Population genetic analysis of a recent range expansion: mechanisms regulating the poleward range limit in the volcano barnacleTetraclita rubescens

Species' attributes predict the relative magnitude of ecological and genetic recovery following mass mortality

Theoretically, species' characteristics should allow estimation of dispersal potential and, in turn, explain levels of population genetic differentiation. However, a mismatch between traits and genetic patterns is often reported for marine species, and interpreted as evidence that life-history traits do not influence dispersal. Here, we couple ecological and genomic methods to test the hypothesis that species with attributes favouring greater dispersal potential-e.g., longer pelagic duration, higher fecundity and larger population size-have greater realized dispersal overall. We used a natural experiment created by a large-scale and multispecies mortality event which created a "clean slate" on which to study recruitment dynamics, thus simplifying a usually complex problem. We surveyed four species of differing dispersal potential to quantify the abundance and distribution of recruits and to genetically assign these recruits to probable parental sources. Species with higher dispersal potential recolonized a broader extent of the impacted range, did so more quickly and recovered more genetic diversity than species with lower dispersal potential. Moreover, populations of taxa with higher dispersal potential exhibited more immigration (71%-92% of recruits) than taxa with lower dispersal potential (17%-44% of recruits). By linking ecological with genomic perspectives, we demonstrate that a suite of interacting life-history and demographic attributes do influence species' realized dispersal and genetic neighbourhoods. To better understand species' resilience and recovery in this time of global change, integrative eco-evolutionary approaches are needed to more rigorously evaluate the effect of dispersal-linked attributes on realized dispersal and population genetic differentiation.

Competition, Trait Variance Dynamics, and the Evolution of a Species' Range

Geographic ranges of communities of species evolve in response to environmental, ecological, and evolutionary forces. Understanding the effects of these forces on species' range dynamics is a major goal of spatial ecology. Previous mathematical models have jointly captured the dynamic changes in species' population distributions and the selective evolution of fitness-related phenotypic traits in the presence of an environmental gradient. These models inevitably include some unrealistic assumptions, and biologically reasonable ranges of values for their parameters are not easy to specify. As a result, simulations of the seminal models of this type can lead to markedly different conclusions about the behavior of such populations, including the possibility of maladaptation setting stable range boundaries. Here, we harmonize such results by developing and simulating a continuum model of range evolution in a community of species that interact competitively while diffusing over an environmental gradient. Our model extends existing models by incorporating both competition and freely changing intraspecific trait variance. Simulations of this model predict a spatial profile of species' trait variance that is consistent with experimental measurements available in the literature. Moreover, they reaffirm interspecific competition as an effective factor in limiting species' ranges, even when trait variance is not artificially constrained. These theoretical results can inform the design of, as yet rare, empirical studies to clarify the evolutionary causes of range stabilization.

Differential Tolerance and Seasonal Adaptation to Temperature and Salinity Stress at a Dynamic Range Boundary Between Estuarine Gastropods

AbstractInsight into how coastal organisms will respond to changing temperature and salinity regimes may be derived from studies of adaptation to fluctuating estuarine environments, especially under stressful range-edge conditions. We characterized a dynamic range boundary between two estuarine sea slugs, Alderia modesta (distributed across the North Pacific and North Atlantic) and Alderia willowi, known from southern and central California. The species overlap from Bodega Bay to San Francisco Bay, where populations are dominated by A. modesta after winter rains but by A. willowi after peak summer temperatures. Laboratory assays confirmed superior tolerance to low salinity for the northern species, A. modesta: encapsulated embryos developed at 8 ppt, larvae survived at 4-6 ppt, and adults survived repeated exposure to 2 ppt, salinities that reduced development or survival for the same stages of A. willowi. Adults did not appreciably differ in their high-temperature threshold, however. Each species showed increased tolerance to either temperature or salinity stress at its range margin, indicating plasticity or local adaptation, but at the cost of reduced tolerance to the other stressor. At its northern limit, A. willowi became more tolerant of low salinity during the winter rainy season, but also less heat tolerant. Conversely, A. modesta became more heat resistant from spring to summer at its southern limit, but less tolerant of low salinity. Trade-offs in stress tolerance may generally constrain adaptation and limit biotic response to a rapidly changing environment, as well as differentiating species niches.

