Enallagma Species Research Articles

Molecular Characterisation of Faecal Bacterial Assemblages Among Four Species of Syntopic Odonates

Factors such as host species, phylogeny, diet, and both timing and location of sampling are thought to influence the composition of gut-associated bacteria in insects. In this study, we compared the faecal-associated bacterial taxa for three Coenagrion and one Enallagma damselfly species. We expected high overlap in representation of bacterial taxa due to the shared ecology and diet of these species. Using metabarcoding based on the 16S rRNA gene, we identified 1513 sequence variants, representing distinct bacterial ‘taxa’. Intriguingly, the damselfly species showed somewhat different magnitudes of richness of ZOTUs, ranging from 480 to 914 ZOTUs. In total, 921 (or 60.8% of the 1513) distinct ZOTUs were non-shared, each found only in one species, and then most often in only a single individual. There was a surfeit of these non-shared incidental ZOTUs in the Enallagma species accounting for it showing the highest bacterial richness and accounting for a sample-wide pattern of more single-species ZOTUs than expected, based on comparisons to the null model. Future studies should address the extent to which faecal bacteria represent non-incidental gut bacteria and whether abundant and shared taxa are true gut symbionts.Graphical Pictures of odonates adopted from Norske Art databank under Creative Commons License (CC BY 4.0).

Levels, bioaccumulation and biomagnification of pesticide residues in a tropical freshwater food web

The study assessed pesticide contamination transfer in Ikpoba River, an important tropical freshwater ecosystem in Southern Nigeria. The study quantified concentrations, bioaccumulation and biomagnification of pesticides in Ikpoba River’s food web, with emphasis on less frequently assessed lower trophic-level organisms. Concentrations of pesticides were quantified in water, sediment and biota (phytoplankton, green algae (Cladophora), macrophyte (Commelina erecta), macrobenthic invertebrates (Lestes species, Caridina africana, Enallagma species, Gerris lacustris, Culex species, Pentaneura species, Sympetrum species, Argyroneta aquatica, Lecane species (Cladocera) and pelagic fish (Tilapia zilli)). Samples were collected at two separate stations and were analyzed using gas chromatography equipped with electron capture detector. Aldrin was the dominant pesticide in the Ikpoba River food web with concentrations accounting for 14.4% of the total pesticide residues in the assessed matrices. Sediment samples had significantly higher pesticide concentrations among the matrices assessed, with a total mean concentration of 0.095 ± 0.02 µg/kg dw. Among the biota samples, total pesticide levels were significantly higher (p < 0.05) in Commelina erecta. BAF and BSAF values were also highest in this species indicating that Commelina erecta may represent a greater reservoir for pesticides and may be a principal factor in subsequent transfer of pesticides along the food web of Ikpoba River. The BMF values for α-HCH, γ-HCH, β-HCH, glyphosate, heptachlor, aldrin, heptachlor epoxide, endosulfan I, endrin, carbofuran and diazinon showed that these pesticides have the potential to biomagnify along the trophic levels. The persistence of these pesticides in Ikpoba River supports the need for continuous monitoring.

Female mate preferences on high-dimensional shape variation for male species recognition traits.

Females in many animal species must discriminate between conspecific and heterospecific males when choosing mates. Such mating preferences that discriminate against heterospecifics may inadvertently also affect the mating success of conspecific males, particularly those with more extreme phenotypes. From this expectation, we hypothesized that female mate choice should cause Enallagma females (Odonata: Coenagrionidae) to discriminate against conspecific males with more extreme phenotypes of the claspers males use to grasp females while mating - the main feature of species mate recognition in these species. To test this, we compared cerci sizes and shapes between males that were captured while mating with females to males that were captured at the same time but not mating in three Enallagma species. In contrast to our hypothesis, we found only one of forty comparisons of shape variation that was consistent with females discriminating against males with more extreme cerci shapes. Instead, differences in cerci shape between mating and single males suggested that females displayed directional preferences on 1-4 aspects of cerci shape in two of the species in our samples. These results suggest that whereas some directional biases in mating based on cerci shape occur, the intraspecific phenotypic variation in male cerci size and shape is likely not large enough for females to express any significant incidental discrimination among conspecifics with more extreme shapes.

