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.     

Chemotaxis and chemical defenses in seaweed susceptibility to herbivory

Recent studies have show that small marine herbivores with limited mobility (mesograzers) often feed on macroalgae chemically defended against fishes or sea-urchins. In order to verify the involved mechanisms of chemotaxis or chemical defense into this process in Brazilian littoral, two species of brown alga Dictyota menstrualis and Dictyota mertensii were studied against the limited mobility herbivores, the amphipod Parhyale hawaiensis and the crab Pachygrapsus transversus. These two species were studied in order to verify the action of their crude extracts in the defense and chemotaxis processes related to limited mobility of these herbivores. Feeding preference assays revealed that P. hawaiensis do not eaten these Dictyota species. P. transversus do not eaten D. menstrualis either, but consumed large amounts of D. mertensii. Chemical deterrent assays showed that extracts of these species act as feeding deterrent to both species of herbivores. In addition, chemotaxis assays demonstrated that both herbivores are significantly negative chemotactic probably due to the presence of complementary metabolites into artificial foods. Considering that both Dictyota species exhibit active extracts against these small herbivores, we suppose that the non-occurrence of these herbivore species in close relationship with the seaweeds D. menstrualis and D. mertensii may explain the defense action of both extracts related to these mesograzers.     

Large mobile versus small sedentary herbivores and their resistance to seaweed chemical defenses

Summary Small, relatively sedentary herbivores like amphipods and polychaetes (mesograzers) often live on the plants they consume and should therefore view plants as both foods and living sites. Large, relatively mobile herbivores like fishes commonly move among, and feed from, many plants; they should view plants primarily as foods and rarely as potential living sites. In marine communities, fishes that consume plants are also important predators on mesograzers. Since seaweeds avoided by fishes should represent safer living sites for small herbivores, mesograzers living on and consuming seaweeds that are not eaten by fishes should have higher fitness than mesograzers living on plants preferred by fishes. In previous work, we demonstrated that seaweed secondary metabolites that deterred feeding by a fish and sea urchin had no effect on feeding by a common amphipod (Hay et al. 1987a). We then hypothesized that mesograzers would, in general, be less affected by seaweed chemical defenses than larger, more mobile herbivores like fishes. In this investigation, we evaluate the generality of this hypothesis by comparing the feeding of an omnivorous fish (Lagodon rhomboides) with that of an omnivorous, tube-building polychaete (Platynereis dumerilii) to see if the mesograzer prefers seaweeds avoided by the fish and if it is less affected by seaweed chemical defense. Platynereis dumerilii fed almost exclusively on Dictyota dichotoma, the seaweed eaten least by Lagodon rhomboides. The diterpene alcohols (dictyol-E and pachydictyol-A) produced by Dictyota significantly deterred feeding by Lagodon but did not affect, or at one concentration stimulated, feeding by Platynereis. Our data support the hypothesis that small, relatively sedentary herbivores that live on plants are more resistant to chemical defenses than are large, relatively mobile herbivores that move among many plants.     

Intraspecific variation in palatability and defensive chemistry of brown seaweeds: effects on herbivore fitness

When offered a choice between brown seaweeds (Phaeophyta) from shallow inshore populations versus deeper offshore populations along the mid-Atlantic coast of the United States of America, the herbivorous amphipod Ampithoe longimana consistently preferred plants from the inshore populations. This was the case for three species (Dictyota menstrualis, Spatoglossum schroederi, and Sargassum filipendula) collected from each of a single inshore and offshore site, and for one species (D. menstrualis) collected from each of three inshore and three offshore sites. Bioassay-guided fractionation of chemical crude extracts from D. menstrualis suggested that the relative unpalatability of the offshore plants was due to the lipid-soluble secondary metabolites 4beta-hydroxydictyodial A and 18, O-dihydro-4beta-hydroxydictyodial A 18-acetate, along with minor compounds that were not fully identified. The inshore-offshore pattern did not appear to result from induction of defenses due to herbivory by mesograzers, as mesograzer densities were higher on the more palatable inshore plants. Herbivore feeding preferences for inshore versus offshore seaweeds matched the effects of those seaweeds on their fitness. When juvenile amphipods were raised on inshore versus offshore tissues of D. menstrualis, amphipod survivorship, growth, and ovulation were significantly suppressed on the offshore compared to the inshore tissues. Few previous investigations have studied intraspecific variance in seaweed palatability. We extend these by showing that between-population differences in palatability can persist for several years and by demonstrating that this variance is chemically based and has dramatic effects on herbivore fitness.     