Environmental association modelling with loci under divergent selection predicts the distribution range of a lizard

During the historical building of a species range, individual colonizers have to confront different ecological challenges, and the capacity of the species to broaden its range may depend on the total amount of adaptive genetic variation supplied by evolution. We set out to increase our understanding of what defines a range and the role of underlying genetics by trying to predict an entire species' range from the geographical distribution of its genetic diversity under selection. We sampled five populations of the western Mediterranean lizard Psammodromus algirus that inhabit a noticeable environmental gradient of temperature and precipitation. We correlated the genotypes of 95 individuals (18-20 individuals per population) for 21 SNPs putatively under selection with environmental scores on a bioclimatic gradient, using 1 × 1 km²  grid cells as sampling units. By extrapolating the resulting model to all possible combinations of alleles, we inferred all the geographic cells that were theoretically suitable for a given amount of genetic variance under selection. The inferred distribution range overlapped to a large extent with the realized range of the species (77.46% of overlap), including an accurate prediction of internal gaps and range borders. Our results suggest an adaptability threshold determined by the amount of genetic variation available that would be required to warrant adaptation beyond a certain limit of environmental variation. These results support the idea that the expansion of a species' range can be ultimately linked to the arising of new variants under selection (either newly selected variants from standing genetic variation or innovative mutations under selection).

Evaluating Assisted Gene Flow in Marginal Populations of a High Mountain Species

Many species cannot either migrate or adapt at the rate of temperature increases due to climate warming. Therefore, they need active conservation strategies to avoid extinction. Facilitated adaptation actions, such assisted gene flow, aim at the increase of the evolutionary resilience of species affected by global change. In elevational gradients, marginal populations at the lower elevation edges are experiencing earlier snowmelt and higher temperatures, which force them to adapt to the new conditions by modifying their phenology. In this context, advancing the onset of flowering and seed germination times are crucial to ensure reproductive success and increase seedling survival prior to summer drought. Assisted gene flow may bring adaptive alleles and increase genetic diversity that can help throughout ontogeny. The main aim of this work is to assess the effects that different gene flow treatments could have on the desired trait changes in marginal populations. Accordingly, we established a common garden experiment in which we assayed four different gene flow treatments between Silene ciliata Pourr. (Caryophyllaceae) populations located in similar and different elevation edges, belonging to the same and different mountains. As a control treatment, within-population crosses of low elevation edge populations were performed. The resulting seeds were sown and the germination and flowering onset dates of the resulting plants recorded, as well as the seedling survival. Gene flow between populations falling on the same mountain and same elevation and gene flow from high-elevation populations from a different mountain to low-elevation populations advanced seed germination time with respect to control crosses. No significant effects of gene flow on seedling survival were found. All the gene flow treatments delayed the onset of flowering with respect to control crosses and this effect was more pronounced in among-mountain gene flows. The results of this study highlight two important issues that should be thoroughly studied before attempting to apply assisted gene flow in practical conservation situations. Firstly, among-populations gene flow can trigger different responses in crucial traits throughout the ontogeny of plant species. Secondly, the population provenance of gene flow is determinant and plays a significant role on the effects of gene flow.

Species better track climate warming in the oceans than on land

There is mounting evidence of species redistribution as climate warms. Yet, our knowledge of the coupling between species range shifts and isotherm shifts remains limited. Here, we introduce BioShifts-a global geo-database of 30,534 range shifts. Despite a spatial imbalance towards the most developed regions of the Northern Hemisphere and a taxonomic bias towards the most charismatic animals and plants of the planet, data show that marine species are better at tracking isotherm shifts, and move towards the pole six times faster than terrestrial species. More specifically, we find that marine species closely track shifting isotherms in warm and relatively undisturbed waters (for example, the Central Pacific Basin) or in cold waters subject to high human pressures (for example, the North Sea). On land, human activities impede the capacity of terrestrial species to track isotherm shifts in latitude, with some species shifting in the opposite direction to isotherms. Along elevational gradients, species follow the direction of isotherm shifts but at a pace that is much slower than expected, especially in areas with warm climates. Our results suggest that terrestrial species are lagging behind shifting isotherms more than marine species, which is probably related to the interplay between the wider thermal safety margin of terrestrial versus marine species and the more constrained physical environment for dispersal in terrestrial versus marine habitats.