Selection on escape performance during ecological speciation driven by predation

Despite the many study systems in which predation has played a major role in phenotypic diversification and speciation, the underlying selective regimes imposed by different predator assemblages have rarely been quantified. We did so for the damselfly genus Enallagma which strongly diverged in antipredator traits when the ancestral species occupying lakes containing fish (hereafter fish lakes) repeatedly invaded fishless lakes with dragonfly larvae as top predators (hereafter dragonfly lakes). In two selection experiments in field enclosures we quantified the selection on two key escape traits of two fish-lake Enallagma species associated with survival selection by fish in the ancestral fish lakes and by dragonfly predators in the invaded fishless, dragonfly lakes. In accordance with the different hunting modes, fish imposed selection for a decreased swimming propensity while dragonfly larvae imposed selection for increased swimming speed in one of the two species. In two complementary quantitative genetic rearing experiments, we found relatively low but significant broad-sense heritabilities for both escape traits. Integrating these estimates for the selection coefficients and the heritabilities suggests that the evolutionary increase in swimming speed associated with the habitat shift may have occurred rapidly. Our study suggests that the phenotypic evolution of ecologically important traits related to habitat shifts may occur at an ecological timescale.

Melanic individuals in color polymorphicEnallagmadamselflies result from phenotypic, not genetic, variation

Genetically determined color polymorphisms have a long history in the study of evolutionary change acting on populations. The Odonata exhibit relatively high levels of sex-specific color polymorphisms in mature adults. In Ischnura and Coenagrion, female-specific polymorphisms are known to be controlled by Mendelian genes. Nearly half of Enallagma species have polymorphic females, but the inheritance of any has yet to be determined. Our aims here were to determine: (1) the inheritance of the color polymorphism in E. hageni; and (2) inherent reproductive characteristics of blue female andromorphs and green heteromorphs reared under controlled conditions as tenerals. Maternal morphs, which developed normal coloration in field enclosures within a week, did not differ in copulation time or clutch size, and their offspring did not differ in sex ratio or survivorship to emergence. Surprisingly, no laboratory-reared offspring developed normal mature coloration. Rather, the initially pale parts of the thorax and a...

Functional Annotation and Comparative Analysis of a Zygopteran Transcriptome

In this paper we present a de novo assembly of the transcriptome of the damselfly (Enallagma hageni) through the use of 454 pyrosequencing. E. hageni is a member of the suborder Zygoptera, in the order Odonata, and Odonata organisms form the basal lineage of the winged insects (Pterygota). To date, sequence data used in phylogenetic analysis of Enallagma species have been derived from either mitochondrial DNA or ribosomal nuclear DNA. This Enallagma transcriptome contained 31,661 contigs that were assembled and translated into 14,813 individual open reading frames. Using these data, we constructed an extensive dataset of 634 orthologous nuclear protein-encoding genes across 11 species of Arthropoda and used Bayesian techniques to elucidate the position of Enallagma in the arthropod phylogenetic tree. Additionally, we demonstrated that the Enallagma transcriptome contains 169 genes that are evolving at rates that differ relative to those of the rest of the transcriptome (29 accelerated and 140 decreased), and, through multiple Gene Ontology searches and clustering methods, we present the first functional annotation of any palaeopteran’s transcriptome in the literature.

Invertebrate predation predicts variation in an autotomy-related trait in larval damselfly

Autotomy, the discarding of a prey appendage grasped by a predator, may evolve when the benefits of immediate survival outweigh the costs of appendage loss. In larval damselflies, joints connecting lamellae to the abdomen vary in size and shape within and among taxa suggesting that they may evolve under selection by invertebrate predators, such as dragonfly larvae. Assuming that joint width is proportional to the force required for autotomy, we tested if invertebrate predation favours smaller lamellar joints for autotomy or larger joints for structural support of lamellae for swimming. We compared the maximum joint widths of larval Lestes and Enallagma among ponds that varied in risk of invertebrate predation. Relative predation risk estimated as the frequency of regenerated lamellae within ponds was weakly and positively related to the relative abundance of larval dragonflies. The allometry of lamellar joint size decreased with increasing risk of invertebrate predation across ponds after controlling for variation in body size in Lestescongener but not in Enallagma species. Both species of Lestes had larger joint sizes than the five species of Enallagma, suggesting that the ancestral divergence of lamellar joints between these genera may influence contemporary responses to invertebrate predation.

