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

Estimating blue mussel (Mytilus edulis) connectivity and settlement capacity in mid-latitude fjord regions

The mussel industry faces challenges such as low and inconsistent levels of larvae settlement and poor-quality spat, leading to variable production. However, mussel farming remains a vital sustainable and environmentally responsible method for producing protein, fostering ecological responsibility in the aquaculture sector. We investigate the population connectivity and larval dispersion of blue mussels (Mytilus edulis) in Scottish waters, as a case study, using a multidisciplinary approach that combined genetic data and particle modelling. This research allows us to develop a thorough understanding of blue mussel population dynamics in mid-latitude fjord regions, to infer gene-flow patterns, and to estimate population divergence. Our findings reveal a primary south-to-north particle transport direction and the presence of five genetic clusters. We discover a significant and continuous genetic material exchange among populations within the study area, with our biophysical model's outcomes aligning with our genetic observations. Additionally, our model reveals a robust connection between the southwest coast and the rest of the west coast. This study will guide the preservation of mussel farming regions, ensuring sustainable populations that contribute to marine ecosystem health and resilience.

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.

SNP genotyping revealed a hybrid zone between Mytilus chilensis and M. platensis in southern South America (the Strait of Magellan, Isla Grande de Tierra del Fuego and the Falkland Islands)

Hybrid zones among mussel species have been extensively studied in the northern hemisphere. In South America, it has only recently become possible to study the natural hybrid zones, due to the clarification of the taxonomy of native mussels of the Mytilus genus. Analysing 54 SNP markers, we show the genetic species composition and admixture in the hybrid zone between M. chilensis and M. platensis in the southern end of South America. Bayesian, non-Bayesian clustering and re-assignment algorithms showed that the natural hybrid zone between M. chilensis and M. platensis in the Strait of Magellan, Isla Grande de Tierra del Fuego and the Falkland Islands shows clinal architecture. The hybrid zone can be divided into three different areas: the first one is on the Atlantic coast where only pure M. platensis and hybrid were found. In the second one, inside the Strait of Magellan, pure individuals of both species and mussels with variable degrees of hybridisation coexist. In the last area at the Strait in front of Punta Arenas City, fjords on the Isla Grande de Tierra del Fuego, and at the Beagle Channel, only M. chilensis and a low number of hybrids were found. According to the proportion of hybrids, bays with protected conditions away from strong currents would give better conditions for hybridisation. We do not find evidence of any other mussel species such as M. edulis, M. galloprovincialis, M. planulatus or M. trossulus in the zone.

An evolutionarily distinct ringed seal in the Ilulissat Icefjord

The Earth's polar regions are low rates of inter- and intraspecific diversification. An extreme mammalian example is the Arctic ringed seal (Pusa hispida hispida), which is assumed to be panmictic across its circumpolar Arctic range. Yet, local Inuit communities in Greenland and Canada recognize several regional variants; a finding supported by scientific studies of body size variation. It is however unclear whether this phenotypic variation reflects plasticity, morphs or distinct ecotypes. Here, we combine genomic, biologging and survey data, to document the existence of a unique ringed seal ecotype in the Ilulissat Icefjord (locally 'Kangia'), Greenland; a UNESCO World Heritage site, which is home to the most productive marine-terminating glacier in the Arctic. Genomic analyses reveal a divergence of Kangia ringed seals from other Arctic ringed seals about 240 kya, followed by secondary contact since the Last Glacial Maximum. Despite ongoing gene flow, multiple genomic regions appear under strong selection in Kangia ringed seals, including candidate genes associated with pelage coloration, growth and osmoregulation, potentially explaining the Kangia seal's phenotypic and behavioural uniqueness. The description of 'hidden' diversity and adaptations in yet another Arctic species merits a reassessment of the evolutionary processes that have shaped Arctic diversity and the traditional view of this region as an evolutionary freezer. Our study highlights the value of indigenous knowledge in guiding science and calls for efforts to identify distinct populations or ecotypes to understand how these might respond differently to environmental change.