Simulating the effects of local adaptation and life history on the ability of plants to track climate shifts

Many studies have examined the impact of dispersal on local adaptation, but much less attention has been paid to how local adaptation influences range shifts. The aim of this study was to test how local adaptation might affect climate-driven range shifts in plants, and if this might differ between plants with different life histories. Simulated range shift dynamics were compared for hypothetical annual, perennial and tree species, each comprised of either one plastic genotype or six locally adapted genotypes. The landscape consists of shifting climate bands made up of 20 × 20 m patches containing multiple individuals. Effects of seed dispersal, breadth of the plastic species' tolerance, steepness of the climate gradient and rate of the climate shift are also examined. Local adaptation increased the equilibrium range size and aided range shifts by boosting fitness near range edges. However, when the rate of climate change was doubled on a steep gradient, locally adapted trees exhibited a higher percent loss of range during the climate shift. The plastic annual species with short dispersal was unable to recover its range size even after the climate stabilized, while the locally adapted annuals tracked climate change well. The results suggest that in most situations local adaptation and longer dispersal distances will be advantageous, though not necessarily sufficient, for tracking suitable climates. However, local adaptation might put species with long generation times at greater risk when climate shifts are very rapid. If confirmed by empirical tests, these results suggest that identifying variation between species in how fitness varies along climate gradients and in these key demographic rates might aid in prioritizing management actions.

Evidence for adaptive responses to historic drought across a native plant species range

As climatic conditions change, species will be forced to move or adapt to avoid extinction. Exacerbated by ongoing climate change, California recently experienced a severe and exceptional drought from 2011 to 2017. To investigate whether an adaptive response occurred during this event, we conducted a "resurrection" study of the cutleaf monkeyflower (Mimulus laciniatus), an annual plant, by comparing trait means and variances of ancestral seed collections ("pre-drought") with contemporary descendant collections ("drought"). Plants were grown under common conditions to test whether this geographically restricted species has the capacity to respond evolutionarily to climate stress across its range. We examined if traits shifted in response to the recent, severe drought and included populations across an elevation gradient, including populations at the low- and high-elevation edges of the species range. We found that time to seedling emergence in the drought generation was significantly earlier than in the pre-drought generation, a response consistent with drought adaptation. Additionally, trait variation in days to emergence was reduced in the drought generation, which suggests selection or bottleneck events. Days to first flower increased significantly by elevation, consistent with climate adaptation across the species range. Drought generation plants were larger and had greater reproduction, which was likely a carryover effect of earlier germination. These results demonstrate that rapid shifts in trait means and variances consistent with climate adaptation are occurring within populations, including peripheral populations at warm and cold climate limits, of a plant species with a relatively restricted range that has so far not shifted its elevation distribution during contemporary climate change. Thus, rapid evolution may mitigate, at least temporarily, range shifts under global climate change. This study highlights the need for better understanding rapid adaptation as a means for plant communities to cope with extraordinary climate events.

Maintenance of a Genetic Cline in the Barnacle Balanus glandula

The barnacle Balanus glandula is a broadly distributed species in the temperate northeastern Pacific that is notable for a robust genetic cline between about 36° and 40° N latitude. Prior work established the evolutionary origins of this pattern and proposed that it is maintained by environmental selection. In recent years, "climate velocity" studies in marine habitats have shown dramatic distributional shifts for many species as they track their preferred temperature range in a warming ocean. We re-sampled B. glandula across its entire geographic range to determine whether there has been any shift in this genetic distribution, a development signaling that temperature or other climate factors are maintaining this genetic cline. Additionally, we asked whether the spatially distributed mitochondrial lineages also vary in reproductive output with latitude, using location as a proxy for temperature and other coastal environmental factors. Here we show that although the distribution of the genetic cline has not appreciably changed, there is a notable association of decreased reproductive output at lower latitudes of the distribution in the "northern" lineage of B. glandula.