SPECIES RECOGNITION AND PATTERNS OF POPULATION VARIATION IN THE REPRODUCTIVE STRUCTURES OF A DAMSELFLY GENUS

The selection pressures imposed by mate choice for species identity should impose strong stabilizing selection on traits that confer species identity to mates. Thus, we expect that such traits should show nonoverlapping distributions among closely related species, but show little to no variance among populations within a species. We tested these predictions by comparing levels of population differentiation in the sizes and shapes of male cerci (i.e., the clasper structures used for species identity during mating) of six Enallagma damselfly species. Cerci shapes were nonoverlapping among Enallagma species, and five of six Enallagma species showed no population variation across their entire species ranges. In contrast, cerci sizes overlapped among species and varied substantially among populations within species. These results, taken with previous studies, suggest that cerci shape is a primary feature used in species recognition used to discriminate conspecific from heterospecifics during mating.

Experimental evidence for neutral community dynamics governing an insect assemblage

The high levels of species diversity observed within many biological communities are captivating, yet the mechanisms that may maintain such diversity remain elusive. Many of the phenotypic differences observed among species cause interspecific tradeoffs that ultimately act to maintain diversity through niche-based coexistence. In contrast, neutral community theory argues that phenotypic differences among species do not contribute to maintaining species diversity because species are ecologically equivalent. Here we provide experimental and observational field evidence that two phylogenetically very distant Enallagma species appear to be ecologically equivalent to one another. Experimental abundance manipulations showed that each species gains no demographic advantage at low relative abundance, whereas manipulations of total Enallagma abundance resulted in large increases in per capita mortality and large decreases in growth for both species. Moreover, demographic rates and relative abundances of multiple Enallagma species were uncorrelated with major environmental gradients in an observational study of 20 natural lakes. These are the expected patterns if species are ecologically equivalent. However, these results do not imply that all damselflies in these lakes are ecologically identical. Previous experimental results have demonstrated the operation of strong coexistence mechanisms maintaining Enallagma and its sister-genus Ischnura in these littoral food webs. Combined with a simple theoretical model we present, these results taken together show how both neutral and niche dynamics can jointly structure communities.

A Tale of Two Diversifications: Reciprocal Habitat Shifts to Fill Ecological Space along the Pond Permanence Gradient

The Enallagma and Lestes damselflies have both diversified and adapted over the past 10-15 million years to the various ecological milieus found along the pond permanence gradient among North American ponds and lakes. Previous articles have explored this diversification process for Enallagma. In this article, we present a phylogenetic hypothesis for the North American Lestes, use this hypothesis to reconstruct Lestes diversification, and compare the diversification processes inferred for Lestes and Enallagma. The results of this study suggest that Lestes began in temporary ponds where large dragonflies are the top predators, while Enallagma began in permanent lakes where fish are the top predators. Starting from these different ancestral habitats, both genera have invaded and adapted to habitats already occupied by the other genus. Moreover, these adaptive habitat shifts involved substantial convergence on the behaviors used to deal with fish and dragonfly predation in both genera and a major life-history shift from diapausing to directly developing eggs in Lestes. However, in Lestes lineages invading fish lakes, swimming speed and morphology did not change to match those of Enallagma species, illustrating that reciprocal shifts between alternative selection regimes are not necessarily evolutionary opposites. Also, the greater sizes and growth rates of Lestes species compared to Enallagma species, which should impart substantial ecological advantages in competition between the genera, were shown to result from phylogenetic inheritance and not from adaptive diversification. This historical analysis of diversification raises new questions about the relationship between the macroevolutionary mechanisms driving lineage diversification and the ecological mechanisms structuring local food webs and regional species assemblages.

