Physiological energetics and biogeographic range limits of three congeneric mussel species

Patterns of admixture and introgression in a mosaic Mytilus galloprovincialis and Mytilus edulis hybrid zone in SW England

The study of hybrid zones offers important insights into speciation. Earlier studies on hybrid populations of the marine mussel species Mytilus edulis and Mytilus galloprovincialis in SW England provided evidence of admixture but were constrained by the limited number of molecular markers available. We use 57 ancestry-informative SNPs, most of which have been mapped genetically, to provide evidence of distinctive differences between admixed populations in SW England and asymmetrical introgression from M. edulis to M. galloprovincialis. We combine the genetic study with analysis of phenotypic traits of potential ecological and adaptive significance. We demonstrate that hybrid individuals have brown mantle edges unlike the white or purple in the parental species, suggesting allelic or non-allelic genomic interactions. We report differences in gonad development stage between the species consistent with a prezygotic barrier between the species. By incorporating results from publications dating back to 1980, we confirm the long-term stability of the hybrid zone despite higher viability of M. galloprovincialis. This stability coincides with a dramatic change in temperature of UK coastal waters and suggests that these hybrid populations might be resisting the effects of global warming. However, a single SNP locus associated with the Notch transmembrane signalling protein shows a markedly different pattern of variation to the others and might be associated with adaptation of M. galloprovincialis to colder northern temperatures.

Evolutionary Adaptation of Protein Turnover in White Muscle of Stenothermal Antarctic Fish: Elevated Cold Compensation at Reduced Thermal Responsiveness

Protein turnover is highly energy consuming and overall relates to an organism's growth performance varying largely between species, e.g., due to pre-adaptation to environmental characteristics such as temperature. Here, we determined protein synthesis rates and capacity of protein degradation in white muscle of the cold stenothermal Antarctic eelpout (Pachycara brachycephalum) and its closely related temperate counterpart, the eurythermal common eelpout (Zoarces viviparus). Both species were exposed to acute warming (P. brachycephalum, 0 °C + 2 °C day-1; Z. viviparus, 4 °C + 3 °C day-1). The in vivo protein synthesis rate (Ks) was monitored after injection of 13C-phenylalanine, and protein degradation capacity was quantified by measuring the activity of cathepsin D in vitro. Untargeted metabolic profiling by nuclear magnetic resonance (NMR) spectroscopy was used to identify the metabolic processes involved. Independent of temperature, the protein synthesis rate was higher in P. brachycephalum (Ks = 0.38-0.614 % day-1) than in Z. viviparus (Ks= 0.148-0.379% day-1). Whereas protein synthesis remained unaffected by temperature in the Antarctic species, protein synthesis in Z. viviparus increased to near the thermal optimum (16 °C) and tended to fall at higher temperatures. Most strikingly, capacities for protein degradation were about ten times higher in the Antarctic compared to the temperate species. These differences are mirrored in the metabolic profiles, with significantly higher levels of complex and essential amino acids in the free cytosolic pool of the Antarctic congener. Together, the results clearly indicate a highly cold-compensated protein turnover in the Antarctic eelpout compared to its temperate confamilial. Constant versus variable environments are mirrored in rigid versus plastic functional responses of the protein synthesis machinery.

Divergent Effects of Ocean Warming On Byssal Attachment in Two Congener Mussel Species

Organisms rely on the integrity of the structural materials they produce to maintain a broad range of processes, such as acquiring food, resisting predators or withstanding extreme environmental forces. The production and maintenance of these biomaterials, which are often modulated by environmental conditions, can therefore have important consequences for fitness in changing climates. One well-known example of such a biomaterial is mussel byssus, an array of collagen-like fibers (byssal threads) that tethers a bivalve mollusk securely to benthic marine substrates. Byssus strength directly influences mortality from dislodgement, predation or competition and depends on the quantity and quality of byssal threads produced. We compared the temperature sensitivity of byssal attachment strength of two mussel species common to the west coast of North America, Mytilus trossulus and M. galloprovincialis, when exposed to seawater temperatures ranging from 10 to 24˚C in the laboratory. We found the two species attached equally strong in seawater ≤ 18˚C, but higher temperatures caused byssal thread production rate and quality (break force and extensibility) to be greatly reduced in M. trossulus and increased in M. galloprovincialis, leading to a 2 to 10-fold difference in overall byssus strength between the two species. Using this threshold value (18˚C), we mapped habitat for each species along the west coast of North America based on annual patterns in sea surface temperature. Estimated ranges are consistent with the current distribution of the two species and suggest a potential mechanism by which ocean warming could facilitate the northern expansion of M. galloprovincialis and displacement of native M. trossulus populations.

