ABSTRACT Ocean warming is currently leading to distributional shifts of species and an alternation of coastal communities. Vulnerable species that are most sensitive to ocean warming are able to use several acclimation mechanisms, with one of the fastest being a shift in and shuffling of their partnerships with symbiotic microorganisms. Assessing symbiosis-focused mechanisms of acclimation and adaptation in response to ocean warming is a technical challenge due to the difficulty of accurately simulating the de novo formation of coastal communities. Here, we use the Kiel Outdoor Benthocosm facility to assess which sponges species are experimentally recruited and whether they exhibit symbiosis-focused mechanisms of acclimation following selection to ocean warming. We observed one sponge species ( Haliclona sp.) and found that this sponge exhibited significant shifts in the membership and composition of its associated microbiome in response to ocean warming, with much of this being attributed to the rare microbiota. Moreover, Haliclona sp. maintained the diversity and dominance of its microbiome members. Four bacteria taxa were differentially abundant at elevated temperatures, with two being a Francisella sp. that is a suspected pathogen and an uncultured Francisellaceae that is most closely related to sulfur-oxidizing endosymbionts. Changes to the Haliclona sp. microbiome are largely consistent with a limited acclimation response, which could indicate that this sponge may use microbial symbionts as part of a mechanism to acclimate and adapt to a warmer future ocean.

Mangroves host many marine species and support fisheries in developing (sub)tropical countries. The suitability of mangrove habitats depends strongly thier the water chemistry. Here, we show how global warming and rising atmospheric CO2 will reduce dissolved oxygen and increase CO2 in mangrove waters. Observations from 23 mangrove-lined estuaries worldwide revealed that most sites already experience mild (34%-43% of the time) or severe (6%-32%) hypercapnic hypoxia, that is, high CO2 and low oxygen conditions. Hypercapnic hypoxia mostly occurs during low tide, at low-salinity sites, and in warm tropical regions. Climate change will decrease oxygen concentrations by 5%-35% and increase CO2 concentrations by 8%-60% in mangrove waters by 2100. Overall, hypercapnic hypoxia events will occur more frequently, last longer, and become more severe. These shifts will reduce mangrove biodiversity and deteriorate habitat quality for commercially valuable fish. The strongest impact is expected in tropical developing countries.

Sea star wasting disease (SSWD) is one of the most severe marine epidemics recorded, affecting numerous asteroid species and causing widespread population declines. Although a bacterial pathogen has recently been proposed for one species, its generality across taxa and regions remains unresolved. Here, we report the first year-round field assessment of SSWD in the Baltic Sea, a rapidly warming, low-salinity ecosystem hosting a single keystone sea star predator, Aste-rias rubens. Using field surveys, image-based monitoring, and laboratory experiments, we characterised disease dy-namics and potential drivers in Kiel Fjord (western Baltic Sea). SSWD-like symptoms were present throughout 2024, with mean prevalence exceeding 40% across seasons and elevated levels during summer and early autumn, when sea surface temperatures approached 22 degrees Celcius and salinity fluctuated between 11 and 17. The mean body radius of asympto-matic individuals declined from 5.6 cm in spring to 2.3 cm in early summer before partially recovering in autumn, consistent with high recruitment of juveniles and selective loss of larger, symptomatic individuals. In a complementary laboratory experiment, survival analyses identified body size as the strongest predictor of SSWD-associated mortality (hazard ratio = 50.8, p < 0.001), with large individuals far more likely to die than small ones. This size-selective mor-tality, together with environmental constraints on recruitment, suggests that SSWD may be reshaping population size structure and reducing predation pressure on blue mussels, with potential consequences for benthic community dynam-ics. Continued monitoring will be essential to assess the long-term impacts of SSWD on A. rubens populations and associated benthic ecosystems under ongoing climate change. ### Competing Interest Statement The authors have declared no competing interest. Deutsche Bundesstiftung Umwelt, https://ror.org/05pmq5w14

Exploring evolutionary and physiological responses to environmental stress is crucial for assessing the effects of climate change on wild populations. Mussels, key inhabitants of the benthos with high ecological and economic value, are a particularly vulnerable species that may be pushed to their ecological limits as warming threatens their survival and population stability. Species within the Mytilus edulis complex are commonly found in temperate regions globally; in the Baltic Sea, populations are formed by M. edulis and M. trossulus hybrids with low levels of M. galloprovincialis introgression. This study investigates the mechanisms through which resilience towards global warming may be fast-tracked in Baltic mussels (Kiel, Germany). For this, we studied two cohorts of juvenile mussels (recently settled animals), one exposed to an extreme heat event early in life and one naïve to this stressor. Both cohorts were later exposed to experimental temperatures ranging from 21°C to 26°C, with animal performance measured after 25 days. Impacts of thermal stress on the genetic composition of each cohort was then assessed by genotyping 50 individuals using a blue mussel 60 K SNP-array. We observed a significant increase in M. edulis genotypes together with a decrease in M. trossulus in the challenged cohort, compared to naive juveniles. We also found exposure to high temperature affected performance of mussel cohorts, reducing dry tissue weight of selected individuals. Results from this study provide insights on how selection through thermal stress impacts performance and genetic composition of key globally distributed intertidal species, with important implications for understanding and managing mussel populations under future warming scenarios.

