posture. Chimyl alcohol-treated shrimp disks caused significant feeding deterrence in sea stars when compared with control disks (solvent plus shrimp treated disks alone).T. belli andC. frankliniana appear to employ a defensive compound that has been found in a variety of temperate and tropical mollusks, where it has been demonstrated to deter fish predators. We provide evidence for further deterrent capabilities of chimyl alcohol and of its trophic relationship in the polar ecosystem of McMurdo Sound, Antarctica.     

Allelopathy as a factor in the success ofHelianthus mollis Lam.

A method to rapidly screen species suspected of producing allelochemics, using results from simple bioassay tests, is presented. By measuring the osmotic potential ofH. mollis extracts and using mannitol solutions of comparable osmotic potential, the influence of osmotic potential in the bioassay was eliminated. Nested analysis of variance was used to examine the separate influences of (1) extract concentration, (2) source of plants used in extract preparation (edge or center of clones) (3) osmotic potential of the extract, and (4) the differential development of radicles and shoots of species used in the bioassay tests. Bioassay tests for allelopathy showed that extracts made of wholeH. mollis plants significantly inhibited both radicle and shoot development of radish and wheat, but only the radicle of little bluestem. There was a significant increase in the inhibition of radish shoots and wheat radicles at high concentration of the extract, but the radicle of little bluestem was inhibited more at the lower concentration. Extracts prepared from plants collected from the clone center inhibited radish radicle development significantly more than extracts made of plants growing at the clone edge.     

Chemotactic tube-foot responses of a spongivorous sea starPerknaster fuscus to organic extracts from antarctic sponges

Hexane, chloroform, and methanol extracts of 18 species of antarctic sponges were tested for their ability to induce sustained tube-foot retraction in the antarctic spongivorous sea starPerknaster fuscus. Extracts were imbedded in silicone and used to coat the tip of a glass rod, which was allowed to contact an extended tube-foot. Retraction times were measured and compared with three controls: contact with a glass rod coated with a hexane extract of fish (feeding stimulant), contact with the glass rod alone (mechanical control), and contact with the glass rod coated with silicone alone (silicone control). Only extracts of the spongeMycale acerata did not elicit significantly longer tube-foot retraction times than controls for at least one of the three organic extracts. Hexane sponge extracts elicited the lowest levels of significant tube-foot responses, with only 39% of the sponge species tested showing activity in this fraction. In contrast, chloroform and methanol extracts elicited a significant tube-foot retraction response in 73% and 78% of the species tested, respectively. This indicates that in this assay repellent metabolites are generally more polar substances. It remains to be determined that secondary metabolites are responsible for all of the tube-foot retraction responses detected in sea stars exposed to sponge extracts; bioactive secondary metabolites have been isolated from a number of these antarctic sponges. It may be of ecological significance that the two rapidly growing sponges,Homaxinella balfourensis andMycale acerata, were either not repellent or had low repellency, and thatM. acerata is the primary dietary item ofPerknaster fuscus.     

Economics of chemical defense in chrysomelinae

Chemical defense in chrysomelid larvae (subtribe Chrysomelina and Phyllodectina) is reviewed. Most species secrete autogenous monoterpenes. The diversity of their secretion is interpreted as a mechanism to reduce adaptation by predacious arthropods. The consequences of a host plant shift to the Salicacae are explored. Salicin from these host plants is used as a precursor for the salicylaldehyde secreted by the larvae of many species. This offers several advantages. It provides the larvae with an inexpensive and efficient defense. The recovery of the glucose moiety of the salicin contributes significantly to the larval energy budget. Adults sequester salicin in the eggs at concentrations which are toxic to ants. Owing to this maternal provisioning, neonate larvae produce salicylaldehyde from hatching onwards, whereas other species secreting monoterpenes are not protected at hatching. The secretion of salicylaldehyde by different species is considered to be chemical mimicry reinforcing visual aposematic signals.     