Phylogenetic and geographic variation in host breadth and composition by herbivorous amphipods in the family Ampithoidae

Predicting the host range for herbivores has been a major aim of research into plant-herbivore interactions and an important model system for understanding the evolution of feeding specialization. Among many terrestrial insects, host range is strongly affected by herbivore phylogeny and long historical associations between particular herbivore and plant taxa. For small herbivores in marine environments, it is known that the evolution of host use is sculpted by several ecological factors (e.g., food quality, value as a refuge from predators, and abiotic forces), but the potential for phylogenetic constraints on host use remains largely unexplored. Here, we analyze reports of host use of herbivorous amphipods from the family Ampithoidae (102 amphipod species from 12 genera) to test the hypotheses that host breadth and composition vary among herbivore lineages, and to quantify the extent to which nonpolar secondary metabolites mediate these patterns. The family as a whole, and most individual species, are found on a wide variety of macroalgae and seagrasses. Despite this polyphagous host use, amphipod genera consistently differed in host range and composition. As an example, the genus Peramphithoe rarely use available macrophytes in the order Dictyotales (e.g., Dictyota) and as a consequence, display a more restricted host range than do other genera (e.g., Ampithoe, Cymadusa, or Exampithoe). The strong phylogenetic effect on host use was independent of the uneven distribution of host taxa among geographic regions. Algae that produced nonpolar secondary metabolites were colonized by higher numbers of amphipod species relative to chemically poor genera, consistent with the notion that secondary metabolites do not provide algae an escape from amphipod herbivory. In contrast to patterns described for some groups of phytophagous insects, marine amphipods that use chemically rich algae tended to have broader, not narrower, host ranges. This result suggests that an evolutionary advantage to metabolite tolerance in marine amphipods may be that it increases the availability of appropriate algal hosts (i.e., enlarges the resource base).     

Host-plant adaptation in an herbivorous marine amphipod: genetic potential not realized in field populations

Evolutionary responses of herbivores to their host plants depend not only on selection from plants, but also on the genetic basis of traits relating to host use. The genetic basis of such traits has been investigated extensively among terrestrial insect herbivores, but has received almost no attention among marine herbivores. We tested whether performance traits in the herbivorous marine amphipod Peramphithoe parmerong display heritable variation and, for the first time for a marine herbivore, whether selection has resulted in local adaptation to host plants on two spatial scales. Peramphithoe parmerong displayed heritable genetic variation for survival on two host macroalgae, the high-quality Sargassum linearifolium and the poor-quality Padina crassa, and for growth on S. linearifolium. Differences in performance on different hosts thus have the potential to select for differential use of hosts by this amphipod. Despite this potential, there was no evidence among field populations of local adaptation to host algae on either scale tested: between hosts within a site or among sites differing in algal species composition. Within a site, amphipods were not more likely to prefer or perform better on the host on which they were collected. Similarly, amphipods collected from sites in which P. crassa was present were not more likely to perform well on this host than amphipods collected from sites where this alga was not found. Ecological factors that may explain the persistence of P. parmerong on P. crassa and the possibility of phylogenetic constraints on host use by P. parmerong are discussed.     

Mesoherbivores reduce net growth and induce chemical resistance in natural seaweed populations

Herbivory on marine macroalgae (seaweeds) in temperate areas is often dominated by relatively small gastropods and crustaceans (mesoherbivores). The effects of these herbivores on the performance of adult seaweeds have so far been almost exclusively investigated under artificial laboratory conditions. Furthermore, several recent laboratory studies with mesoherbivores indicate that inducible chemical resistance may be as common in seaweeds as in vascular plants. However, in order to further explore and test the possible ecological significance of induced chemical resistance in temperate seaweeds, data are needed that address this issue in natural populations. We investigated the effect of grazing by littorinid herbivorous snails (Littorina spp.) on the individual net growth of the brown seaweed Ascophyllum nodosum in natural field populations. Furthermore, the capacity for induced resistance in the seaweeds was assessed by removing herbivores and assaying for relaxation of defences. We found that ambient densities of gastropod herbivores significantly reduced net growth by 45% in natural field populations of A. nodosum. Seaweeds previously exposed to grazing in the field were less consumed by gastropod herbivores in feeding bioassays. Furthermore, the concentration of phlorotannins (polyphenolics), which have been shown to deter gastropod herbivores, was higher in the seaweeds that were exposed to gastropod herbivores in the field. This field study corroborates earlier laboratory experiments and demonstrates that it is important to make sure that the lack of experimental field data on marine mesoherbivory does not lead to rash conclusions about the lack of significant effects of these herbivores on seaweed performance. The results strongly suggest that gastropods exert a significant selection pressure on the evolution of defensive traits in the seaweeds, and that brown seaweeds can respond to attacks by natural densities of these herbivores through increased chemical resistance to further grazing.     