Widespread shifts in the coastal biota of northern California during the 2014-2016 marine heatwaves

During 2014-2016, severe marine heatwaves in the northeast Pacific triggered well-documented disturbances including mass mortalities, harmful algal blooms, and declines in subtidal kelp beds. However, less attention has been directed towards understanding how changes in sea surface temperature (SST) and alongshore currents during this period influenced the geographic distribution of coastal taxa. Here, we examine these effects in northern California, USA, with a focus on the region between Point Reyes and Point Arena. This region represents an important biogeographic transition zone that lies <150 km north of Monterey Bay, California, where numerous southern species have historically reached their northern (poleward) range limits. We report substantial changes in geographic distributions and/or abundances across a diverse suite of 67 southern species, including an unprecedented number of poleward range extensions (37) and striking increases in the recruitment of owl limpets (Lottia gigantea) and volcano barnacles (Tetraclita rubescens). These ecological responses likely arose through the combined effects of extreme SST, periods of anomalous poleward flow, and the unusually long duration of heatwave events. Prolonged marine heatwaves and enhanced poleward dispersal may play an important role in longer-term shifts in the composition of coastal communities in northern California and other biogeographic transition zones.

A coalescent sampler successfully detects biologically meaningful population structure overlooked by F-statistics

Assessing the geographic structure of populations has relied heavily on Sewell Wright's F-statistics and their numerous analogues for many decades. However, it is well appreciated that, due to their nonlinear relationship with gene flow, F-statistics frequently fail to reject the null model of panmixia in species with relatively high levels of gene flow and large population sizes. Coalescent genealogy samplers instead allow a model-selection approach to the characterization of population structure, thereby providing the opportunity for stronger inference. Here, we validate the use of coalescent samplers in a high gene flow context using simulations of a stepping-stone model. In an example case study, we then re-analyze genetic datasets from 41 marine species sampled from throughout the Hawaiian archipelago using coalescent model selection. Due to the archipelago's linear nature, it is expected that most species will conform to some sort of stepping-stone model (leading to an expected pattern of isolation by distance), but F-statistics have only supported this inference in ~10% of these datasets. Our simulation analysis shows that a coalescent sampler can make a correct inference of stepping-stone gene flow in nearly 100% of cases where gene flow is ≤100 migrants per generation (equivalent to F ST = 0.002), while F-statistics had mixed results. Our re-analysis of empirical datasets found that nearly 70% of datasets with an unambiguous result fit a stepping-stone model with varying population sizes and rates of gene flow, although 37% of datasets yielded ambiguous results. Together, our results demonstrate that coalescent samplers hold great promise for detecting weak but meaningful population structure, and defining appropriate management units.

Contemporary range expansion of the Virginia opossum (Didelphis virginiana) impacted by humans and snow cover

Range expansions are key demographic events driven by factors such as climate change and human intervention that ultimately influence the genetic composition of peripheral populations. The expansion of the Virginia opossum (Didelphis virginiana Kerr, 1792) into Michigan has been documented over the past 200 years, indicating relatively new colonizations in northern Michigan. Although most contemporary expansions are a result of shifts in climate regimes, the opossum has spread beyond its hypothesized climate niche, offering an opportunity to examine the compounding influence that climate change and humans have on a species’ distribution. The genetic consequences of two range expansions were investigated using genotypic data for nine microsatellite markers from opossums collected in Michigan, Ohio, and Wisconsin, USA. Two genetic clusters were identified: one on either side of Lake Michigan. Using general linear models, we found that measurements of genetic diversity across 15 counties are best explained by days of snow on the ground. Next best models incorporate anthropogenic covariates including farm density. These models suggest that opossum expansion may be facilitated by agricultural land development and at the same time be limited by their inability to forage in snow.

Population genetics and adaptation to climate along elevation gradients in invasive Solidago canadensis

Gene flow between populations may either support local adaptation by supplying genetic variation on which selection may act, or counteract it if maladapted alleles arrive faster than can be purged by selection. Although both such effects have been documented within plant species’ native ranges, how the balance of these forces influences local adaptation in invasive plant populations is less clear, in part because introduced species often have lower genetic variation initially but also tend to have good dispersal abilities. To evaluate the extent of gene flow and adaptation to local climate in invasive populations of Solidago canadensis, and the implications of this for range expansion, we compared population differentiation at microsatellite and chloroplast loci for populations across Switzerland and assessed the effect of environmental transfer distance using common gardens. We found that while patterns of differentiation at neutral genetic markers suggested that populations are connected through extensive pollen and seed movement, common-garden plants nonetheless exhibited modest adaptation to local climate conditions. Growth rate and flower production declined with climatic distance from a plant's home site, with clones from colder home sites performing better at or above the range limit. Such adaptation in invasive species is likely to promote further spread, particularly under climate change, as the genotypes positioned near the range edge may be best able to take advantage of lengthening growing seasons to expand the range.