SNP discovery and genetic structure in blue mussel species using low coverage sequencing and a medium density 60 K SNP-array

Blue mussels from the genus Mytilus are an abundant component of the benthic community, found in the high latitude habitats. These foundation species are relevant to the aquaculture industry, with over 2 million tonnes produced globally each year. Mussels withstand a wide range of environmental conditions and species from the Mytilus edulis complex readily hybridize in regions where their distributions overlap. Significant effort has been made to investigate the consequences of environmental stress on mussel physiology, reproductive isolation, and local adaptation. Yet our understanding on the genomic mechanisms underlying such processes remains limited. In this study, we developed a multi species medium-density 60 K SNP-array including four species of the Mytilus genus. SNPs included in the platform were called from 138 mussels from 23 globally distributed mussel populations, sequenced using a whole-genome low coverage approach. The array contains polymorphic SNPs which capture the genetic diversity present in mussel populations thriving across a gradient of environmental conditions (~59 K SNPs) and a set of published and validated SNPs informative for species identification and for diagnosis of transmissible cancer (610 SNPs). The array will allow the consistent genotyping of individuals, facilitating the investigation of ecological and evolutionary processes in these taxa. The applications of this array extend to shellfish aquaculture, contributing to the optimization of this industry via genomic selection of blue mussels, parentage assignment, inbreeding assessment and traceability. Further applications such as genome wide association studies (GWAS) for key production traits and those related to environmental resilience are especially relevant to safeguard aquaculture production under climate change.

Evidence of separate influence of moon and sun on light synchronization of mussel's daily rhythm during the polar night

Marine organisms living at high latitudes are faced with a light climate that undergoes drastic annual changes, especially during the polar night (PN) when the sun remains below the horizon for months. This raises the question of a possible synchronization and entrainment of biological rhythms under the governance of light at very low intensities. We analyzed the rhythms of the mussel Mytilus sp. during PN. We show that (1) mussels expressed a rhythmic behavior during PN; (2) a monthly moonlight rhythm was expressed; (3) a daily rhythm was expressed and influenced by both sunlight and moonlight; and (4) depending on the different times of PN and moon cycle characteristics, we were able to discriminate whether the moon or the sun synchronize the daily rhythm. Our findings fuel the idea that the capability of moonlight to synchronize daily rhythms when sunlight is not sufficient would be a crucial advantage during PN.

Hitchhiking into the Deep: How Microplastic Particles are Exported through the Biological Carbon Pump in the North Atlantic Ocean

Understanding residence times of plastic in the ocean is a major knowledge gap in plastic pollution studies. Observations report a large mismatch between plastic load estimates from worldwide production and disposal and actual plastics floating at the sea surface. Surveys of the water column, from the surface to the deep sea, are rare. Most recent work, therefore, addressed the "missing plastic" question using modeling or laboratory approaches proposing biofouling and degradation as the main removal processes in the ocean. Through organic matrices, plastic can affect the biogeochemical and microbial cycling of carbon and nutrients. For the first time, we provide in situ measured vertical fluxes of microplastics deploying drifting sediment traps in the North Atlantic Gyre from 50 m down to 600 m depth, showing that through biogenic polymers plastic can be embedded into rapidly sinking particles also known as marine snow. We furthermore show that the carbon contained in plastic can represent up to 3.8% of the total downward flux of particulate organic carbon. Our results shed light on important pathways regulating the transport of microplastics in marine systems and on potential interactions with the marine carbon cycle, suggesting microplastic removal through the "biological plastic pump".

Infection cycle of Marteilia pararefringens in blue mussels Mytilus edulis in a heliothermic marine oyster lagoon in Norway