A neolignoid feeding deterrent againstLuehdorfia puziloi larvae (lepidoptera: Papilionidae) fromHeterotropa aspera,a host plant of sibling species,L. japonica

A feeding deterrent against larvae of a papilionid butterfly,Luehdorfia puziloi (Parnassiinae), a specialist onAsiasarum plants (Aristolochiaceae), was isolated from another aristolochiaceous plant,Heterotropa aspera. Antifeedant activity was exhibited by then-hexane-soluble fraction that proved to contain at least two active components. One of the deterrents was identified as a neolignan compound, asatone. The concentration of asatone inH. aspera was estimated at approximately 225 ppm, and larval feeding ofL. puziloi was significantly deterred at concentrations over 90 ppm. By contrast, asatone was not detectable (< 1 ppm, if any) in its host plant,Asiasarum sieboldii.     

Genetic component of variation in chemical defense ofOreina gloriosa (Coleoptera: Chrysomelidae)

Defensive secretions of adultOreina gloriosa, liberated at the surface of the pronotum and elytra, contain a complex mixture of cardenolides, and ethanolamine. Proportions and concentrations of constituents determined by reverse-phase HPLC show considerable variation among individual beetles. Heritabilities of proportions of five main components were estimated by mother-offspring regression providing a validation of the less reliable full-sib correlation estimates. Average heritabilities based on the two methods were 0.51 and 0.58, respectively, estimated by using offspring of two age groups. Regression estimates of 2- and 10-week-old offspring differed significantly for one secretion constituent (RT16). Heritability estimates of concentrations of 16 secretion components were calculated by full-sib correlation analysis. Average heritability was 0.45, indicating a significant genetic component. Estimates did not differ significantly between the two age groups. We also estimated heritabilities of concentrations by a two-way model including data from offspring of both age groups. Heritability estimates based on this model are thought to correspond approximately to estimates based on samples from natural populations. The average of these estimates was lower (h ² =0.31) than the average heritability of each age group separately (h ² =0.45), suggesting a developmental effect on variation in chemical defense ofO. gloriosa.     

Physiological sources of variation in chemical defense ofOreina gloriosa (Coleoptera: Chrysomelidae)

The defensive secretion of the alpine chrysomelidOreina gloriosa is a complex mixture of mainly cardenolides and tyrosine betaine. Individually sampled secretions of adult laboratory-reared and field-collected beetles were analyzed by reverse-phase HPLC; 16 secretion components were quantified. Quantities and concentrations of different components were significantly affected by the age, sex, and reproductive status of individual beetles. Aging was correlated with marked increases (up to 4.4-fold) and decreases (up to 2.7-fold) of quantities and concentrations of several components. Differences between the sexes were smaller, but quantities of all components and concentrations of several components were larger in laboratory-reared females than in males. There was less of one component of the secretion in mated than unmated females, but the concentrations of four secretion components were higher (up to 1.6-fold) in mated females.     

Evidence for chemical defense in tropical green algaCaulerpa ashmeadii (Caulerpaceae: Chlorophyta): Isolation of new bioactive sesquiterpenoids

Results of field feeding preference studies with 12 species of tropical green algae of the genusCaulerpa showed thatC. ashmeadii was preferred least by herbivorous fishes. Chemical investigations ofC. ashmeadii demonstrated the presence of high concentrations of sesquiterpenoid metabolites. The chemical isolation and structural elucidation of five majorC. ashmeadii metabolites, as well as the results of field feeding preference, antimicrobial, and ichthyotoxicity assays demonstrating the biological activities of these metabolites are reported here.     