In the face of escalating heatwaves, accurately forecasting ectotherm population mortality is a pressing ecological challenge. Current Thermal Tolerance Landscape (TTL) models, while surpassing single-threshold metrics by incorporating individual survival times, are constrained by frequentist regression parametrization reliant on constant-temperature experiments, omitting probabilistic outcomes. This study addresses these limitations by pioneering the application of Approximate Bayesian Computation-Sequential Monte Carlo (ABC-SMC) to analyze survival data from Baltic Mytilus mussels subjected to both microcosm (constant temperature) and mesocosm (dynamic temperature) heatwave regimes. The ABC-SMC yields probabilistic predictions of individual lethality buildup and population survival trajectories, closely aligned with observed survival data across both experimental conditions. Informed by more realistic dynamic data, the TTL model predicts local mussel resilience against the most extreme summer heatwaves projected for this century, albeit with considerations for sublethal impacts and potential recruitment declines. Our approach can enhance the predictive accuracy concerning the sensitivity of key marine populations amidst intensifying heatwaves, addressing the urgent need for accurate modeling tools to inform conservation practices and ecosystem management, ultimately aiding in the preservation of marine biodiversity. ### Competing Interest Statement The authors have declared no competing interest.

Mussels from the genus Mytilus, key inhabitants of the benthos, are important for the aquaculture industry and one of the most sustainable sources of animal protein available. Species within the Mytilus edulis complex (M. edulis, M. galloprovincialis and M. trossulus) are commonly found in temperate regions globally and can easily hybridise whenever their geographic distributions overlap. In the Baltic Sea, populations are formed by M. edulis and M. trossulus hybrids with low levels of M. galloprovincialis introgression. Given the economic and ecological relevance of mussels, this study aimed to investigate mechanisms through which their resilience towards global warming may be fast-tracked. For this, we developed two cohorts of juvenile mussels (i.e. recently settled animals) from the Baltic Sea (Kiel, Germany), one exposed to an extreme heat event early in life and one naive to this stressor. Both cohorts were then exposed to experimental temperatures at the proposed upper thermal limit for this population, 21oC to 26oC, with animal performance measured after 25 days. We then assessed the impacts of thermal stress on the genetic composition of each cohort by genotyping 50 individuals using the blue mussel 60K SNP-array. We observed a significant increase in M. edulis genotypes together with a decrease in M. trossulus in the S cohort in comparison with NS juveniles. We also found that exposure to high temperature has an effect on the performance of mussel cohorts, reducing dry tissue weight of the selected individuals. Results from this study provide relevant insights on how selection through thermal stress impacts performance and genetic composition of blue mussel juveniles, with key implications for understanding and managing mussel populations under future warming scenarios. ### Competing Interest Statement The authors have declared no competing interest.

The simulation of deep-sea conditions in laboratories is technically challenging but necessary for experiments that aim at a deeper understanding of physiological mechanisms or host-symbiont interactions of deep-sea organisms. In a proof-of-concept study, we designed a recirculating system for long-term culture (>2 years) of deep-sea mussels Gigantidas childressi (previously Bathymodiolus childressi). Mussels were automatically (and safely) supplied with a maximum stable level of ~60 µM methane in seawater using a novel methane-air mixing system. Experimental animals also received daily doses of live microalgae. Condition indices of cultured G. childressi remained high over years, and low shell thickness growth could be detected, which is indicative of positive energy budgets. Using stable isotope data, we demonstrate that G. childressi in our culture system gained energy, both, from digestion of methane oxidizing endosymbionts and from digesting particulate food (microalgae). Limitations of the system, as well as opportunities for future experimental approaches involving deep-sea mussels are discussed.