Interactive effects of ZnO nanoparticles and temperature on molecular and cellular stress responses of the blue mussel Mytilus edulis

Temperature is an important abiotic factor that modulates all aspects of ectotherm physiology, including sensitivity to pollutants. Nanoparticles are emerging pollutants in coastal environments, and their potential to cause toxicity in marine organisms is a cause for concern. Here we studied the interactive effects of temperature (including seasonal and experimental warming) on sublethal toxicity of ZnO nanoparticles (nano-ZnO) in a model marine bivalve, the blue mussel Mytilus edulis. Molecular markers were used to assess the pollutant-induced cellular stress responses in the gills and the digestive gland of mussels exposed for 21 days to 10 μg l^-1 and 100 μg l^-1 of nano-ZnO or dissolved Zn under different temperature regimes including ambient temperature (10 °C and 15 °C in winter and summer, respectively) or experimental warming (+5 °C). Exposure to high concentration (100 μg l^-1) of nano-ZnO caused oxidative injury to proteins and lipids and induced a marked apoptotic response indicated by increased transcript levels of apoptosis-related genes p53, caspase 3 and the MAPK pathway (JNK and p38) and decreased mRNA expression of anti-apoptotic Bcl-2. No significant induction of inflammatory cytokine-related response (TGF-β and NF-κB) of tissues was observed in nano-ZnO exposed-mussels. Furthermore, the oxidative injury and apoptotic response could differentiate the effects of nano-ZnO from those of dissolved Zn in the mussels. This study revealed that oxidative stress and stress-related transcriptional responses to nano-ZnO were strongly modified by warming and season in the mussels. No single biomarker could be shown to consistently respond to nano-ZnO in all experimental groups, which implies that multiple biomarkers are needed to assess nano-ZnO toxicity to marine organisms under the variable environmental conditions of coastal habitats.

Two lineages of bivalve transmissible neoplasia affect the blue mussel Mytilus trossulus Gould in the subarctic Sea of Okhotsk

There are increasing findings of the bivalve transmissible neoplasia derived from the Pacific mussel Mytilus trossulus (MtrBTN) in populations of different Mytilus species worldwide. The Subarctic is an area where this disease has not yet been sought despite the fact that Mytilus spp. are widespread there, and M. trossulus itself is a boreal species. We used flow cytometry of the hemolymph, hemocytology and histology to diagnose disseminated neoplasia in a sample of M. trossulus from Magadan in the subarctic Sea of Okhotsk. Neoplasia was identified in 11 of 214 mussels studied. Using mtDNA COI sequencing, we revealed genotypes identical or nearly identical to known MtrBTN ones in the hemolymph of most of the diseased mussels. Both MtrBTN evolutionary lineages have been identified, the widespread MtrBTN2, and MtrBTN1, so far only known from M. trossulus in British Columbia on the other side of the Pacific from Magadan. In addition, MtrBTN2 was represented by two common diverged mtDNA haplolineages. These conclusions were confirmed for selected cancerous mussels by molecular cloning of COI and additional nuclear and mtDNA genes. On the background of high genetic diversity, different cancers were similar in terms of ploidy (range 4.0 - 5.8n) and nuclear to cell ratio. Our study provides the first description of neoplasia and MtrBTN in mussels from the Sea of Okhotsk and from the Subarctic, of both MtrBTN1 and MtrBTN2 in the same mussel population, and the first direct comparison between these transmissible cancers.

Does the environmental history of mussels have an effect on the physiological response to additional stress under experimental conditions?

Expected effects on marine biota of the ongoing elevation of water temperature and high latitudes is of major concern when considering the reliability of coastal ecosystem production. To compare the capacity of coastal organisms to cope with a temperature increase depending on their environmental history, responses of adult blue mussels (Mytilus spp.) taken from two sites differentially exposed to chemical pollution were investigated during an experimental exposure to a thermal stress. Immune parameters were notably altered by extreme warming and transcriptional changes for a broad selection of genes were associated to the temperature increase following a two-step response pattern. Site-specific responses suggested an influence of environmental history and support the possibility of a genetic basis in the physiological response. However no meaningful difference was detected between the response of hybrids and M galloprovincialis. This study brings new information about the capacity of mussels to cope with the ongoing elevation of water temperature in these coastal ecosystems.