Soldier frontal glands of the termiteMacrotermes subhyalinus: Morphology,chemical composition,and use in defense

Soldiers of the East African fungus-growing termiteMacrotermes subhyalinus (Rambur) (Termitidae: Macrotermitinae) employ both mechanical and chemical defenses. Soldiers release a chemical secretion composed of long-chain saturated and monounsaturated hydrocarbons into wounds inflicted by their powerful mandibles. Chemical analysis of the secretion shows the paraffin fraction to consist primarily ofn-tricosane,n-pentacosane, 3- and 5-methylpentacosane, and 5-methylheptacosane. The major oleflns were identified as (Z)-9-heptacosene and (Z)-9-nonacosene. The secretion originates from the frontal glands of both major and minor soldiers; however, the hypertrophied gland of the major soldiers contains 500-fold more secretion than that of the minor soldiers. This secretion appears to impair the healing of wounds in test ants, and thus could represent a valuable supplement to the mechanical defense mechanism.     

Distribution of birch (Betula SPP.), willow (Salix SPP.), and poplar (Populus SPP.) secondary metabolites and their potential role as chemical defense against herbivores

Isoprenoids and phenolics, major metabolites of important browse species, are reviewed in regard to concentrations, distribution within tissues, and between species. Seasonal variation of specific substances and changes with age of the plant are also considered. The distribution of substances may affect food selection and feeding behavior of animals. Wild mammalian herbivores tend to avoid plant parts rich in these substances, in spite of high nutritional content of the plant tissue. Possible mechanisms for defense by plants against depredation by mammalian herbivores are discussed within the framework of the plants' biochemistry.     

Ecdysteroids fromPycnogonum litorale (Arthropoda,Pantopoda) act as chemical defense againstCarcinus maenas (Crustacea,Decapoda)

Pycnogonum litorale (Ström) is unpalatable to the common shore crabCarcinus maenas, a generalist predator in the pycnogonid's habitat. A feeding bioassay reveals that the crabs are deterred by ecdysteroids that occur in high levels in all developmental stages ofP. litorale. The total ecdysteroids in the pycnogonids reach 5.9×10^–4 M. The 20-hydroxyecdysone 22-acetate (20E22A), which is the predominant ecdysteroid in the pycnogonids, and 20-hydroxyecdysone (20E), the arthropod molting hormone, were tested for their antifeedant effect onC. maenas. When contained in food pellets in homogeneous concentrations, 20E and 20E22A significantly reduced food consumption at 1.25×10^–4 and 5.0×10^–4 molar levels, respectively. The present results demonstrate for the first time chemical defense in arthropods in a marine predator-prey relationship. Furthermore, they provide evidence that ES contained in one animal can act as feeding deterrents on another animal.     

Extreme intraspecific chemical variability in soldier defense secretions of allopatric and sympatric colonies ofLongipeditermes longipes

The chemical defense secretions of major and minor soldiers of over 18 colonies of the primitive glue-squirting nasute termiteLongipeditermes longipes (Isoptera, Termitidae, Nasutitermitinae) were analyzed chormatographically. The colonies, collected from four rainforest sites in peninsular Malaysia, showed monoterpene patterns rich in pinenes and limonene but with few quantitative differences between colonies. In marked contrast, the diterpene chemistry is high variable, and includes tricyclic (trinervitane), tetracyclic (rippertane), bicyclic (secotrinervitane), and a new spirotetracyclic (longipane) skeleton. Three new natural products are included in this remarkable and unprecedented example of structural diversity among different colonies of a single species.Fellow of the Alfred P. Sloan Foundation (1981–85) and Camille and Henry Dreyfus Teacher-Scholar (1981–86).     

Mate choice and toxicity in two species of leaf beetles with different types of chemical defense

Evidence for the use of defensive compounds for sexual purposes is scarce, even though sexual selection might have some importance for the evolution of defensive traits. This study investigates the effect of defense-related traits and body size on mating success in two sister species of leaf beetle differing in their type of chemical defense. Oreina gloriosa produces autogenous cardenolides, whereas O. cacaliae sequesters pyrrolizidine alkaloids from its food plant. Larger O. gloriosa males with more toxin or higher toxin concentration had a mating advantage, likely due to direct or indirect female choice. In the laboratory, particular pairings recurred repeatedly in this species, indicating mate fidelity. O. gloriosa females were also subject to sexual selection, possibly by male choice, because larger females and those with higher toxin concentration mated more readily and more often. In O. cacaliae, in contrast, sexual selection for toxicity and body size was not detected, or at best was much weaker. Because toxicity is heritable in O. gloriosa but environment-dependent in O. cacaliae, individuals of the former species could be choosing well-defended partners with good genes. Our study suggests that sexual selection may contribute to the maintenance of heritable defensive traits.     