Agapollen is a traditional heliothermic marine oyster lagoon in western Norway, representing the northernmost site of any Marteilia sp. protists detected in Europe. The semi-closed lagoon is a unique site to study the life cycle and development of M. pararefringens in naïve mussels. Two baskets with uninfected mussels were deployed in the lagoon outlet in May and October 2018, respectively, and sampled every 6 wk. The parasite was first detected in the mussels by PCR in early July and by histology in late August. By then, M. pararefringens had developed into mature stages, indicating a rapid development during mid-summer. Sporulation occurred during autumn. Mussels deployed in October never became infected, indicating that transmission was restricted to the warmest period of the year. Pronounced pathology was observed in infected mussels, including degenerated digestive tubules and infiltration of haemocytes. Mussel mortality was observed in the baskets, but whether this was due to infections of M. pararefringens or other environmental factors could not be determined. Plankton samples from the lagoon were also collected for PCR analysis. These samples, dominated by copepods, were positive for M. pararefringens in summer. In sorted samples, M. pararefringens was detected in the Acartia spp. and Paracartia grani fractions between July and October. These plankton copepods are therefore potentially involved in the life cycle of M. pararefringens.

Environmental risk of nickel in aquatic Arctic ecosystems

The Arctic faces many environmental challenges, including the continued exploitation of its mineral resources such as nickel (Ni). The responsible development of Ni mining in the Arctic requires establishing a risk assessment framework that accounts for the specificities of this unique region. We set out to conduct preliminary assessments of Ni exposure and effects in aquatic Arctic ecosystems. Our analysis of Ni source and transport processes in the Arctic suggests that fresh, estuarine, coastal, and marine waters are potential Ni-receiving environments, with both pelagic and benthic communities being at risk of exposure. Environmental concentrations of Ni show that sites with elevated Ni concentrations are located near Ni mining operations in freshwater environments, but there is a lack of data for coastal and estuarine environments near such operations. Nickel bioavailability in Arctic freshwaters seems to be mainly driven by dissolved organic carbon (DOC) concentrations with bioavailability being the highest in the High Arctic, where DOC levels are the lowest. However, this assessment is based on bioavailability models developed from non-Arctic species. At present, the lack of chronic Ni toxicity data on Arctic species constitutes the greatest hurdle toward the development of Ni quality standards in this region. Although there are some indications that polar organisms may not be more sensitive to contaminants than non-Arctic species, biological adaptations necessary for life in polar environments may have led to differences in species sensitivities, and this must be addressed in risk assessment frameworks. Finally, Ni polar risk assessment is further complicated by climate change, which affects the Arctic at a faster rate than the rest of the world. Herein we discuss the source, fate, and toxicity of Ni in Arctic aquatic environments, and discuss how climate change effects (e.g., permafrost thawing, increased precipitation, and warming) will influence risk assessments of Ni in the Arctic.

Vitellogenin gene expression in marine mussels exposed to ethinylestradiol: No induction at the transcriptional level

Vitellogenin (Vtg), a large multidomain protein precursor of egg-yolk proteins, is used as an endocrine disruption biomarker in fish, and in the last decades, its use has been extended to invertebrates like mollusks. However, it remains unclear whether invertebrate endocrine system produces Vtg in response to estrogens, like it occurs in oviparous vertebrates. In a previous study, no evidence of induction of Vtg expression at protein level was found in gonads of the marine mussel Mytilus galloprovincialis after exposure to the estrogenic chemical 17α-ethinylestradiol (EE2). In the present follow-up study, it was investigated whether there is any effect of EE2 on Vtg abundance at transcriptional level in M. galloprovincialis gonads. To this aim, RT-qPCR analysis targeting three different domains of Vtg transcript was performed on gonads of mussels that were exposed either 4 or 24 days to 100 ng/L EE2. In addition, several reference genes were analysed and a selection of these for potential use in further RT-qPCR analyses on mussel male and female gonads is provided. Results showed higher expression in females than in males for the three analysed Vtg domains, and no evidence of Vtg mRNA induction due to EE2 either in females or males. The present results, together with those obtained from previous analysis at protein level, support that Vtg is not an adequate biomarker for xenoestrogenicity in marine mussels. Additionally, nucleotide sequences of Vtg transcripts of three closely-related species from Mytilus edulis complex (M. galloprovincialis, M. edulis and M. trossulus) are provided and compared with Vtg sequences from other mollusk species to assess the level of conservation and evolutionary relationships among 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.