Mytilus trossulus introgression and consequences for shell traits in longline cultivated mussels

Mussels belonging to the Mytilus species complex (M. edulis, ME; M. galloprovincialis, MG; and M. trossulus, MT) often occur in sympatry, facilitating introgressive hybridization. This may be further promoted by mussel aquaculture practices, with MT introgression often resulting in commercially unfavourable traits such as low meat yield and weak shells. To investigate the relationship between genotype and shell phenotype, genetic and morphological variability was quantified across depth (1 m to 7 m) along a cultivation rope at a mussel farm on the West coast of Scotland. A single nuclear marker (Me15/16) and a novel panel of 33 MT-diagnostic single nucleotide polymorphisms were used to evaluate stock structure and the extent of MT introgression across depth. Variation in shell strength, determined as the maximum compression force for shell puncture, and shell shape using geometric morphometric analysis were evaluated in relation to cultivation depth and the genetic profiles of the mussels. Overall, ME was the dominant genotype across depth, followed by ME × MG hybrids and smaller quantities of ME × MT hybrids and pure MT individuals. In parallel, we identified multiple individuals that were either predominantly homozygous or heterozygous for MT-diagnostic alleles, likely representing pure MT and first-generation ME × MT hybrids, respectively. Both the proportion of individuals carrying MT alleles and MT allele frequency declined with depth. Furthermore, MT-introgressed individuals had significantly weaker and more elongate shells than nonintrogressed individuals. This study provides detailed insights into stock structure along a cultivation rope and suggests that practical methods to assess shell strength and shape of cultivated mussels may facilitate the rapid identification of MT, limiting the impact of this commercially damaging species.

Resilience in Greenland intertidal Mytilus: The hidden stress defense

The Arctic is experiencing particularly rapid rates of warming, consequently invasive boreal species are now able to survive the less extreme Arctic winter temperatures. Whilst persistence of intertidal and terrestrial species in the Arctic is primarily determined by their ability to tolerate the freezing winters, air temperatures in the Arctic summer can reach 36 °C in the intertidal, which is beyond the upper thermal limits of many marine species. This is normally lethal for the conspicuous ecosystem engineer Mytilus edulis. Transcriptomic analyses were undertaken on both in situ collected and experimentally warmed animals to understand whether M. edulis is able to tolerate these very high summer temperatures. Surprisingly there was no significant enrichment for Gene Ontology terms (GO) when comparing the inner and outer fjord intertidal animals with outer fjord subtidal (control) animals, representing animals collected at 27 °C, 19 °C and 3 °C respectively. This lack of differentiation indicated a wide acclimation ability in this species. Conversely, significant enrichment for processes such as signal transduction, cytoskeleton and cellular protein modification was identified in the expression profiles of the 22 °C and 32 °C experimentally heated animals. This difference in gene expression between in situ collected and experimentally warmed animals was almost certainly due to the former being acclimated to a fluctuating, but predictable, temperature regime, which has increased their thermal tolerances. Interestingly, there was no evidence for enrichment of the classical cellular stress response in any of the animals sampled. Identification of a massive expansion of the HSPA12 heat shock protein 70 kDa gene family presented the possibility of these genes acting as intertidal regulators underpinning thermal resilience. This expansion has resulted in a modified cellular stress response, as an evolutionary adaptation to the rigour of the invasive intertidal life style. Thus, M. edulis appear to have considerable capacity to withstand the current rates of Arctic warming, and the very large attendant thermal variation.

Bioenergetics in environmental adaptation and stress tolerance of aquatic ectotherms: linking physiology and ecology in a multi-stressor landscape

Energy metabolism (encompassing energy assimilation, conversion and utilization) plays a central role in all life processes and serves as a link between the organismal physiology, behavior and ecology. Metabolic rates define the physiological and life-history performance of an organism, have direct implications for Darwinian fitness, and affect ecologically relevant traits such as the trophic relationships, productivity and ecosystem engineering functions. Natural environmental variability and anthropogenic changes expose aquatic ectotherms to multiple stressors that can strongly affect their energy metabolism and thereby modify the energy fluxes within an organism and in the ecosystem. This Review focuses on the role of bioenergetic disturbances and metabolic adjustments in responses to multiple stressors (especially the general cellular stress response), provides examples of the effects of multiple stressors on energy intake, assimilation, conversion and expenditure, and discusses the conceptual and quantitative approaches to identify and mechanistically explain the energy trade-offs in multiple stressor scenarios, and link the cellular and organismal bioenergetics with fitness, productivity and/or ecological functions of aquatic ectotherms.

Season-dependent effects of ZnO nanoparticles and elevated temperature on bioenergetics of the blue mussel Mytilus edulis

Input of ZnO nanoparticles (nZnO) from multiple sources have raised concerns about the potential toxic effects on estuarine and coastal organisms. The toxicity of nZnO and its interaction with common abiotic stressors (such as elevated temperature) are not well understood in these organisms. Here, we examined the bioenergetics responses of the blue mussel Mytilus edulis exposed for 21 days to different concentrations of nZnO or dissolved zinc (Zn²⁺) (0, 10, 100 μg l^-1) and two temperatures (ambient and 5 °C warmer) in winter and summer. Exposure to nZnO had little effect on the protein and lipid levels, but led to a significant depletion of carbohydrates and a decrease in the electron transport system (ETS) activity. Qualitatively similar but weaker effects were found for dissolved Zn. In winter mussels, elevated temperature (15 °C) led to elevated protein and lipid levels increasing the total energy content of the tissues. In contrast, elevated temperature (20 °C) resulted in a decrease in the lipid and carbohydrate levels and suppressed ETS in summer mussels. These data indicate that moderate warming in winter (but not in summer) might partially compensate for the bioenergetics stress caused by nZnO toxicity in M. edulis from temperate areas such as the Baltic Sea.