Large-scale gene flow in the barnacle Jehlius cirratus and contrasts with other broadly-distributed taxa along the Chilean coast

We evaluate the population genetic structure of the intertidal barnacle Jehlius cirratus across a broad portion of its geographic distribution using data from the mitochondrial cytochrome oxidase I (COI) gene region. Despite sampling diversity from over 3,000 km of the linear range of this species, there is only slight regional structure indicated, with overall Φ CT of 0.036 (p < 0.001) yet no support for isolation by distance. While these results suggest greater structure than previous studies of J. cirratus had indicated, the pattern of diversity is still far more subtle than in other similarly-distributed species with similar larval and life history traits. We compare these data and results with recent findings in four other intertidal species that have planktotrophic larvae. There are no clear patterns among these taxa that can be associated with intertidal depth or other known life history traits.

Microsatellite loci discovery from next-generation sequencing data and loci characterization in the epizoic barnacle Chelonibia testudinaria (Linnaeus, 1758)

Microsatellite markers remain an important tool for ecological and evolutionary research, but are unavailable for many non-model organisms. One such organism with rare ecological and evolutionary features is the epizoic barnacle Chelonibia testudinaria (Linnaeus, 1758). Chelonibia testudinaria appears to be a host generalist, and has an unusual sexual system, androdioecy. Genetic studies on host specificity and mating behavior are impeded by the lack of fine-scale, highly variable markers, such as microsatellite markers. In the present study, we discovered thousands of new microsatellite loci from next-generation sequencing data, and characterized 12 loci thoroughly. We conclude that 11 of these loci will be useful markers in future ecological and evolutionary studies on C. testudinaria.

Plasticity in fecundity highlights the females' importance in the spiny-cheek crayfish invasion mechanism

Invasion is one of the most consequential phenomena affecting the distribution of native species. Few in number of species, European crayfish are losing the competition with introduced North American crayfish. The spiny-cheek crayfish, Orconectes limosus, is an outstanding example, successfully competing against the native narrow-clawed crayfish, Astacus leptodactylus. For four years, we collected data regarding crayfish occurrences, their relative abundance, and the structure of populations in the ongoing colonisation process of O. limosus in the lower Danube. The mature females of both invasive and indigenous crayfish species were analysed with respect to biometry and production of oocytes in relation to the dynamics of invasion. The interspecific comparisons showed no significant differences regarding body size, with an average of approximately 102 mm total length and 31 g wet weight for both species. However, the fecundity of the indigenous species was found to be constant throughout the investigated area, whereas the number of eggs produced by the invasive females was significantly increased at the active front of the invasion. The maximum number of ovarian eggs found was 887 and 1156 in the indigenous species and the invasive species, respectively. We propose the scenario that the invasive species, which carries the deadly crayfish plague, creates an ecological advantage by reducing the populations of indigenous crayfish. Subsequently, the invasive females opportunistically use the available resources to enhance their fecundity, resulting in the acute growth of populations. However, the long-term competitiveness and colonisation success of O. limosus still remain in question.

Phylogeographic structure and northward range expansion in the barnacle Chthamalus fragilis

The barnacle Chthamalus fragilis is found along the US Atlantic seaboard historically from the Chesapeake Bay southward, and in the Gulf of Mexico. It appeared in New England circa 1900 coincident with warming temperatures, and is now a conspicuous member of rocky intertidal communities extending through the northern shore of Cape Cod, Massachusetts. The origin of northern C. fragilis is debated. It may have spread to New England from the northern end of its historic range through larval transport by ocean currents, possibly mediated by the construction of piers, marinas, and other anthropogenic structures that provided new hard substrate habitat. Alternatively, it may have been introduced by fouling on ships originating farther south in its historic distribution. Here we examine mitochondrial cytochrome c oxidase I sequence diversity and the distribution of mitochondrial haplotypes of C. fragilis from 11 localities ranging from Cape Cod, to Tampa Bay, Florida. We found significant genetic structure between northern and southern populations. Phylogenetic analysis revealed three well-supported reciprocally monophyletic haplogroups, including one haplogroup that is restricted to New England and Virginia populations. While the distances between clades do not suggest cryptic speciation, selection and dispersal barriers may be driving the observed structure. Our data are consistent with an expansion of C. fragilis from the northern end of its mid-19th century range into Massachusetts.