Freshening increases the susceptibility to heat stress in intertidal mussels (Mytilus edulis) from the Arctic

Temperatures in the Arctic are increasing at a faster pace than at lower latitudes resulting in range expansion of boreal species. In Greenland, the warming also drives accelerating melt of the Greenland Ice Sheet resulting in more meltwater entering Greenland fjords in summer. Our aim was to determine if increasing summer temperatures combined with lower salinity can induce the expression of stress-related proteins, for example, heat shock protein, in boreal intertidal mussels in Greenland, and whether low salinity reduces the upper thermal limit at which mortality occurs. We conducted a mortality experiment, using 12 different combinations of salinity and air temperature treatments during a simulated tidal regime, and quantified the change in mRNA levels of five stress-related genes (hsp24, hsp70, hsp90, sod and p38) in surviving mussels to discern the level of sublethal stress. Heat-induced mortality occurred in mussels exposed to an air temperature of 30°C and mortality was higher in treatments with lowered salinity (5 and 15‰), which confirms that low habitat salinity decreases the upper thermal limit of Mytilus edulis. The gene expression analysis supported the mortality results, with the highest gene expression found at combinations of high temperature and low salinity. Combined with seasonal measurements of intertidal temperatures in Greenland, we suggest heat stress occurs in low salinity intertidal area, and that further lowered salinity in coastal water due to increased run-off can make intertidal bivalves more susceptible to summer heat stress. This study thus provides an example of how different impacts of climate warming can work synergistically to stress marine organisms.

Early detection of marine non-indigenous species on Svalbard by DNA metabarcoding of sediment

Non-indigenous species (NIS) in the Arctic have an increased likelihood of arrival from ship traffic in the region, while the survival potential of the species becomes more likely in a warming environment. Monitoring is essential to detect the rate and magnitude of the establishment of NIS. In this study, a list of 123 potential marine NIS for Svalbard was drafted and the presence of marine NIS in soft sediment of Kongsfjorden in Svalbard was assessed using molecular metabarcoding techniques. For 37 species, including eight potential Arctic NIS, we generated new 18S and/or COI barcode sequences to improve the available online reference databases. In total, 299 species were identified in the sediment samples, including seven potential NIS. Three of these potential NIS have not been reported before in Svalbard: the harpacticoid copepod Euterpina acutifrons, and the ascidians Botrylloides violaceus and Molgula manhattensis. Another novel observation for Svalbard was the polychaete Chone mollis. Additional studies are needed to assess whether the NIS have been established on Svalbard and what their potential impact on the local system may be. Metabarcoding proved to be an effective monitoring tool to detect the presence of new species in Svalbard marine waters. We advise its use to set up a baseline record for the presence of NIS at points of entry, especially harbours. This approach is also valuable for biodiversity monitoring, in particular the detection of small organisms and life stages that are hard to identify using current visual techniques.

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.

Growth and behaviour of blue mussels, a re-emerging polar resident, follow a strong annual rhythm shaped by the extreme high Arctic light regime

Polar regions are currently warming at a rate above the global average. One issue of concern is the consequences on biodiversity in relation to the Northward latitudinal shift in distribution of temperate species. In the present study, lasting almost two years, we examined two phenological traits, i.e. the shell growth and behavioural rhythm of a recently re-established species in the high Arctic, the blue mussel Mytilus sp. We compared this with a native species, the Islandic scallop Chlamys islandica. We show marked differences in the examined traits between the two species. In Mytilus sp., a clear annual pattern of shell growth strongly correlated to the valve behaviour rhythmicity, whereas C. islandica exhibited a shell growth pattern with a total absence of annual rhythmicity of behaviour. The shell growth was highly correlated to the photoperiod for the mussels but weaker for the scallops. The water temperature cycle was a very weak parameter to anticipate the phenology traits of both species. This study shows that the new resident in the high Arctic, Mytilus sp., is a highly adaptive species, and therefore a promising bioindicator to study the consequences of biodiversity changes due to global warming.