Detrimental impacts of climate change may be exacerbated by density-dependent population regulation in blue mussels

The climate on our planet is changing and the range distributions of organisms are shifting in response. In aquatic environments, species might not be able to redistribute poleward or into deeper water when temperatures rise because of barriers, reduced light availability, altered water chemistry or any combination of these. How species respond to climate change may depend on physiological adaptability, but also on the population dynamics of the species. Density dependence is a ubiquitous force that governs population dynamics and regulates population growth, yet its connections to the impacts of climate change remain little known, especially in marine studies. Reductions in density below an environmental carrying capacity may cause compensatory increases in demographic parameters and population growth rate, hence masking the impacts of climate change on populations. On the other hand, climate-driven deterioration of conditions may reduce environmental carrying capacities, making compensation less likely and populations more susceptible to the effects of stochastic processes. Here we investigate the effects of climate change on Baltic blue mussels using a 17-year dataset on population density. Using a Bayesian modelling framework, we investigate the impacts of climate change, assess the magnitude and effects of density dependence, and project the likelihood of population decline by the year 2030. Our findings show negative impacts of warmer and less saline waters, both outcomes of climate change. We also show that density dependence increases the likelihood of population decline by subjecting the population to the detrimental effects of stochastic processes (i.e. low densities where random bad years can cause local extinction, negating the possibility for random good years to offset bad years). We highlight the importance of understanding, and accounting for both density dependence and climate variation when predicting the impact of climate change on keystone species, such as the Baltic blue mussel.

Thermal sensitivity of cell metabolism of different Antarctic fish species mirrors organism temperature tolerance

Despite cold adaptation, Antarctic fish show lower growth than expected from the van’t Hoff’s Q10 rule. Protein synthesis is one of the main energy-consuming processes, which is downregulated under energy deficiency. Considering the effect of temperature on growth performance, we tested if temperature-dependent cellular energy allocation to protein synthesis correlates with temperature-dependent whole-animal growth and thus thermal tolerance. Cell respiration and energy expenditure for protein synthesis were determined in hepatocytes of the circumpolar-distributed Antarctic eelpout Pachycara brachycephalum after warm acclimation (0 °C vs 5 °C) and, of two notothenioids the sub-Antarctic Lepidonotothen squamifrons and the high-Antarctic icefish Chionodraco hamatus. We used intermittent-flow respirometry to analyse cellular response to acute warming from 5 to 10 °C (P. brachycephalum) and from 1 to 5 °C (L. squamifrons, C. hamatus). Warming-induced rise in respiration was similar between 0- and 5 °C-acclimated P. brachycephalum and between L. squamifrons and C. hamatus. Irrespective of acclimation, warming decreased energy expenditure for protein synthesis in P. brachycephalum, which corresponds to reduced whole-animal growth at temperatures > 5 °C. Warming doubled energy expenditure for protein synthesis in L. squamifrons but had no effect on C. hamatus indicating that L. squamifrons might benefit from warmer waters. The species-specific temperature effect on energy expenditure for protein synthesis is discussed to mirror thermal sensitivity of whole-animal growth performance, thereby paralleling the degree of cold adaptation. Clearly more data are necessary including measurements at narrower temperature steps particularly for C. hamatus and an increased species’ number per ecotype to reinforce presented link between cellular and whole-animal thermal sensitivity.

Replicated anthropogenic hybridisations reveal parallel patterns of admixture in marine mussels

Human-mediated transport creates secondary contacts between genetically differentiated lineages, bringing new opportunities for gene exchange. When similar introductions occur in different places, they provide informally replicated experiments for studying hybridisation. We here examined 4,279 Mytilus mussels, sampled in Europe and genotyped with 77 ancestry-informative markers. We identified a type of introduced mussels, called "dock mussels," associated with port habitats and displaying a particular genetic signal of admixture between M. edulis and the Mediterranean lineage of M. galloprovincialis. These mussels exhibit similarities in their ancestry compositions, regardless of the local native genetic backgrounds and the distance separating colonised ports. We observed fine-scale genetic shifts at the port entrance, at scales below natural dispersal distance. Such sharp clines do not fit with migration-selection tension zone models, and instead suggest habitat choice and early-stage adaptation to the port environment, possibly coupled with connectivity barriers. Variations in the spread and admixture patterns of dock mussels seem to be influenced by the local native genetic backgrounds encountered. We next examined departures from the average admixture rate at different loci, and compared human-mediated admixture events, to naturally admixed populations and experimental crosses. When the same M. galloprovincialis background was involved, positive correlations in the departures of loci across locations were found; but when different backgrounds were involved, no or negative correlations were observed. While some observed positive correlations might be best explained by a shared history and saltatory colonisation, others are likely produced by parallel selective events. Altogether, genome-wide effect of admixture seems repeatable and more dependent on genetic background than environmental context. Our results pave the way towards further genomic analyses of admixture, and monitoring of the spread of dock mussels both at large and at fine spacial scales.