Synthesis of pungenin,a foliar constituent of some spruce species,and investigation of its efficacy as a feeding deterrent for spruce budworm [Choristoneura fumiferana (Clem.)]

Pungenin was synthesized from 3,4-dihydroxyacetophenone by a short sequence involving manipulation of protecting groups on the 3 and 4 hydroxyl functions. Bioassays indicated that the glucoside is a modest feeding deterrent for sixth-instar spruce budworm larvae, but it does not appear to retard the development of small larvae or lead to increased mortality.     

Seasonal and spatial variations of alkaloids in Merendera montana in relation to chemical defense and phenology

Merendera montana (Liliaceae) is an endemic plant broadly, but exclusively, occurring in the Iberian Peninsula where it is one of the most abundant species in rangelands grazed by sheep and on the routes used by migrating sheep herds. In spite of showing a peculiar phenology adapted to Mediterranean regions (leaves remain green from fall to spring and are absent in summer), this species has also successfully colonized high altitude environments (up to 2300 m), and abundant populations can be found in montane and subalpine pastures. Chemical analysis has shown a high forage quality as well as the pre- sence of tropolone alkaloids (similar to other species of the genus Merendera), mainly colchicine, 3-demethylcolchicine, and colchicoside. In general terms, the alkaloid content was significantly lower in the subterranean corms than in the leaves. No remarkable seasonal variations in total alkaloids was found, although an increase of colchicoside in the corms and a decrease of colchicine was noted in summer. Both chemical deterrents and phenology could be the cause of the widespread ecological adaptation of Merendera montana. The alkaloid content would prevent intake by wild herbivores during winter, while the absence of aerial organs in summer, coinciding exactly with the highest domestic herbivore pressure, would permit this species to colonize rich soil habitats where there is low plant competition.     

Effects of diet breadth on autogenous chemical defense of a generalist grasshopper

The lubber grasshopper,Romalea guttata, produces a metathoracic defensive secretion containing primarily phenolics and quinones. This insect feeds on a wide range of plant species. Insects reared on an artificial diet and a diet of onion,Allium canadense, had secretions that contained fewer compounds, lower concentrations of compounds, and markedly altered relative composition of components compared to insects reared on a varied diet of 26 plant species that included onion. The study demonstrates that diet breadth has a major effect on the quality and quantity of the autogenous defensive secretion of this generalist herbivore. The results are compared to diet effects known in chemically defended specialists. Two possible mechanisms explaining the effects of diet breadth are proposed: one involves changes in precursor availability with changing diet breadth; the other suggests that physiological stress due to diet restriction changes allocation of resources to chemical defense.     

Robber bees (Lestrimelitta limao) and their host chemical and visual cues in nest defense byTrigona (Tetragonisca) angustula (Apidae: Meliponinae)

The nest of the stingless bee,Trigona (Tetragonisca) angustula, is guarded by bees positioned in the nest entrance and others hovering in front of it. Hovering guard bees track returning foragers sideways along the last 10 cm in front of the nest, but intercept and incapacitate nest intruders by clinging with mandibles to wings and legs. When attacked by the cleptobiotic stingless beeLestrimelitta limao, the colony strengthens its aerial defense with hundreds of additional hoverers. To test our hypothesis that this reaction is due to interspecific chemical communication based on kairomone effects, we presented synthetic cephalic volatiles of both species at the nest entrance and counted the number of bees leaving the nest and taking up hovering positions. We conclude that guard bees recognizeL. limao by the major terpenoids of their volatile cephalic secretions, geranial, neral (=citral) and 6-methyl-5-hepten-2-one; other components may fine-tune this recognition. The effect of chemical stimuli is not significantly enhanced by combination with a dummy ofL. limao. Guard bees, we hypothesize, respond to this kairomone by secreting a species specific alarm pheromone; a major component of this pheromone, benzaldehyde, recruits additional bees to defend the nest.     