PARALLEL EVOLUTION IN ECOLOGICAL AND REPRODUCTIVE TRAITS TO PRODUCE CRYPTIC DAMSELFLY SPECIES ACROSS THE HOLARCTIC

The damselfly genus Enallagma originated in the Nearctic, and two Nearctic lineages recently underwent radiations partly associated with multiple independent habitat shifts from lakes dominated by fish predators into lakes dominated by dragonfly predators. A previous molecular study of four Palearctic morphospecies and all representative Nearctic species identified the presence of two cryptic species sets, with each set having Palearctic and Nearctic representatives. However, the cryptic species within each set are not sibling species. Here, we present quantitative data on ecologically important larval morphologies and behaviors involved in predator avoidance and on adult male morphological structures involved in mate recognition to quantify the phenotypic relationships among these cryptic species sets. For the adult stage, our data indicate strong parallel evolution of the structures involved in specific mate recognition-the male cerci. For the larval stage, morphometric analyses show that the Palearctic species evolved a nearly identical morphology to the sibling-clade members in the Nearctic that live in waters where dragonflies are the top predators. This implicates the importance of dragonfly predation in the history of the Palearctic clade. Behavioral analyses suggest population differentiation in response to the actual predator environment in the Palearctic clade, consistent with the species differentiation seen in the Nearctic. Our results suggest parallel evolution of adult traits that influence specific mate choice and larval traits that influence ecological performance underlie the striking similarity of Enallagma species across continents. This concurrent parallel evolution in both stages of a complex life cycle, especially when both stages do not share the same selective environment, may be a very unusual mechanism generating cryptic species.

The growth/predation risk trade-off: so what is the mechanism?

Among damselflies in natural lakes, Ischnura species grow faster than coexisting Enallagma species, but Enallagma species have higher survival under predation than Ischnura species. This growth/predation risk trade-off apparently allows these taxa to coexist in ponds and lakes across the Holarctic. However, laboratory studies presented here show that the mechanism assumed by most theoretical and empirical studies to mediate this trade-off, namely activity simultaneously modulating foraging returns and predation risk, does not operate in this system. Ischnura verticalis larvae were more active than larvae of Enallagma species in a short-term behavioral experiment, which explains why Ischnura experiences greater mortality from predation. However, this greater activity did not translate into higher feeding rates. Ischnura verticalis and Enallagma species ate comparable amounts of food in both the short-term behavioral experiment and a longer feeding and digestion experiment. In spite of no difference in the amount of food ingested or assimilated, I. verticalis larvae grew faster than Enallagma larvae because they were better able to physiologically convert assimilated food into their own biomass in the presence of mortality threats. From these studies we understand the phenotypic mechanisms determining the antagonistic patterns of relative growth and survival between these two genera, but why these patterns exist remains unclear.

EVOLUTION OF PREY BEHAVIOR IN RESPONSE TO CHANGES IN PREDATION REGIME: DAMSELFLIES IN FISH AND DRAGONFLY LAKES

Abstract In a large behavioral experiment we reconstructed the evolution of behavioral responses to predators to explore how interactions with predators have shaped the evolution of their prey—behavior. All Enallagma damselfly species reduced both movement and feeding in the presence of coexisting predators. Some Enallagma species inhabit water bodies with both fish and dragonflies, and these species responded to the presence of both predators, whereas other Enallagma species inhabit water bodies that have only large dragonflies as predators, and these species only responded to the presence of dragonflies. Lineages that shifted to live with large dragonflies showed no evolution in behaviors expressed in the presence of dragonflies, but they evolved greater movement in the absence of predators and greater movement and feeding in the presence of fish. These results suggest that Enallagma species have evolutionarily lost the ability to recognize fish as a predator. Because species coexisting with only dragonfly predators have also evolved the ability to escape attacking dragonfly predators by swimming, the decreased predation risk associated with foraging appears to have shifted the balance of the foraging/predation risk trade-off to allow increased activity in the absence of mortality threats to evolve in these lineages. Our results suggest that evolution in response to changes in predation regime may have greater consequences for characters expressed in the absence of mortality threats because of how the balance between the conflicting demands of growth and predation risk are altered.

Evolution of prey behavior in response to changes in predation regime: damselflies in fish and dragonfly lakes.