Phylogeography of a Marine Insular Endemic in the Atlantic Macaronesia: The Azorean Barnacle, Megabalanus azoricus (Pilsbry, 1916)

The Azorean barnacle, Megabalanus azoricus (Pilsbry, 1916), is a Macaronesian endemic whose obscure taxonomy and the unknown relationships among forms inhabiting isolated Northern Atlantic oceanic islands is investigated by means of molecular analysis herein. Mitochondrial data from the 16S rRNA and COX1 genes support its current species status, tropical ancestry, and the taxonomic homogeneity throughout its distribution range. In contrast, at the intraspecific level and based on control region sequences, we detected an overall low level of genetic diversity and three divergent lineages. The haplogroups α and γ were sampled in the Azores, Madeira, Canary, and Cabo Verde archipelagos; whereas haplogroup β was absent from Cabo Verde. Consequently, population analysis suggested a differentiation of the Cabo Verde population with respect to the genetically homogenous northern archipelagos generated by current oceanographic barriers. Furthermore, haplogroup α, β, and γ demographic expansions occurred during the interglacial periods MIS5 (130 Kya - thousands years ago -), MIS3 (60 Kya), and MIS7 (240 Kya), respectively. The evolutionary origin of these lineages is related to its survival in the stable southern refugia and its demographic expansion dynamics are associated with the glacial-interglacial cycles. This phylogeographic pattern suggests the occurrence of genetic discontinuity informative to the delimitation of an informally defined biogeographic entity, Macaronesia, and its generation by processes that delineate genetic diversity of marine taxa in this area.

Ecological niche transferability using invasive species as a case study

Species distribution modeling is widely applied to predict invasive species distributions and species range shifts under climate change. Accurate predictions depend upon meeting the assumption that ecological niches are conserved, i.e., spatially or temporally transferable. Here we present a multi-taxon comparative analysis of niche conservatism using biological invasion events well documented in natural history museum collections. Our goal is to assess spatial transferability of the climatic niche of a range of noxious terrestrial invasive species using two complementary approaches. First we compare species' native versus invasive ranges in environmental space using two distinct methods, Principal Components Analysis and Mahalanobis distance. Second we compare species' native versus invaded ranges in geographic space as estimated using the species distribution modeling technique Maxent and the comparative index Hellinger's I. We find that species exhibit a range of responses, from almost complete transferability, in which the invaded niches completely overlap with the native niches, to a complete dissociation between native and invaded ranges. Intermediate responses included expansion of dimension attributable to either temperature or precipitation derived variables, as well as niche expansion in multiple dimensions. We conclude that the ecological niche in the native range is generally a poor predictor of invaded range and, by analogy, the ecological niche may be a poor predictor of range shifts under climate change. We suggest that assessing dimensions of niche transferability prior to standard species distribution modeling may improve the understanding of species' dynamics in the invaded range.

Control region sequences indicate that multiple externae represent multiple infections by Sacculina carcini (Cirripedia: Rhizocephala)

The rhizocephalan barnacle, Sacculina carcini, is a common parasite of the European shore crab, Carcinus maenas, in which it causes significant detrimental physical and behavioral modifications. In the vast majority of cases, the external portion of the parasite is present in the form of a single sac-like externa; in rare cases, double or even triple externae may occur on the same individual host. Here, we use a highly variable DNA marker, the mitochondrial control region (CR), to investigate whether multiple externae in S. carcini represent infection by multiple parasites or asexual cloning developed by a single parasite individual. Sequences for multiple externae from C. maenas hosts from the Danish inlet, Limfjorden, and from the mud flates at Roscoff, France, were compared. In almost all cases, double or triple externae from an individual host yielded different haplotypes. In the few cases where identical haplotypes were identified from externae on a multiple-infected host, this always represented the most commonly found haplotype in the population. This indicates that in Sacculina carcini, the presence of multiple externae on a single host reflects infection by different individual parasites. A haplotype network of CR sequences also suggests a degree of geographical partitioning, with no shared haplotypes between the Limfjorden and Roscoff. Our data represent the first complete CR sequences for a rhizocephalan, and a unique gene order was also revealed. Although the utility of CR sequences for population-level work must be investigated further, the CR has proved a simple to use and highly variable marker for studies of S. carcini and can easily be applied to a variety of studies in this important parasite.