Genome-wide insights into introgression and its consequences for genome-wide heterozygosity in the Mytilus species complex across Europe

The three mussel species comprising the Mytilus complex are widespread across Europe and readily hybridize when they occur in sympatry, resulting in a mosaic of populations with varying genomic backgrounds. Two of these species, M. edulis and M. galloprovincialis, are extensively cultivated across Europe, with annual production exceeding 230,000 tonnes. The third species, M. trossulus, is considered commercially damaging as hybridization with this species results in weaker shells and poor meat quality. We therefore used restriction site associated DNA sequencing to generate high-resolution insights into the structure of the Mytilus complex across Europe and to resolve patterns of introgression. Inferred species distributions were concordant with the results of previous studies based on smaller numbers of genetic markers, with M. edulis and M. galloprovincialis predominating in northern and southern Europe respectively, while introgression between these species was most pronounced in northern France and the Shetland Islands. We also detected traces of M. trossulus ancestry in several northern European populations, especially around the Baltic and in northern Scotland. Finally, genome-wide heterozygosity, whether quantified at the population or individual level, was lowest in M. edulis, intermediate in M. galloprovincialis, and highest in M. trossulus, while introgression was positively associated with heterozygosity in M. edulis but negatively associated with heterozygosity in M. galloprovincialis. Our study will help to inform mussel aquaculture by providing baseline information on the genomic backgrounds of different Mytilus populations across Europe and by elucidating the effects of introgression on genome-wide heterozygosity, which is known to influence commercially important traits such as growth, viability, and fecundity in mussels.

Trans-Atlantic Distribution and Introgression as Inferred from Single Nucleotide Polymorphism: Mussels Mytilus and Environmental Factors

Large-scale climate changes influence the geographic distribution of biodiversity. Many taxa have been reported to extend or reduce their geographic range, move poleward or displace other species. However, for closely related species that can hybridize in the natural environment, displacement is not the only effect of changes of environmental variables. Another option is subtler, hidden expansion, which can be found using genetic methods only. The marine blue mussels Mytilus are known to change their geographic distribution despite being sessile animals. In addition to natural dissemination at larval phase—enhanced by intentional or accidental introductions and rafting—they can spread through hybridization and introgression with local congeners, which can create mixed populations sustaining in environmental conditions that are marginal for pure taxa. The Mytilus species have a wide distribution in coastal regions of the Northern and Southern Hemisphere. In this study, we investigated the inter-regional genetic differentiation of the Mytilus species complex at 53 locations in the North Atlantic and adjacent Arctic waters and linked this genetic variability to key local environmental drivers. Of seventy-nine candidate single nucleotide polymorphisms (SNPs), all samples were successfully genotyped with a subset of 54 SNPs. There was a clear interregional separation of Mytilus species. However, all three Mytilus species hybridized in the contact area and created hybrid zones with mixed populations. Boosted regression trees (BRT) models showed that inter-regional variability was important in many allele models but did not prevail over variability in local environmental factors. Local environmental variables described over 40% of variability in about 30% of the allele frequencies of Mytilus spp. For the 30% of alleles, variability in their frequencies was only weakly coupled with local environmental conditions. For most studied alleles the linkages between environmental drivers and the genetic variability of Mytilus spp. were random in respect to “coding” and “non-coding” regions. An analysis of the subset of data involving functional genes only showed that two SNPs at Hsp70 and ATPase genes correlated with environmental variables. Total predictive ability of the highest performing models (r² between 0.550 and 0.801) were for alleles that discriminated most effectively M. trossulus from M. edulis and M. galloprovincialis, whereas the best performing allele model (BM101A) did the best at discriminating M. galloprovincialis from M. edulis and M. trossulus. Among the local environmental variables, salinity, water temperature, ice cover and chlorophyll a concentration were by far the greatest predictors, but their predictive performance varied among different allele models. In most cases changes in the allele frequencies along these environmental gradients were abrupt and occurred at a very narrow range of environmental variables. In general, regions of change in allele frequencies for M. trossulus occurred at 8–11 psu, 0–10 °C, 60%–70% of ice cover and 0–2 mg m⁻³ of chlorophyll a, M. edulis at 8–11 and 30–35 psu, 10–14 °C and 60%–70% of ice cover and for M. galloprovincialis at 30–35 psu, 14–20 °C.