Effects of osmotic and thermal shock on the invasive aquatic mudsnail Potamopyrgus antipodarum: mortality and physiology under stressful conditions

Invasive freshwater species, such as the exotic mollusc Potamopyrgus antipodarum (New Zealand mudsnail), can frequently survive under harsh conditions, including brackish and hypoxic environments. We experimentally assessed the effects of osmotic (0, 10, 20, 25 and 30 psu) and thermal (20 °C) shock on mortality, activity and physiology of P. antipodarum collected at Capitol Lake, Olympia, Washington, USA, during winter and spring seasons when environmental temperature was 5 and 10 °C respectively. We measured standard metabolic rate and enzymatic activities (malate dehydrogenase, lactate dehydrogenase, alanopine dehydrogenase) in snails after a 10-day acclimation period at high salinity. Significantly higher mortalities were observed at higher salinities; the strongest effects occurred on snails collected at the end of winter, and exposed to 30 psu and 20 °C (100% mortality in 3 days). When snails were collected during the spring, 100% mortality was observed after 40 days at 30 psu and 20 °C. Standard metabolic rates were significantly lower when snails were exposed to salinities of 25 and 30 psu, even after 10 days of acclimation. Enzymatic activities showed small but significant declines after 10 days at 30 psu reflecting the declines observed in overall metabolism. The physiological tolerances to temperature and salinity displayed by this population of P. antipodarum make its eradication from Capital Lake difficult to achieve.

Local cold adaption increases the thermal window of temperate mussels in the Arctic

Species expand towards higher latitudes in response to climate warming, but the pace of this expansion is related to the physiological capacity to resist cold stress. However, few studies exist that have quantified the level of inter-population local adaptation in marine species freeze tolerance, especially in the Arctic. We investigated the importance of cold adaptation and thermal window width towards high latitudes from the temperate to the Arctic region. We measured upper and lower lethal air temperatures (i.e. LT and LT50) in temperate and Arctic populations of blue mussels (Mytilus edulis), and analysed weather data and membrane fatty acid compositions, following emersion simulations. Both populations had similar upper LT (~38 °C), but Arctic mussels survived 4°C colder air temperatures than temperate mussels (-13 vs. -9°C, respectively), corresponding to an 8% increase in their thermal window. There were strong latitudinal relationships between thermal window width and local air temperatures, indicating Arctic mussels are highly adapted to the Arctic environment where the seasonal temperature span exceeds 60°C. Local adaptation and local habitat heterogeneity thus allow leading-edge M. edulis to inhabit high Arctic intertidal zones. This intraspecific pattern provides insight into the importance of accounting for cold adaptation in climate change, conservation and biogeographic studies.

Assessing the sensitivity of bivalve populations to global warming using an individual-based modelling approach

Climate change exposes benthic species populations in coastal ecosystems to a combination of different stressors (e.g., warming, acidification and eutrophication), threatening the sustainability of the ecological functions they provide. Thermal stress appears to be one of the strongest drivers impacting marine ecosystems, acting across a wide range of scales, from individual metabolic performances to geographic distribution of populations. Accounting for and integrating the response of species functional traits to thermal stress is therefore a necessary step in predicting how populations will respond to the warming expected in coming decades. Here, we developed an individual-based population model using a mechanistic formulation of metabolic processes within the framework of the dynamic energy budget theory. Through a large number of simulations, we assessed the sensitivity of population growth potential to thermal stress and food conditions based on a climate projection scenario (Representative Concentration Pathway; RCP8.5: no reduction of greenhouse gas emissions). We focused on three bivalve species with contrasting thermal tolerance ranges and distinct distribution ranges along 5,000 km of coastline in the NE Atlantic: the Pacific oyster (Magallana gigas), and two mussel species: Mytilus edulis and Mytilus galloprovincialis. Our results suggest substantial and contrasting changes within species depending on local temperature and food concentration. Reproductive phenology appeared to be a core process driving the responses of the populations, and these patterns were closely related to species thermal tolerances. The nonlinear relationship we found between individual life-history traits and response at the population level emphasizes the need to consider the interactions resulting from upscaling across different levels of biological organisation. These results underline the importance of a process-based understanding of benthic population response to seawater warming, which will be necessary for forward planning of resource management and strategies for conservation and adaptation to environmental changes.