Within-plant variation in seaweed palatability and chemical defenses: optimal defense theory versus the growth-differentiation balance hypothesis

Within-plant variation in the concentration of secondary metabolites, nutritive value, toughness, and susceptibility to herbivory was assessed for the brown alga Dictyota ciliolata. When young apices and older tissue from the same plant were offered in equal abundance to the herbivorous amphipod Ampithoe longimana and the sea urchin Arbacia punctulata, young apices were consumed about 2 times more than older tissue. Compared to young apices, the less preferred older tissue had a less palatable lipophilic extract, significantly higher concentrations of two secondary metabolites (another secondary metabolite did not differ significantly), 33% more soluble protein, and was 233% tougher. Higher levels of chemical defenses in older tissues, and not tissue toughness or nutritive value, appear to be responsible for the preference of Ampithoe longimana for young apices. The pattern of lower levels of chemical defenses in young than older tissues of D. ciliolata is the opposite of the pattern observed in coenocytic seaweeds and most vascular terrestrial and marine plants, all of which have translocation systems for moving materials among plant portions. Unlike these other plants, which preferentially allocate chemical defenses to young tissues, D. ciliolata cannot readily translocate secondary metabolites. The growth-differentiation balance hypothesis suggests that actively dividing and expanding cells are less able to produce secondary metabolites. This hypothesis may help explain why older tissues are better defended than young, rapidly growing apices.     

Marine-terrestrial contrasts in the ecology of plant chemical defenses against herbivores

Small marine herbivores that live on the plants they consume often selectively eat seaweeds that are chemically defended from fishes. Their feeding is unaffected or stimulated by the plant metabolites that deter fishes, and these small herbivores dramatically reduce their susceptibility to predation by associating with host plants that are noxious to fishes. Ecological similarities between these small marine herbivores and numerous terrestrial insects suggest that herbivorous insects also may have evolved a preference for toxic plants because this diminishes their losses to predators, parasites and pathogens. Although marine and terrestrial plants and herbivores evolved in strikingly different environments, the ease of experimentation in some marine systems makes them ideal for addressing certain questions of fundamental importance to both terrestrial and marine workers.     

Chemical defense of brown algae (Dictyopteris spp.) against the herbivorous amphipod Ampithoe longimana

Terpenoids, polyphenols, and C₁₁ metabolites are broadly distributed among brown seaweeds. Terpenoids and polyphenols have often been investigated as chemical defenses against herbivores, while there are only few investigations of the fatty-acid-derived C₁₁ hydrocarbons and C₁₁ sulfur compounds as potential defenses. We investigated effects of C₁₁ sulfur metabolites from the cosmopolitan brown alga Dictyopteris membranacea on feeding and fitness of the herbivorous amphipod Ampithoe longimana. In choice tests between freshly collected thalli of D. hoytii (which lacks C₁₁ sulfur compounds) and D. membranacea (which contains C₁₁ sulfur compounds) amphipods consumed about 4 times more of the species lacking the C₁₁ sulfur compounds. The same feeding preference was observed when these plants were finely ground and embedded in an agar matrix to destroy morphological differences. When a diet made from field-collected thalli of D. membranacea containing C₁₁ sulfur compounds was tested against a diet made from a laboratory culture of D. membranacea that had lost the ability to produce C₁₁ sulfur compounds, the same magnitude of preference was observed for the population lacking the sulfur compounds. In addition to the C₁₁ sulfur compounds, a water-soluble C9-oxo acid that appears to be a by-product in the biosynthesis of the C₁₁ metabolites also suppressed amphipod feeding to a comparable extent. Both classes of compound may contribute to the effective chemical protection of D. membranacea. When juvenile amphipods were reared for 28 days on artificial diets containing the above compounds, their survivorship (⢪%) closely resembled that of a starved treatment, but differed dramatically from a control treatment (60%) consisting of the same food, but without the metabolites. Most other classes of brown algal secondary metabolites are defensive against a broad spectrum of larger herbivores, but relatively ineffective against the amphipod studied here. In contrast, the fatty-acid-derived sulfur compounds and the C9-oxo acid strongly deter Ampithoe-like mesograzers but appear less effective against other herbivores, suggesting that these metabolites could be ecologically important in defending zygotes and germlings against these small consumers.     