1. As climate change intensifies heatwaves, quantifying associated mortality within ectothermic populations is crucial for effective conservation. Thermal tolerance landscape (TTL) models are useful predictive tools that assume exponentially decreasing survival durations in individuals with increasing temperatures. This assumption has been validated through regression analyses on data from constant temperature experiments, primarily focusing on adult-stage individuals. However, this approach does not allow for direct model validation with data from dynamic, real-world heatwave events and overlooks early recruitment stage vulnerabilities. 2. This study aimed to address these gaps using the blue mussel Mytilus, a foundation species forming extensive reefs along temperate coasts, as a model organism. We monitored survival rates of mussels (juveniles and adults) under constant heatwave (CHW) conditions in a laboratory experiment and under dynamic heatwave (DHW) scenarios simulated in an outdoor mesocosm experiment. Post-heatwaves, we also assessed recruitment rates within the mesocosms. TTL models were parametrised by employing Approximate Bayesian Computation with Sequential Monte Carlo (ABC-SMC) on each dataset separately. 3. The parameter distributions were similar across both experiments, and the ABC-SMC model predictions closely matched the observed survival declines, validating these models. In comparison, we found a lower predictive performance when using a Bayesian regression approach. Additionally, our best-fit model predicted that warming across the non-fatal DHW regimes would increase sublethal effects on mussels. The observed impact on the recruitment stage was more pronounced, with the recruitment rate following an exponential decay as sublethal effects increased. Our model projected minor (<4%) sublethal effects in adult mussels during the century's five warmest summer temperature regimes, corresponding to 0%-32% declines in recruitment rates. 4. Our research extends the TTL model validation, demonstrates the resilience of subtidal Baltic Mytilus to future extreme heatwaves and offers an approach to predict heatwave-induced population mortalities, applicable to other species and sensitive systems. Read the free Plain Language Summary for this article on the Journal blog.

Pocillopora damicornis (Linnaeus, 1758), a species complex, consists of several genetic lineages, some of which likely represent reproductively isolated species, including the species Pocillopora acuta Lamarck, 1816. Pocillopora acuta can exhibit similar morphological characteristics as P. damicornis, thus making it difficult to identify species-level taxonomic units. To determine whether the P. damicornis-like colonies on the reefs in the Andaman Sea (previously often identified as P. damicornis) consist of different species, we sampled individual colonies at five sites along a 50 km coastal stretch at Phuket Island and four island sites towards Krabi Province, Thailand. We sequenced 210 coral samples for the mitochondrial open reading frame and identified six distinct haplotypes, all belonging to P. acuta according to the literature. Recently, P. acuta was observed to efficiently recolonize heat-damaged reefs in Thailand as well as globally, making it a potentially important coral species in future reefs. Specifically in the light of global change, this study underscores the importance of high-resolution molecular species recognition, since taxonomic units are important factors for population genetic studies, and the latter are crucial for management and conservation efforts.

Abstract Background The widespread Indo-Pacific coral species Pocillopora acuta Lamarck, 1816 displays varying levels of asexual versus sexual reproduction, with strong repercussions on genetic diversity, connectivity and genetic structuring within and among populations. For many geographic regions, baseline information on genetic diversity is still lacking, particularly in the Andaman Sea. The region suffered a massive heat-induced bleaching event in 2010 with high coral cover loss of branching coral species such as P. acuta. A subsequent bleaching in 2016, however, revealed a mild bleaching response in pocilloporids compared to other coral taxa in the region, suggesting that rare, heat tolerant genotypes had been selected by the 2010 bleaching event. In order to test whether this potential ‘evolutionary rescue’ event has led to a low genetic diversity, we conducted a population genetic survey covering a total of nine different P. acuta populations (336 individuals) along a 50 km coastal stretch around Phuket Island, Thailand. We used six microsatellite markers to assess genotypic diversity and to determine the prevalent mode of reproduction (i.e. sexual or asexual recruitment). Results In contrast to other Indian Ocean P. acuta populations, the majority of corals in this study adopted a sexual reproduction mode (75% across all populations). At the same time, substantial regional gene flow was observed around Phuket Island with strong genetic differentiation as indicated by three genetic clusters that were separated by only a few kilometers. Patterns of isolation by distance over 0.7 – 40 km suggest small-scale genetic barriers, such as changing currents throughout each monsoonal season, potentially contributing to locally restricted dispersal of P. acuta larvae. Conclusions The occurrence of distinct genetic clusters within short coastal stretches suggests that the 2010 bleaching event has not led to extreme genetic impoverishment. While more in-depth genomic analyses are necessary to investigate changes in genetic diversity following extreme bleaching events, our results will help guide conservation efforts to maintain genetic diversity of a coral species that likely will be dominant in future, warmer Andaman Sea reefs.

An ongoing loss of experts in marine cellular biochemistry and physiology (CBP) is stagnating the generation of knowledge upon which rapidly growing "omics" approaches rely, ultimately hampering our ability to predict organismal responses to climate change.