Historical and recent processes shaping the geographic range of a rocky intertidal gastropod: phylogeography, ecology, and habitat availability

Factors shaping the geographic range of a species can be identified when phylogeographic patterns are combined with data on contemporary and historical geographic distribution, range-wide abundance, habitat/food availability, and through comparisons with codistributed taxa. Here, we evaluate range dynamism and phylogeography of the rocky intertidal gastropod Mexacanthina lugubris lugubris across its geographic range – the Pacific coast of the Baja peninsula and southern California. We sequenced mitochondrial DNA (CO1) from ten populations and compliment these data with museum records, habitat availability and range-wide field surveys of the distribution and abundance of M. l. lugubris and its primary prey (the barnacle Chthamalus fissus). The geographic range of M. l. lugubris can be characterized by three different events in its history: an old sundering in the mid-peninsular region of Baja (∼ 417,000 years ago) and more recent northern range expansion and southern range contraction. The mid-peninsular break is shared with many terrestrial and marine species, although M. l. lugubris represents the first mollusc to show it. This common break is often attributed to a hypothesized ancient seaway bisecting the peninsula, but for M. l. lugubris it may result from large habitat gaps in the southern clade. Northern clade populations, particularly near the historical northern limit (prior to the 1970s), have high local abundances and reside in a region with plentiful food and habitat – which makes its northern range conducive to expansion. The observed southern range contraction may result from the opposite scenario, with little food or habitat nearby. Our study highlights the importance of taking an integrative approach to understanding the processes that shape the geographic range of a species via combining range-wide phylogeography data with temporal geographic distributions and spatial patterns of habitat/food availability.

Rapid range expansion increases genetic differentiation while causing limited reduction in genetic diversity in a damselfly

Many ectothermic species are currently expanding their geographic range due to global warming. This can modify the population genetic diversity and structure of these species because of genetic drift during the colonization of new areas. Although the genetic signatures of historical range expansions have been investigated in an array of species, the genetic consequences of natural, contemporary range expansions have received little attention, with the only studies available focusing on range expansions along a narrow front. We investigate the genetic consequences of a natural range expansion in the Mediterranean damselfly Coenagrion scitulum, which is currently rapidly expanding along a broad front in different directions. We assessed genetic diversity and genetic structure using 12 microsatellite markers in five centrally located populations and five recently established populations at the edge of the geographic distribution. Our results suggest that, although a marginal significant decrease in the allelic richness was found in the edge populations, genetic diversity has been preserved during the range expansion of this species. Nevertheless, edge populations were genetically more differentiated compared with core populations, suggesting genetic drift during the range expansion. The smaller effective population sizes of the edge populations compared with central populations also suggest a contribution of genetic drift after colonization. We argue and document that range expansion along multiple axes of a broad expansion front generates little reduction in genetic diversity, yet stronger differentiation of the edge populations.

Dispersal at a snail's pace: historical processes affect contemporary genetic structure in the exploited wavy top snail (Megastraea undosa)

We used population genetics to assess historical and modern demography of the exploited wavy top snail, Megastraea undosa, which has a 5-10 day pelagic larval duration. Foot tissue was sampled from an average of 51 individuals at 17 sites across the range of M. undosa. Genetic structure at the mtDNA locus is strikingly high (ΦST of 0.19 across 1000 km), and a major cline occurs in northern Baja California (ΦCT of 0.29 between northern and southern populations). Genetic data indicate that the northern region is highly connected through larval dispersal, whereas the southern region exhibits low genetic structure. However, additional analyses based on patterns of haplotype diversity and relationships among haplotypes indicate that M. undosa has likely recently expanded into the Southern California Bight or expanded from a small refugial population, and analysis using isolation by distance to calculate dispersal distance indicates surprisingly short estimates of dispersal from 30 m to 3 km. This scenario of a northward expansion and limited larval dispersal is supported by coalescent-based simulations of genetic data. The different patterns of genetic variation between northern and southern populations are likely artifacts of evolutionary history rather than differences in larval dispersal and this may have applications to management of this species. Specifically, these data can help to inform the scale at which this species should be managed, and given the potentially very small dispersal distances, this species should be managed at local scales. Consideration of the evolutionary history of target species allows for a more accurate interpretation of genetic data for management.