Biogeography of sponge chemical ecology: comparisons of tropical and temperate defenses

Examples from both marine and terrestrial systems have supported the hypothesis that predation is higher in tropical than in temperate habitats and that, as a consequence, tropical species have evolved more effective defenses to deter predators. Although this hypothesis was first proposed for marine sponges over 25 years ago, our study provides the first experimental test of latitudinal differences in the effectiveness of sponge chemical defenses. We collected 20 common sponge species belonging to 14 genera from tropical Guam and temperate Northeast Spanish coasts (Indo-Pacific and Mediterranean biogeographic areas) and conducted field-based feeding experiments with large and small fish predators in both geographic areas. We use the term global deterrence to describe the deterrent activity of a sponge extract against all of the predators used in our experiments and to test the hypothesis that sponges from Guam are chemically better defended than their Mediterranean counterparts. Sympatric and allopatric deterrence refer to the average deterrent activity of a sponge against sympatric or allopatric predators. All of the sponges investigated in this study showed deterrent properties against some predators. However, 35% of the sponge species were deterrent in at least one but not in all the experiments, supporting the idea that predators can respond to chemical defenses in a species-specific manner. Tropical and temperate sponges have comparable global, sympatric, and allopatric deterrence, suggesting not only that chemical defenses from tropical and temperate sponges are equally strong but also that they are equally effective against sympatric and allopatric predators. Rather than supporting geographic trends in the production of chemical defenses, our data suggest a recurrent selection for chemical defenses in sponges as a general life-history strategy.     

Compensatory growth of the kelp Macrocystis integrifolia (Phaeophyceae, Laminariales) against grazing of Peramphithoe femorata (Amphipoda, Ampithoidae) in northern-central Chile

Compensation of tissue loss has been considered an alternative strategy for seaweeds that have no or only minor chemical or structural defense against herbivory. Compensatory responses are facilitated by resource transfer among different tissues and have been suggested for large kelps. Macrocystis integrifolia (Bory) is a common kelp species from northern-central Chile, which is characterized by high growth rates and the absence of lipophilic chemical defenses against herbivore grazing. Herein, we used the giant kelp M. integrifolia to test for compensatory growth in response to grazing by the nest-dwelling amphipod Peramphithoe femorata (Krøyer). Amphipods were allowed to graze inside nests on subapical blades of M. integrifolia sporophytes for 14 days. We measured growth and chemical composition (C, N, laminaran and mannitol) of apical and subapical blades of grazed and ungrazed (control) sporophytes. Our results revealed the capability of M. integrifolia to maintain elongation rates in grazed subapical blades, which were similar to those of subapical blades from ungrazed sporophytes. Apical blades grew slower in grazed than in ungrazed sporophytes indicating a trade-off between apical and subapical blades when herbivores were present. Thus, compensation occurs in blades directly attacked by grazers and is probably mediated by vertical resource allocation within sporophytes to subapical blades, a suggestion supported by the fact that stipe internodes in these regions grew more on grazed sporophytes. In general, our study indicates that M. integrifolia exhibits compensatory growth against the herbivore amphipod P. femorata, and we suggest that this could be an important strategy of large kelp species to tolerate moderate grazing intensities.     