In a large behavioral experiment we reconstructed the evolution of behavioral responses to predators to explore how interactions with predators have shaped the evolution of their prey's behavior. All Enallagma damselfly species reduced both movement and feeding in the presence of coexisting predators. Some Enallagma species inhabit water bodies with both fish and dragonflies, and these species responded to the presence of both predators, whereas other Enallagma species inhabit water bodies that have only large dragonflies as predators, and these species only responded to the presence of dragonflies. Lineages that shifted to live with large dragonflies showed no evolution in behaviors expressed in the presence of dragonflies, but they evolved greater movement in the absence of predators and greater movement and feeding in the presence of fish. These results suggest that Enallagma species have evolutionarily lost the ability to recognize fish as a predator. Because species coexisting with only dragonfly predators have also evolved the ability to escape attacking dragonfly predators by swimming, the decreased predation risk associated with foraging appears to have shifted the balance of the foraging/predation risk trade-off to allow increased activity in the absence of mortality threats to evolve in these lineages. Our results suggest that evolution in response to changes in predation regime may have greater consequences for characters expressed in the absence of mortality threats because of how the balance between the conflicting demands of growth and predation risk are altered.

Habitat requirements and local persistence of three damselfly species (odonata: coenagrionidae)

Habitat requirements and population persistence were investigated in three damselfly species, all coastal plain pond specialists: Enallagma recurvatum, E. laterale, and E. pictum. Because of geographic restriction, two are of special concern to conservation, E. recurvatum and E. laterale. We surveyed more than 70 ponds on Cape Cod, Massachusetts, and collected adult presence-absence data during the summers of 1999 and 2000. We achieved a detection rate approaching 100% for each species by visiting each pond up to three times. We looked for relationships between the presence of each damselfly species and presence of specific aquatic vegetation, the presence of the other Enallagma species, and the number of ponds within various distances of the 72 surveyed ponds. Using stepwise logistic regression, we found the following significant associations: E. recurvatum with the rush Juncus militaris; E. laterale with water lilies (Nuphar variegatum and Brasenia schreberi) the damselfly E. pictum, and the number of ponds within 2 km; and E. pictum with the water lily Nymphaea odorata, the damselfly E. laterale, and the number of ponds within 1.5 km and 2.5 km. Presence-absence data were used to calculate turnover and local extinction rates for each species between the two years. E. recurvatum's turnover and local extinction rates (33.3% and 41% respectively) were much higher than either E. laterale (9.8%, 11.5%) or E. pictum (7.7%, 10%). These results suggest that E. recurvatum occurs in a metapopulation, and that patch colonization rates might be important to local population persistence.

PHYSIOLOGICAL AND BEHAVIORAL RESPONSES TO PREDATORS SHAPE THE GROWTH/PREDATION RISK TRADE-OFF IN DAMSELFLIES

Most organisms must simultaneously find enough food for themselves while trying not to become food for some other organism. Previous field experiments have shown that larvae of Enallagma and Ischnura species are able to coexist in the littoral zones of lakes because they resolve this growth/predation risk trade-off differently: Ischnura species grow more quickly than Enallagma species, but Ischnura species suffer higher mortality rates than Enallagma. We performed a series of laboratory studies to explore the mechanistic basis for the difference in growth between the genera. When held in complete isolation and with unlimited food, larvae of a number of Enallagma species that coexist with fish accumulated mass at much faster rates than Ischnura species. This difference in isolation was due to the fish-lake Enallagma simply ingesting more food. In contrast, when held in the presence of other damselflies or a fish predator, Ischnura had significantly higher growth rates than Enallagma species from fish lakes. All species decreased the amount of food they ingested in the presence of the fish predator as compared to when fish were absent, which resulted in decreased growth in the presence of the predator for all species. However, the interspecific differences in growth rate were due primarily to differences in the abilities of the species to convert ingested food into their own biomass; in the presence of fish, comparably sized larvae ingested nearly identical amounts of food, but Ischnura larvae grew faster because they converted significantly more ingested food into their own biomass than did larvae of Enallagma species from fish lakes. This difference in conversion efficiency between the genera was not apparent when larvae were raised in complete isolation. These results indicate that Enallagma and Ischnura species differ in physiological stress responses to the presence of predators, and this difference may facilitate the coexistence of Enallagma and Ischnura species in the field.