Interactive effects of salinity variation and exposure to ZnO nanoparticles on the innate immune system of a sentinel marine bivalve, Mytilus edulis

ZnO nanoparticles (nZnO) are released into the coastal environment from multiple sources, yet their toxicity to marine organisms is not well understood. We investigated the interactive effects of salinity (normal 15, low 5, and fluctuating 5-15) and nZnO (100 μg l-1) on innate immunity of the blue mussels Mytilus edulis from a brackish area of the Baltic Sea. Exposure to ionic Zn (100 μg l-1) was used to test whether the toxic effects of nZnO can be attributed to the potential release of Zn2+. Functional parameters and the expression of key immune-related genes were investigated in the mussels exposed to nZnO or ionic Zn under different salinity regimes for 21 days. nZnO exposures elevated hemocyte mortality, suppressed adhesion, stimulated phagocytosis, and led to an apparent increase in lysosomal volume. At salinity 15, nZnO suppressed the mRNA expression of the Toll-like receptors TLRb and c, C-lectin, and the complement system component C3q indicating impaired ability for pathogen recognition. In contrast, the mRNA levels of an antimicrobial peptide defensin increased during nZnO exposure at salinity 15. At fluctuating salinity (5-15), nZnO exposure increased expression of multiple immune-related genes in hemocytes including the complement system components C1 and C3q, and the Toll-like receptors TLRa, b and c. Low salinity (5) had strong immunosuppressive effects on the functional and molecular immune traits of M. edulis that overshadowed the effects of nZnO. The salinity-dependent modulation of immune response to nZnO cannot be attributed to the differences in the aggregation or solubility of nZnO, and likely reflects the interaction of the toxic effects of nanoparticles and physiological effects of the osmotic stress. These findings have implications for the environmental risk assessment of nanomaterials and the development of the context-specific biomarker baselines for coastal pollution monitoring.

First mussel settlement observed in Antarctica reveals the potential for future invasions

Global biodiversity is both declining and being redistributed in response to multiple drivers characterizing the Anthropocene, including synergies between biological invasions and climate change. The Antarctic marine benthos may constitute the last biogeographic realm where barriers (oceanographic currents, climatic gradients) have not yet been broken. Here we report the successful settlement of a cohort of Mytilus cf. platensis in a shallow subtidal habitat of the South Shetland Islands in 2019, which demonstrates the ability of this species to complete its early life stages in this extreme environment. Genetic analyses and shipping records show that this observation is consistent with the dominant vectors and pathways linking southern Patagonia with the Antarctic Peninsula and demonstrates the potential for impending invasions of Antarctic ecosystems.

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.

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.

Comparison between single and multi-locus approaches for specimen identification in Mytilus mussels

Mytilus mussels have been the object of much research given their sentinel role in coastal ecosystems and significant value as an aquaculture resource appreciated for both, its flavour and nutritional content. Some of the most-studied Mytilus species are M. edulis, M. galloprovincialis, M. chilensis and M. trossulus. As species identification based on morphological characteristics of Mytilus specimens is difficult, molecular markers are often used. Single-locus markers can give conflicting results when used independently; not all markers differentiate among all species, and the markers target genomic regions with different evolutionary histories. We evaluated the concordance between the PCR-RFLP markers most commonly-used for species identification in mussels within the Mytilus genus (Me15-16, ITS, mac-1, 16S rRNA and COI) when used alone (mono-locus approach) or together (multi-locus approach). In this study, multi-locus strategy outperformed the mono-locus methods, clearly identifying all four species and also showed similar specimen identification performance than a 49 SNPs panel. We hope that these findings will contribute to a better understanding of DNA marker-based analysis of Mytilus taxa. These results support the use of a multi-locus approach when studying this important marine resource, including research on food quality and safety, sustainable production and conservation.