Potential role of ascorbate oxidase as a plant defense protein against insect herbivory

Ascorbic acid is essential for both nutritive and antioxidant functions in phytophagous insects; however, maintaining sufficient quantities of reduced ascorbate may be problematical for them. In this investigation, we show that the plant enzyme ascorbate oxidase retains activity in the digestive system of the herbivoreHelicoverpa zea. High levels of the enzyme are present in several host plants ofH. zea, including cotton, tomato, soybean, crimson clover, and vetch. The enzyme oxidizesL-ascorbic acid to dehydro-L-ascorbic acid, a potentially toxic product. The oxidation of ascorbic acid also produces active oxygen species such as the highly reactive hydroxyl radical. The nutritional quality of protein for larvalH. zea was significantly reduced by treatment with ascorbate and ascorbate oxidase. Oxidative damage to the protein was indicated by decreased lysine content, increased carbonyl formation, and the occurrence of protein fragmentation and polymerization. Furthermore, the oxidative loss of ascorbate in the herbivore's digestive system prevents ascorbate from functioning as an important antioxidant against a plethora of dietary prooxidants.     

Role of secondary metabolites in feeding associations between a predatory nudibranch,two grazing nudibranchs,and a bryozoan

The carnivorous nudibranchRoboastra tigris preys preferentially upon two nudibranchs,Tambja abdere andT. eliora, that in turn feed upon the bryozoanSessibugula translucens. All four organisms contain tambjamines A–D (I–IV) that were shown to be fish feeding inhibitors. When attacked byRoboastra, T. abdere secretes a distasteful mucus containing a total of 3 mg of the tambjamines that sometimes causes theRoboastra to break off the attack. Under similar circumstancesT. eliora attempts to swim away; it presumably contains insufficient of the tambjamines to deterRoboastra. Roboastra follows the slime trail of nudibranchs using contact chemoreception and reverses direction when the trail is broken. The slime trail ofT. abdere contains low concentrations of the tambjamines. In Y-maze experiments,T. eliora was attracted towards seawater containingS. translucens and seawater containing 10^–10 M tambjamines A and B (11) but was repelled by seawater containing > 10^–8 M tambjamines A and B. At higher concentrations the mixture of tambjamines may be recognized as an alarm pheromone.     

A complex,cross-taxon,chemical releaser of antipredator behavior in amphibians

Prey species show diverse antipredator responses to chemical cues signaling predation threat. Among terrestrial vertebrates, the red-backed salamander, Plethodon cinereus, is an important species in the study of these chemical defenses. During the day and early evening, this species avoids rinses from garter snakes, Thamnophis sirtalis, independent of snake diet, but late at night, avoids only those rinses from garter snakes that have recently eaten P. cinereus. We tested whether the selective, late-night response requires the ingestion or injury of salamanders. In three experiments, we tested P. cinereus for their responses to separate or combined rinses from salamanders (undisturbed, distressed, and injured P. cinereus) and snakes (unfed, earthworm fed, and salamander-fed T. sirtalis). When paired against a water control, only rinses from salamander-fed snakes were avoided. When salamander treatments (undisturbed or distressed) were combined with the snake treatments (unfed or earthworm-fed) and tested against a water control, the combinations elicited avoidance. When selected treatments were paired against the standard rinse from salamander-fed snakes, only the combined rinses from salamanders and snakes nullified the avoidance response to the standard rinse. These data reveal a prey defense mechanism involving chemical elements from both the predator and prey that does not require injury or ingestion of the prey in the formation of the cue.