Resilience to climate change in coastal marine ecosystems

Ecological resilience to climate change is a combination of resistance to increasingly frequent and severe disturbances, capacity for recovery and self-organization, and ability to adapt to new conditions. Here, we focus on three broad categories of ecological properties that underlie resilience: diversity, connectivity, and adaptive capacity. Diversity increases the variety of responses to disturbance and the likelihood that species can compensate for one another. Connectivity among species, populations, and ecosystems enhances capacity for recovery by providing sources of propagules, nutrients, and biological legacies. Adaptive capacity includes a combination of phenotypic plasticity, species range shifts, and microevolution. We discuss empirical evidence for how these ecological and evolutionary mechanisms contribute to the resilience of coastal marine ecosystems following climate change-related disturbances, and how resource managers can apply this information to sustain these systems and the ecosystem services they provide.

Introduction to the symposium--barnacle biology: essential aspects and contemporary approaches

Barnacles have evolved a number of specialized features peculiar for crustaceans: they produce a calcified, external shell; they exhibit sexual strategies involving dioecy and androdioecy; and some have become internal parasites of other Crustacea. The thoroughly sessile habit of adults also belies the highly mobile and complex nature of their larval stages. Given these and other remarkable innovations in their natural history, it is perhaps not surprising that barnacles present a spectrum of opportunities for study. This symposium integrates research on barnacles in the areas of larval biology, biofouling, reproduction, biogeography, speciation, population genetics, ecological genomics, and phylogenetics. Pioneering comparisons are presented of metamorphosis among barnacles from three major lineages. Biofouling is investigated from the perspectives of biochemical and biomechanical mechanisms. Tradeoffs in reproductive specializations are scrutinized through theoretical modeling and empirical validation. Patterns of endemism and diversity are delineated in Australia and intricate species boundaries in the genus Chthamalus are elucidated for the Indo-Pacific. General methodological concerns with population expansion studies in crustaceans are highlighted using barnacle models. Data from the first, draft barnacle genome are employed to examine location-specific selection. Lastly, barnacle evolution is framed in a deep phylogenetic context and hypothetical origins of defined characters are outlined and tested.

Examining an outlier: molecular diversity in the cirripedia

Despite the typical assumption in studies of mitochondrial diversity that such data are useful for approximating population size and demography, studies of sequence diversity in mitochondrial DNA across the Metazoa have shown a surprising excess of rare alleles, a pattern associated either with strong selection or population growth. Previous work has shown that this bias toward an excess of rare alleles is typical across the Crustacea, and in particular, in the Cirripedia (barnacles). Here, we directly evaluate sequence data from studies of barnacle populations to ensure that inclusion of cryptic species is not the cause of this pattern. The results shown here reinforce previous studies that suggest caution in interpreting such patterns of allele frequencies, as they are likely to be influenced both by demographic changes and selection.

Impacts associated with the recent range shift of the aeolid nudibranch Phidiana hiltoni (Mollusca, Opisthobranchia) in California

In 1977, Phidiana hiltoni (O'Donoghue in J. Entomol Zool (Pomona College, Claremont, California) 19:77-119, 1927) began spreading northward from Monterey, California. By 1992, it had reached Duxbury Reef (37° 53' 23″ N, 122° 41' 59″ W), 100 km to the north, where other nudibranchs subsequently appeared to decline. The role of P. hiltoni in this decline was investigated through diet analysis, feeding trials, and comparison of historical and recent abundance data. In the wild, P. hiltoni preyed largely on hydroids, but also showed evidence of predation on nudibranchs. In the laboratory, P. hiltoni attacked most of the dendronotid and aeolid nudibranchs presented to it, ingesting small individuals whole. The pooled abundance of nudibranchs vulnerable to attack by P. hiltoni declined an average of two-thirds at Duxbury Reef since its arrival, compared to (1) no change in the non-vulnerable species and (2) no change in either group at two other sites where P. hiltoni was one to two orders of magnitude less abundant. Phidianahiltoni therefore appears to have caused this decline, likely through a combination of direct predation and competition for prey. A brief larval period, combined with cyclonic re-circulation in the lee of Point Reyes, may be driving self-recruitment of P. hiltoni at Duxbury Reef, as well as hindering further northward spread.