The effects of ZnO nanostructures of different morphology on bioenergetics and stress response biomarkers of the blue mussels Mytilus edulis

Biofouling causes massive economical losses in the maritime sector creating an urgent need for effective and ecologically non-harmful antifouling materials. Zinc oxide (ZnO) nanorod coatings show promise as an antifouling material; however, the toxicity of ZnO nanorods to marine organisms is not known. We compared the toxicity of suspended ZnO nanorods (NR) with that of ZnO nanoparticles (NP) and ionic Zn²⁺ in a marine bivalve Mytilus edulis exposed for two weeks to 10 or 100 μg Zn L^-1 of ZnO NPs, NRs or Zn²⁺, or to immobilized NRs. The multi-biomarker assessment included bioenergetics markers (tissue energy reserves, activity of mitochondrial electron transport system and autophagic enzymes), expression of apoptotic and inflammatory genes, and general stress biomarkers (oxidative lesions, lysosomal membrane stability and metallothionein expression). Exposure to ZnO NPs, NRs and Zn²⁺ caused accumulation of oxidative lesions in proteins and lipids, stimulated autophagy, and led to lysosomal membrane destabilization indicating toxicity. However, these responses were not specific for the form of Zn (NPs, NR or Zn²⁺) and showed no monotonous increase with increasing Zn concentrations in the experimental exposures. No major disturbance of the energy status was found in the mussels exposed to ZnO NPs, NRs, or Zn²⁺. Exposure to ZnO NPs and NRs led to a strong induction of apoptosis- and inflammation-related genes, which was not seen in Zn²⁺ exposures. Based on the integrated biomarker response, the overall toxicity as well as the pro-apoptotic and pro-inflammatory action was stronger in ZnO NPs compared with the NRs. Given the stability of ZnO NR coatings and the relatively low toxicity of suspended ZnO NR, ZnO NR coating might be considered a promising low-toxicity material for antifouling paints.

Seasonal and spatial variations in biomarker baseline levels within Arctic populations of mussels (Mytilus spp.)

Climate change and pollution resulting from human activities in the Arctic require reliable monitoring systems in sentinel species. Mytilus spp. are used as sentinel species all around the world. The use of Mytilus spp. in environmental monitoring requires knowledge about natural variations in pollution biomarkers. Seasonal variations in baseline levels of biomarkers were studied over a year in the mussels from both upper and lower littoral zones in Rakkfjorden, Norway, as they underwent their annual reproductive cycle. Spatial variations of these biomarker baseline levels were measured in five mussel populations within a 60-km radius from Rakkfjorden to investigate universality of the results from the specific population of Rakkfjorden at a regional scale. Seasonal variations in biomarker baseline levels were revealed and seemed to be related to the reproductive state of the mussels and the tidal zone. The mussels appeared to be more sensitive to oxidative stress during gametogenesis in autumn and winter, when having lower lysosome membrane stability and lower baseline levels of antioxidant biomarkers. An increase in baseline levels of these biomarkers was reported during spawning in spring, however, it was not possible to reveal whether these changes were due to spawning, or to a higher metabolic activity in mussels in response to elevated water temperature and food abundance. The differences between the tidal zones were reflected in reduced size of the mussels from the upper littoral zone, their late spawning in the season and increased baseline levels of antioxidant biomarkers during the coldest month, indicating a more challenging environment in the upper littoral zone. The spatial study indicated that the biomarker baseline levels measured in Rakkfjorden were no different from the levels measured in the mussels from five other sites and thus, are representative for mussels on a regional scale.

Phylogeography of the highly invasive sugar beet nematode, Heterodera schachtii (Schmidt, 1871), based on microsatellites

Plant-parasitic nematodes (PPNs) threaten crop production worldwide. Yet few studies have examined their intraspecific genetic diversity or patterns of invasion, critical data for managing the spread of these cryptic pests. The sugar beet nematode Heterodera schachtii, a global invader that parasitizes over 200 plant species, represents a model for addressing important questions about the invasion genetics of PPNs. Here, a phylogeographic study using 15 microsatellite markers was conducted on 231 H. schachtii individuals sampled from four continents, and invasion history was reconstructed through an approximate Bayesian computation approach, with emphasis on the origin of newly discovered populations in Korea. Multiple analyses confirmed the existence of cryptic lineages within this species, with the Korean populations comprising one group (group 1) and the populations from Europe, Australia, North America, and western Asia comprising another (group 2). No multilocus genotypes were shared between the two groups, and large genetic distance was inferred between them. Population subdivision was also revealed among the populations of group 2 in both population comparison and STRUCTURE analyses, mostly due to different divergent times between invasive and source populations. The Korean populations showed substantial genetic homogeneity and likely originated from a single invasion event. However, none of the other studied populations were implicated as the source. Further studies with additional populations are needed to better describe the distribution of the potential source population for the East Asian lineage.