BIOCHEMICAL EVOLUTION ASSOCIATED WITH ANTIPREDATOR ADAPTATION IN DAMSELFLIES.

Previous studies have shown that at least two lineages of Enallagma damselflies (Odonata: Coenagrionidae) shifted from inhabiting lakes with fish as top predators to inhabiting ponds and lakes with large dragonflies as the top predators. In adapting to living with the new predator type, these lineages evolved much greater swimming speeds to avoid attacking dragonflies. In this paper, I test whether biochemical adaptations to fuel swimming arose in concert with previously identified morphological changes that increase swimming speed. I assayed the mass-specific enzyme activities of three enzymes involved in fueling strenuous activity: pyruvate kinase and lactate dehydrogenase (enzymes involved in glycolysis) and arginine kinase (the enzyme that recharges the ATP pool). Enzyme activities were determined for 14 Enallagma species from across the genus. Species that coexist with dragonfly predators had significantly higher mass-specific arginine kinase activities than species that coexist with fish, and the results of evolutionary contrasts analyses indicate that this difference between the two groups is the result of evolutionary change associated with the habitat shifts of lineages from fish lakes to dragonfly lakes. Although significant evolution was documented for lactate dehydrogenase and pyruvate kinase across the genus, evolutionary change in the activities of these enzymes was not consistent with adaptation to coexisting with dragonfly predators. Swimming bouts to avoid dragonfly predators last for only a few seconds, and the action of arginine kinase to phosphorylate ADP to make ATP will extend the duration of maximal exertion for swimming for a few seconds. However, much longer time periods (over 45 sec) are required to generate ATP via glycolysis. Therefore, selection may have favored adaptation only at the arginine kinase locus.

THE CONSEQUENCES OF CHANGING THE TOP PREDATOR IN A FOOD WEB: A COMPARATIVE EXPERIMENTAL APPROACH

Changing the top predator in a food web often results in dramatic changes in species composition at lower trophic levels; many species are extirpated and replaced by new species in the presence of the new top predator. These shifts in species composition also often result in substantial alterations in the strengths of species interactions. However, some species appear to be little affected by these changes that cause species turnover at other positions in the food web. An example of such a difference in species responses is apparent in the distributions of coenagrionid damselflies (Odonata: Zygoptera) among permanent water bodies with and without fish as top predators. Enallagma species segregate between ponds and lakes that do and do not support fish populations, with each lake type having a characteristic Enallagma assemblage. In contrast, species of Ischnura, the sister genus to Enallagma, are common to both fish and fishless ponds and lakes. Previous research has shown that Enallagma species segregate because they are differentially vulnerable to the top predators in each lake type: dragonflies in fishless lakes and fish in fish lakes. This paper reports the results of a series of laboratory and field experiments quantifying the mortality and growth effects of interactions in the food webs surrounding Enallagma and Ischnura species in both lake types. These results are compared to determine how features of the food web change to force segregation of Enallagma species between the lake types but permit Ischnura species to inhabit both. The results of experiments conducted in a fishless lake show that damselflies are not food limited in this lake type, but that they do strongly compete via interference mechanisms. Interference effects between the genera are symmetrical. Ischnura species have substantially higher growth rates than Enallagma species under all conditions in fishless lakes. Although both Enallagma and Ischnura experience substantial mortality from predation by dragonflies (Anax and Aeshna species, the top predators in fishless lakes), these dragonflies display a significant bias towards feeding on Ischnura. Mortality rates due to dragonfly predation are not density dependent. The results of experiments done in a fish lake indicate that damselflies are food limited and thus compete for resources in fish lakes. Ischnura growth rates are also substantially higher than Enallagma species in the fish-lake system. Dragonfly species that coexist with fish (Basiaeschna and Epitheca species) do not impose significant mortality on coexisting damselflies, but they do compete for resources with the damselflies, and they may also generate feeding interference in the damselflies. Fish impose significantly higher mortality on Ischnura species than on coexisting Enallagma species, and this mortality is negatively density dependent. The coexistence of Enallagma and Ischnura species is fostered in both lake types by trade-offs in their abilities to avoid predators and to utilize resources. Native Enallagma species are better at avoiding coexisting predators in each lake type, but these abilities come at the expense of the ability to utilize resources effectively and to avoid the predator found in the other lake type. In contrast, Ischnura are better at utilizing resources in both lake types, but these abilities come at the expense of effectively avoiding both fish and dragonflies. Understanding the trade-offs faced by species at similar trophic positions within a food web is critical to predicting changes in food webs following major environmental perturbations such as changing the top predator.