Overview of the chemical ecology of benthic marine invertebrates along the western Antarctic peninsula

Thirteen years ago in a review that appeared in the American Zoologist, we presented the first survey of the chemical and ecological bioactivity of Antarctic shallow-water marine invertebrates. In essence, we reported that despite theoretical predictions to the contrary the incidence of chemical defenses among sessile and sluggish Antarctic marine invertebrates was widespread. Since that time we and others have significantly expanded upon the base of knowledge of Antarctic marine invertebrates' chemical ecology, both from the perspective of examining marine invertebrates in new, distinct geographic provinces, as well as broadening the evaluation of the ecological significance of secondary metabolites. Importantly, many of these studies have been framed within established theoretical constructs, particularly the Optimal Defense Theory. In the present article, we review the current knowledge of chemical ecology of benthic marine invertebrates comprising communities along the Western Antarctic Peninsula (WAP), a region of Antarctica that is both physically and biologically distinct from the rest of the continent. Our overview indicates that, similar to other regions of Antarctica, anti-predator chemical defenses are widespread among species occurring along the WAP. In some groups, such as the sponges, the incidence of chemical defenses against predation is comparable to, or even slightly higher than, that found in tropical marine systems. While there is substantial knowledge of the chemical defenses of benthic marine invertebrates against predators, much less is known about chemical anti-foulants. The sole survey conducted to date suggests that secondary metabolites in benthic sponges are likely to be important in the prevention of fouling by benthic diatoms, yet generally lack activity against marine bacteria. Our understanding of the sensory ecology of Antarctic benthic marine invertebrates, despite its great potential, remains in its infancy. For example, along the WAP, community-level non-consumptive effects occur when amphipods chemically sense fish predators and respond by seeking refuge in chemically-defended macroalgae. Such interactions may be important in releasing amphipods from predation pressure and facilitating their unusually high abundances along the WAP. Moreover, recent studies on the sensory biology of the Antarctic keystone sea star Odontaster validus indicate that chemotactile-mediated interactions between conspecifics and other sympatric predatory sea stars may have significant ramifications in structuring community dynamics. Finally, from a global environmental perspective, understanding how chemical ecology structures marine benthic communities along the WAP must increasingly be viewed in the context of the dramatic impacts of rapid climatic change now occurring in this biogeographic region.     

Detritus and Epibiota on Artificial Substrata: The Possible Role of Food in Structuring Hawaiian Epiphytal Amphipod Communities

Epiphytal amphipods prefer artificial substrata (bottle brushes) which accumulated epibiotic coatings while submerged in the sea for a period of up to 15 days. There is evidence that these amphipods eat epibiota on algae (predominately Padina japonica YAMADEA 1931 and Dictyota crenulata AGARDH 1847) and that food availability may be a factor which helps structure epiphytal amphipod communities.     

Stream mosses as chemically-defended refugia for freshwater macroinvertebrates

Marine and terrestrial studies show that small, sedentary herbivores that utilize plants as both food and habitat can gain enemy-free space by living on hosts that are chemically defended from larger, generalist consumers. Although large herbivores are increasingly recognized as important consumers of macrophytes in freshwater communities, the potential indirect effects of herbivory on plant-associated macroinvertebrates have rarely been studied. Here, we show that the large, generalist consumers in a riverine system, Canada geese, Branta canadensis , and crayfish, Procambarus spiculifer , both selectively consumed riverweed, Podostemum ceratophyllum , over an aquatic moss, Fontinalis novae-angliae, even though moss comprised 89% of the total plant biomass on riverine rocky shoals. Moss supported twice as many plant-associated macroinvertebrates as riverweed, suggesting that it might provide a spatial refuge from consumption by these larger consumers. Bioassay-guided fractionation of moss extracts led to the isolation of a C₁₈ acetylenic acid, octadeca-9,12-dien-6-ynoic acid, that deterred crayfish feeding. In contrast to results with Canada geese and crayfish, both the amphipod Crangonyx gracilis and the isopod Asellus aquaticus consumed significant amounts of moss but rejected riverweed in laboratory feeding assays. Moreover, neither amphipod nor isopod feeding was deterred by the crude organic extract of Fontinalis , suggesting that these mesograzers tolerate or circumvent the chemical defenses that deterred larger consumers. Thus, herbivory by large, generalist herbivores may drive freshwater plant community structure towards chemically defended plants and favor the ecological specialization of smaller, less mobile herbivores on unpalatable hosts that represent enemy-free space.     