Blue mussel shell shape plasticity and natural environments: a quantitative approach

Shape variability represents an important direct response of organisms to selective environments. Here, we use a combination of geometric morphometrics and generalised additive mixed models (GAMMs) to identify spatial patterns of natural shell shape variation in the North Atlantic and Arctic blue mussels, Mytilus edulis and M. trossulus, with environmental gradients of temperature, salinity and food availability across 3980 km of coastlines. New statistical methods and multiple study systems at various geographical scales allowed the uncoupling of the developmental and genetic contributions to shell shape and made it possible to identify general relationships between blue mussel shape variation and environment that are independent of age and species influences. We find salinity had the strongest effect on the latitudinal patterns of Mytilus shape, producing shells that were more elongated, narrower and with more parallel dorsoventral margins at lower salinities. Temperature and food supply, however, were the main drivers of mussel shape heterogeneity. Our findings revealed similar shell shape responses in Mytilus to less favourable environmental conditions across the different geographical scales analysed. Our results show how shell shape plasticity represents a powerful indicator to understand the alterations of blue mussel communities in rapidly changing environments.

Blue mussel shell shape plasticity and natural environments: a quantitative approach

Shape variability represents an important direct response of organisms to selective environments. Here, we use a combination of geometric morphometrics and generalised additive mixed models (GAMMs) to identify spatial patterns of natural shell shape variation in the North Atlantic and Arctic blue mussels, Mytilus edulis and M. trossulus, with environmental gradients of temperature, salinity and food availability across 3980 km of coastlines. New statistical methods and multiple study systems at various geographical scales allowed the uncoupling of the developmental and genetic contributions to shell shape and made it possible to identify general relationships between blue mussel shape variation and environment that are independent of age and species influences. We find salinity had the strongest effect on the latitudinal patterns of Mytilus shape, producing shells that were more elongated, narrower and with more parallel dorsoventral margins at lower salinities. Temperature and food supply, however, were the main drivers of mussel shape heterogeneity. Our findings revealed similar shell shape responses in Mytilus to less favourable environmental conditions across the different geographical scales analysed. Our results show how shell shape plasticity represents a powerful indicator to understand the alterations of blue mussel communities in rapidly changing environments.

Naturally acidified habitat selects for ocean acidification-tolerant mussels

Ocean acidification severely affects bivalves, especially their larval stages. Consequently, the fate of this ecologically and economically important group depends on the capacity and rate of evolutionary adaptation to altered ocean carbonate chemistry. We document successful settlement of wild mussel larvae (Mytilus edulis) in a periodically CO₂-enriched habitat. The larval fitness of the population originating from the CO₂-enriched habitat was compared to the response of a population from a nonenriched habitat in a common garden experiment. The high CO₂-adapted population showed higher fitness under elevated Pco₂ (partial pressure of CO₂) than the non-adapted cohort, demonstrating, for the first time, an evolutionary response of a natural mussel population to ocean acidification. To assess the rate of adaptation, we performed a selection experiment over three generations. CO₂ tolerance differed substantially between the families within the F₁ generation, and survival was drastically decreased in the highest, yet realistic, Pco₂ treatment. Selection of CO₂-tolerant F₁ animals resulted in higher calcification performance of F₂ larvae during early shell formation but did not improve overall survival. Our results thus reveal significant short-term selective responses of traits directly affected by ocean acidification and long-term adaptation potential in a key bivalve species. Because immediate response to selection did not directly translate into increased fitness, multigenerational studies need to take into consideration the multivariate nature of selection acting in natural habitats. Combinations of short-term selection with long-term adaptation in populations from CO₂-enriched versus nonenriched natural habitats represent promising approaches for estimating adaptive potential of organisms facing global change.