Estimation of fitness from energetics and life-history data: An example using mussels

Changing environments have the potential to alter the fitness of organisms through effects on components of fitness such as energy acquisition, metabolic cost, growth rate, survivorship, and reproductive output. Organisms, on the other hand, can alter aspects of their physiology and life histories through phenotypic plasticity as well as through genetic change in populations (selection). Researchers examining the effects of environmental variables frequently concentrate on individual components of fitness, although methods exist to combine these into a population level estimate of average fitness, as the per capita rate of population growth for a set of identical individuals with a particular set of traits. Recent advances in energetic modeling have provided excellent data on energy intake and costs leading to growth, reproduction, and other life-history parameters; these in turn have consequences for survivorship at all life-history stages, and thus for fitness. Components of fitness alone (performance measures) are useful in determining organism response to changing conditions, but are often not good predictors of fitness; they can differ in both form and magnitude, as demonstrated in our model. Here, we combine an energetics model for growth and allocation with a matrix model that calculates population growth rate for a group of individuals with a particular set of traits. We use intertidal mussels as an example, because data exist for some of the important energetic and life-history parameters, and because there is a hypothesized energetic trade-off between byssus production (affecting survivorship), and energy used for growth and reproduction. The model shows exactly how strong this trade-off is in terms of overall fitness, and it illustrates conditions where fitness components are good predictors of actual fitness, and cases where they are not. In addition, the model is used to examine the effects of environmental change on this trade-off and on both fitness and on individual fitness components.

Genetic diversity and connectivity within Mytilus spp. in the subarctic and Arctic

Climate changes in the Arctic are predicted to alter distributions of marine species. However, such changes are difficult to quantify because information on present species distribution and the genetic variation within species is lacking or poorly examined. Blue mussels, Mytilus spp., are ecosystem engineers in the coastal zone globally. To improve knowledge of distribution and genetic structure of the Mytilus edulis complex in the Arctic, we analyzed 81 SNPs in 534 Mytilus spp. individuals sampled at 13 sites to provide baseline data for distribution and genetic variation of Mytilus mussels in the European Arctic. Mytilus edulis was the most abundant species found with a clear genetic split between populations in Greenland and the Eastern Atlantic. Surprisingly, analyses revealed the presence of Mytilus trossulus in high Arctic NW Greenland (77°N) and Mytilus galloprovincialis or their hybrids in SW Greenland, Svalbard, and the Pechora Sea. Furthermore, a high degree of hybridization and introgression between species was observed. Our study highlights the importance of distinguishing between congener species, which can display local adaptation and suggests that information on dispersal routes and barriers is essential for accurate predictions of regional susceptibility to range expansions or invasions of boreal species in the Arctic.

Physiologically grounded metrics of model skill: a case study estimating heat stress in intertidal populations

Models of ecological responses to climate change fundamentally assume that predictor variables, which are often measured at large scales, are to some degree diagnostic of the smaller-scale biological processes that ultimately drive patterns of abundance and distribution. Given that organisms respond physiologically to stressors, such as temperature, in highly non-linear ways, small modelling errors in predictor variables can potentially result in failures to predict mortality or severe stress, especially if an organism exists near its physiological limits. As a result, a central challenge facing ecologists, particularly those attempting to forecast future responses to environmental change, is how to develop metrics of forecast model skill (the ability of a model to predict defined events) that are biologically meaningful and reflective of underlying processes. We quantified the skill of four simple models of body temperature (a primary determinant of physiological stress) of an intertidal mussel, Mytilus californianus, using common metrics of model performance, such as root mean square error, as well as forecast verification skill scores developed by the meteorological community. We used a physiologically grounded framework to assess each model's ability to predict optimal, sub-optimal, sub-lethal and lethal physiological responses. Models diverged in their ability to predict different levels of physiological stress when evaluated using skill scores, even though common metrics, such as root mean square error, indicated similar accuracy overall. Results from this study emphasize the importance of grounding assessments of model skill in the context of an organism's physiology and, especially, of considering the implications of false-positive and false-negative errors when forecasting the ecological effects of environmental change.