Rapid experimental shift in host use traits of a polyphagous marine herbivore reveals fitness costs on alternative hosts

The host breadth of any particular herbivore reflects a compromise between evolutionary forces that promote specialism and those that promote polyphagy. Because most terrestrial herbivorous insects specialize, explorations of this evolutionary balance have focused largely on specialist than on polyphagous herbivores. Here, we experimentally tested whether fitness-based tradeoffs in utilizing alternative hosts can be detected within a polyphagous marine herbivore. The marine amphipod Ampithoe longimana occurs on multiple seaweeds year-round (especially the genera Sargassum, Ulva and Hypnea), but is particularly abundant on the diterpene-rich genus Dictyota during warmer summer months. If fitness-based tradeoffs in using these alternative hosts are present, A. longimana may experience fluctuating selection across seasons. To test this possibility, we performed a controlled natural-selection experiment in which amphipods were isolated on Dictyota or a mixed seaweed assemblage that did not include Dictyota. Within 15 weeks (less than five overlapping generations), Dictyota-lines had greater feeding tolerance for Dictyota and its secondary metabolites than did mixed-seaweed-lines. Dictyota-line females reproduced more quickly than did mixed-seaweed-line females on Dictyota, but mixed-seaweed-line juveniles had greater growth on Sargassum and Ulva and higher fecundity on all hosts than did Dictyota-line juveniles. While experimental shifts in preference and performance are likely genetically-mediated, our experimental protocol does not preclude a role for phenotypic plasticity. The presence of a fitness cost to evolving greater preference for Dictyota suggests that fluctuating selection may operate on feeding preference across seasons, but our test of this hypothesis was equivocal. We suggest that one reason that polyphagy persists within A. longimana and potentially other marine grazers is because polyphagy broadens resource use across seasons, and this benefit outweighs the fitness-based costs that can favor specialism. Our results also reinforce the notion that timescales of ecological and evolutionary dynamics can overlap.     

Impact of acanthocephalan parasites on aggregation behavior of amphipods (Gammarus pseudolimnaeus)

Acanthocephalan parasites can manipulate the behavior of their amphipod intermediate hosts in ways that increase the amphipod's risk of being eaten by a predator that serves as the final host for the parasite. Some asocial amphipod species have been shown to increase the likelihood of aggregation in response to chemical cues associated with predators. If such aggregation has anti-predation benefits, it might be subject to manipulation by parasites. We tested this hypothesis by comparing the preference of parasitized and unparasitized amphipods (Gammarus pseudolimnaeus) for associating with a group of unparasitized conspecifics, both in the presence and absence of chemical cues from predatory brook sticklebacks (Culaea inconstans). Amphipods with encysted parasites (Corynosoma sp.) avoided aggregating, whereas unparasitized amphipods preferred to aggregate. We also found that the risk of predation by sticklebacks faced by an individual amphipod was significantly lower when the amphipod was in a group compared to when it was alone. This suggests that the aggregation response of unparasitized amphipods is an adaptive response to escape predation. This study provides evidence for a novel parasitic manipulation of intermediate host behavior that is likely to increase transmission to the definitive host.     

Genetic isolation of populations of the gammaridean amphipod, Corophium volutator, in the Bay of Fundy, Canada

Selection experiments suggest that evolutionary modifications in amphipod demography can respond to local environmental changes and that local races of amphipods may be common. We tested this hypothesis in mudflat populations of Corophium volutator in the Bay of Fundy, Canada. Due to the unique topography of the Bay of Fundy, distinctive environmental conditions are prevalent in different branches of the Bay, while the impact of shorebird predation has also been shown to vary between populations. Several hypotheses have been suggested to explain ecological evidence which indicates that Corophium volutator (Pallas), a common amphipod crustacean, exhibits extensive life history variation in Bay of Fundy populations. We used RAPD-PCR techniques to examine populations of C. volutator in an investigation of genetic isolation in marine environments. Our data suggest that variation in selection pressures have played a significant role in the genetic divergence of populations of C. volutator in the Bay of Fundy.     

Do predatory fishes affect the structure of an epiphytal amphipod assemblage on a protected algal reef in Hawaii?

On a sheltered Hawaiian algal reef seaweeds are abundant and large herbivorous fishes are absent. Epiphytal amphipods are abundant on their seaweed habitats. Predator exclusion experiments were conducted to determine if predatory fishes affect the abundance, diversity, and size distribution of the epiphytal amphipod community living on plant substrata. Results suggested that amphipod abundance, expected number of species, and size distribution were all independent of predation pressure.     

Confrontation of cryptic diversity and mate discrimination within Gammarus pulex and Gammarus fossarum species complexes

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