Adaptation to Predators in a New Community: Swimming Performance and Predator Avoidance in Damselflies

Species evolve in response to the abiotic and biotic interactions they experience in a community, and so studying the adaptation of species in a community context is a necessary component for understanding the development of community structure. Previous studies have suggested that Enallagma damselflies (Odonata: Coenagrionidae) invaded aquatic habitats with large dragonflies as the top predators at least twice, and large changes in morphological structures (e.g., caudal lamellae and abdomen), which should increase swimming speed, are associated with these invasions. This is significant because these species swim away from attacking predators, whereas species in the ancestral habitat (habitats with fish as the top predators) do not swim from attacking predators. We quantified swimming speed over a large ontogenetic size range for larvae of seven Enallagma species, two in dragonfly lakes and five in fish lakes. Results of analysis of covariance and a modification of Felsenstein's method of evolutionary contrasts indicate that a large evolutionary increase in the slope of the ontogenetic relationship between swimming speed and loge(body length) is associated with invasions into dragonfly lakes, and this shift results in larvae of dragonfly—lake species being faster swimmers, especially in larger size classes than larvae of fish—lake species. We also present experimental results demonstrating that damselflies with larger lamellae are faster swimmers and have higher survival when exposed to dragonfly predation. The results of this study indicate that the change in selective environment associated with habitat shifts into dragonfly lakes promoted adaptations for increased swimming speed in Enallagma lineages. The potential ramifications of this adaptation for community structure are discussed.

Behavioral Differences between Enallagma Species (Odonata) Influencing Differential Vulnerability to Predators

In previous research I have shown that fish predation contributes to the exclusion of a group of Enallagma species (Odonata: Zygoptera) from lakes containing fish, whereas dragonfly predation contributes to the exclusion of another group of Enallagma species from fishless lakes. In the study reported in this paper I performed a series of laboratory experiments to identify behavioral differences between the two groups that could contribute to their differential vulnerabilities to the two predators. In one experiment I observed the behaviors of four Enallagma species, two from fishless lakes and two from lakes containing fish, in the presence of no predators, dragonfly larvae (Anax junius), and fish (Lepomis macrochirus) to identify behavioral differences that may cause the species to be differentially detected by the predators. In another experiment, I observed Aeshna dragonflies feeding on Enallagma larvae to evaluate whether species in the two groups differ in how they attempt to avoid attacking dragonflies. The Enallagma species from fishless lakes walked in rapid bursts of 2—3 s and frequently performed a conspicuous behavioral display, while the Enallagma species from lakes containing fish walked very slowly for durations of 20—30 s and almost never performed the conspicuous display. The species from lakes containing fish also swam for longer durations and remained motionless longer following swims than species from fishless lakes. The species from fishless lakes also greatly reduced their movement and responsiveness toward prey and increased the duration of motionless periods in the presence of dragonflies, but only reduced the number and duration of the conspicuous display in the presence of fish. In contrast, the species from lakes containing fish reduced their movement and responsiveness toward prey and increased the duration of motionless periods in the presence of both fish and dragonflies. In videotaped feeding trials, larvae of an Enallagma species from fishless lakes usually swam away from attacking Aeshna dragonflies. Larvae of an Enallagma species from lakes containing fish usually did not respond to attacking dragonflies, and were usually captured without attempting to evade the dragonfly. These behavioral differences are consistent with observed differences in their vulnerabilities to predators, and probably contribute to causing the pattern of habitat separation observed in the field.