Spatio-Temporal Variation in Effects of Upwelling on the Fatty Acid Composition of Benthic Filter Feeders in the Southern Benguela Ecosystem: Not All Upwelling Is Equal

Variability in mesoscale nearshore oceanographic conditions plays an important role in the distribution of primary production and food availability for intertidal consumers. Advection of nutrient rich waters by upwelling usually allows the proliferation of diatoms, later replaced by dinoflagellates. We examined upwelling effects on the fatty acid (FA) signature of a benthic intertidal filter feeder to identify its response to pulsed variability in food availability. The study took place in two contrasting seasons and at two upwelling and two non-upwelling sites interspersed within the southern Benguela upwelling system of South Africa. We investigated the FA composition of the adductor muscles and gonads of the mussel Mytilus galloprovincialis to assess how FA are apportioned to the different tissues and whether this changes between upwelling and non-upwelling conditions. In situ temperature loggers used to identify upwelling conditions at the four sites indicated that such events occurred only at the upwelling centres and only in summer. Tissues differed strongly, with gonads presenting a higher proportion of essential FAs. This could reflect the faster turnover rate of gonad tissue or preferential retention of specific FA for reproductive purposes. FA composition did not vary as a direct function of upwelling, but there were strong dissimilarities among sites. Upwelling influenced mussel diets at one upwelling site while at the other, the expected signature of upwelling was displaced downstream of the core of upwelling. Condition Index (CI) and Gonad Index (GI) differed among sites and were not influenced by upwelling, with GI being comparable among sites. In addition, FA proportions were consistent among sites, indicating similar food quality and quantity over time and under upwelling and non-upwelling conditions. This suggests that the influence of upwelling on the west coast of South Africa is pervasive and diffuse, rather than discrete; while nearshore retention or advection of upwelled water is critical and site-specific so that the effects of upwelling differ even among sites categorised as upwelling centres.

Mytilus hybridisation and impact on aquaculture: A minireview

The three species in the blue mussel complex (Mytilus edulis, Mytilus galloprovincialis and Mytilus trossulus) show varying levels of hybridisation wherever they occur sympatrically. The spatial variation in hybridisation patterns is potentially governed by environmental conditions, larval dispersal and aquaculture practices. Commercial mussel cultivation has been shown to increase hybridisation through introduction of non-native species or spat transfer. There is evidence that mussel cultivation may promote commercially less desirable phenotypes (e.g. fragile shells), however, to what extent hybridisation impacts aquaculture is currently not clear. The aim of this review is to summarize the available information on Mytilus hybridisation patterns in Europe and their promotion through aquaculture practices in order to shed light on the overall implications for the aquaculture industry.

Genetic, Ecological and Morphological Distinctness of the Blue Mussels Mytilus trossulus Gould and M. edulis L. in the White Sea

Two blue mussel lineages of Pliocene origin, Mytilus edulis (ME) and M. trossulus (MT), co-occur and hybridize in several regions on the shores of the North Atlantic. The two species were distinguished from each other by molecular methods in the 1980s, and a large amount of comparative data on them has been accumulated since that time. However, while ME and MT are now routinely distinguished by various genetic markers, they tend to be overlooked in ecological studies since morphological characters for taxonomic identification have been lacking, and no consistent habitat differences between lineages have been reported. Surveying a recently discovered area of ME and MT co-occurrence in the White Sea and employing a set of allozyme markers for identification, we address the issue whether ME and MT are true biological species with distinct ecological characteristics or just virtual genetic entities with no matching morphological and ecological identities. We find that: (1) in the White Sea, the occurrence of MT is largely concentrated in harbors, in line with observations from other subarctic regions of Europe; (2) mixed populations of ME and MT are always dominated by purebred individuals, animals classified as hybrids constituting only ca. 18%; (3) in terms of shell morphology, 80% of MT bear a distinct uninterrupted dark prismatic strip under the ligament while 97% of ME lack this character; (4) at sites of sympatry MT is more common on algal substrates while ME mostly lives directly on the bottom. This segregation by the substrate may contribute to maintaining reproductive isolation and decreasing competition between taxa. We conclude that while ME and MT are not fully reproductively isolated, they do represent clearly distinguishable biological, ecological and morphological entities in the White Sea. It remains to be documented whether the observed morphological and ecological differences are of a local character, or whether they have simply been overlooked in other contact zones.

The environmentally tuned transcriptomes of Mytilus mussels

Transcriptomics is a powerful tool for elucidating the molecular mechanisms that underlie the ability of organisms to survive and thrive in dynamic and changing environments. Here, we review the major contributions in this field, and we focus on studies of mussels in the genus Mytilus, which are well-established models for the study of ecological physiology in fluctuating environments. Our review is organized into four main sections. First, we illustrate how the abiotic forces of the intertidal environment drive the rhythmic coupling of gene expression to diel and tidal cycles in Mytilus californianus. Second, we discuss the challenges and pitfalls of conducting transcriptomic studies in field-acclimatized animals. Third, we examine the link between transcriptomic responses to environmental stress and biogeographic distributions in blue mussels, Mytilus trossulus and Mytilus galloprovincialis. Fourth, we present a comparison of transcriptomic datasets and identify 175 genes that share common responses to heat stress across Mytilus species. Taken together, these studies demonstrate that transcriptomics can provide an informative snapshot of the physiological state of an organism within an environmental context. In a comparative framework, transcriptomics can reveal how natural selection has shaped patterns of transcriptional regulation that may ultimately influence biogeography.

Cold adaptation shapes the robustness of metabolic networks in Drosophila melanogaster

When ectotherms are exposed to low temperatures, they enter a cold-induced coma (chill coma) that prevents resource acquisition, mating, oviposition, and escape from predation. There is substantial variation in time taken to recover from chill coma both within and among species, and this variation is correlated with habitat temperatures such that insects from cold environments recover more quickly. This suggests an adaptive response, but the mechanisms underlying variation in recovery times are unknown, making it difficult to decisively test adaptive hypotheses. We use replicated lines of Drosophila melanogaster selected in the laboratory for fast (hardy) or slow (susceptible) chill-coma recovery times to investigate modifications to metabolic profiles associated with cold adaptation. We measured metabolite concentrations of flies before, during, and after cold exposure using nuclear magnetic resonance (NMR) spectroscopy to test the hypotheses that hardy flies maintain metabolic homeostasis better during cold exposure and recovery, and that their metabolic networks are more robust to cold-induced perturbations. The metabolites of cold-hardy flies were less cold responsive and their metabolic networks during cold exposure were more robust, supporting our hypotheses. Metabolites involved in membrane lipid synthesis, tryptophan metabolism, oxidative stress, energy balance, and proline metabolism were altered by selection on cold tolerance. We discuss the potential significance of these alterations.

Mussels as a model system for integrative ecomechanics

Mussels form dense aggregations that dominate temperate rocky shores, and they are key aquaculture species worldwide. Coastal environments are dynamic across a broad range of spatial and temporal scales, and their changing abiotic conditions affect mussel populations in a variety of ways, including altering their investments in structures, physiological processes, growth, and reproduction. Here, we describe four categories of ecomechanical models (biochemical, mechanical, energetic, and population) that we have developed to describe specific aspects of mussel biology, ranging from byssal attachment to energetics, population growth, and fitness. This review highlights how recent advances in these mechanistic models now allow us to link them together across molecular, material, organismal, and population scales of organization. This integrated ecomechanical approach provides explicit and sometimes novel predictions about how natural and farmed mussel populations will fare in changing climatic conditions.

Metabolic physiology of the invasive clam, Potamocorbula amurensis: the interactive role of temperature, salinity, and food availability

In biological systems energy serves as the ultimate commodity, often determining species distributions, abundances, and interactions including the potential impact of invasive species on native communities. The Asian clam Potamocorbula amurensis invaded the San Francisco Estuary (SFE) in 1986 and is implicated in the decline of native fish species through resource competition. Using a combined laboratory/field study we examined how energy expenditure in this clam is influenced by salinity, temperature and food availability. Measures of metabolism were made at whole organism (metabolic rate) and biochemical (pyruvate kinase (PK) and citrate synthase (CS) enzyme activities) levels. We found in the field, over the course of a year, the ratio of PK to CS was typically 1.0 suggesting that aerobic and fermentative metabolism were roughly equivalent, except for particular periods characterized by low salinity, higher temperatures, and intermediate food availabilities. In a 30-day laboratory acclimation experiment, however, neither metabolic rate nor PK:CS ratio was consistently influenced by the same variables, though the potential for fermentative pathways did predominate. We conclude that in field collected animals, the addition of biochemical measures of energetic state provide little additional information to the previously measured whole organism metabolic rate. In addition, much of the variation in the laboratory remained unexplained and additional variables, including reproductive stage or body condition may influence laboratory-based results. Further study of adult clams must consider the role of organismal condition, especially reproductive state, in comparisons of laboratory experiments and field observations.

Climate change, species distribution models, and physiological performance metrics: predicting when biogeographic models are likely to fail

Modeling the biogeographic consequences of climate change requires confidence in model predictions under novel conditions. However, models often fail when extended to new locales, and such instances have been used as evidence of a change in physiological tolerance, that is, a fundamental niche shift. We explore an alternative explanation and propose a method for predicting the likelihood of failure based on physiological performance curves and environmental variance in the original and new environments. We define the transient event margin (TEM) as the gap between energetic performance failure, defined as CT_max, and the upper lethal limit, defined as LT_max. If TEM is large relative to environmental fluctuations, models will likely fail in new locales. If TEM is small relative to environmental fluctuations, models are likely to be robust for new locales, even when mechanism is unknown. Using temperature, we predict when biogeographic models are likely to fail and illustrate this with a case study. We suggest that failure is predictable from an understanding of how climate drives nonlethal physiological responses, but for many species such data have not been collected. Successful biogeographic forecasting thus depends on understanding when the mechanisms limiting distribution of a species will differ among geographic regions, or at different times, resulting in realized niche shifts. TEM allows prediction of the likelihood of such model failure.