Low-salinity stress can severely affect the fitness of marine organisms. As desalination has been predicted for many coastal areas with ongoing climate change, it is crucial to gain more insight in mechanisms that constrain salinity acclimation ability. Low-salinity induced depletion of the organic osmolyte pool has been suggested to set a critical boundary in osmoconforming marine invertebrates. Whether inorganic ions also play a persistent role during low-salinity acclimation processes is currently inconclusive. We investigated the salinity tolerance of six marine invertebrate species following a four-week acclimation period around their low-salinity tolerance threshold. To obtain complete osmolyte budgets, we quantified organic and inorganic osmolytes and determined fitness proxies. Our experiments corroborated the importance of the organic osmolyte pool during low-salinity acclimation. Methylamines constituted a large portion of the organic osmolyte pool in molluscs, whereas echinoderms exclusively utilized free amino acids. Inorganic osmolytes were involved in long-term cellular osmoregulation in most species, thus are not just modulated with acute salinity stress. The organic osmolyte pool was not depleted at low salinities, whilst fitness was severely impacted. Instead, organic and inorganic osmolytes often stabilized at low-salinity. These findings suggest that low-salinity acclimation capacity cannot be simply predicted from organic osmolyte pool size. Rather, multiple parameters (i.e. osmolyte pools, net growth, water content and survival) are necessary to establish critical salinity ranges. However, a quantitative knowledge of cellular osmolyte systems is key to understand the evolution of euryhalinity and to characterize targets of selection during rapid adaptation to ongoing desalination.

Salinity is a major environmental factor shaping the distribution and abundance of marine organisms. Climate change is predicted to alter salinity in many coastal regions due to sea level rise, evaporation, and changes in freshwater input. This exerts significant physiological stress on coastal invertebrates whose body fluid osmolality follows that of seawater (‘osmoconformers’). In this study, we conducted a systematic review and meta-analysis of osmolytes (both organic and inorganic) utilized by osmoconforming marine invertebrates during a >14-day acclimation to reduced salinity. Of the 2,389 studies screened, a total of 56 fulfilled the search criteria. Thirty-eight studies reported tissue osmolyte. Following acclimation to reduced salinity, tissue concentrations of six organic compounds and sodium were consistently reduced across phyla. This suggests that intracellular inorganic ions are not only utilized as a rapid response system during acute exposure to low salinity stress but also, in concert with reductions in organic osmolyte concentrations, during longer-term acclimation. Our systematic review demonstrates that only a few studies (n = 13) have quantified salinity-induced long-term changes in intracellular ion concentrations. In addition, no study has compiled a complete intracellular osmolyte budget. Alanine, betaine, glycine, and taurine are the major organic osmolytes that are universally employed across five phyla. The characterization of organic osmolytes was heavily weighted towards free amino acids (FAAs) and derivatives—neglecting methylamines and methylsulfonium compounds, which can be as important as FAAs in modulating intracellular osmolality. As a consequence, we suggest best-practice guidelines to streamline experimental designs and protocols in osmoregulation research in order to better understand the conserved mechanisms that define the limits of salinity acclimation in marine invertebrates. To our best knowledge, this is the first systematic review and meta-analysis on osmolyte concentrations in osmoconformers acclimated to low salinity. It creates a valuable baseline for future research and reveals large research gaps. Our meta-analysis suggests that there are common osmolyte actors employed across phyla but no uniform concept since osmolyte pool composition and proportions were taxon-specific. In light of future salinity changes and their potential consequences, it becomes more important to understand salinity tolerance capacities and limits.

Predicting the implications of ongoing ocean climate warming demands a better understanding of how short-term thermal variability impacts marine ectotherms, particularly at beyond-optimal average conditions during summer heatwaves. Using a globally important model species, the blue mussel Mytilus, in a 5-week-long experiment, we (a) assessed growth performance traits under 12 scenarios, consisting of four thermal averages (18.5, 21, 23.5 and 26celcius) imposed as constant or daily fluctuating regimes with amplitudes of 2 or 4celcius. Additionally, we conducted a short-term assay using different mussel individuals to (b) test for the species capacity for suppression and recovery of metabolic performance traits (feeding and aerobic respiration) when exposed to a 1-day thermal fluctuation regime (16.8-30.5celcius). Using this high-resolution data, we (c) generated short-term thermal metabolic performance curves to predict and explain growth responses observed in the long-term experiment. We found that daily high-amplitude thermal cycles (4celcius) improved mussel growth when fluctuations were imposed around an extreme average temperature of 26celcius, representing end-of-century heatwaves. In contrast, thermal cycles negatively affected mussel growth at a less extreme average temperature of 23.5celcius, resembling current peak summer temperature scenarios. These results suggest that fluctuations ameliorate heat stress impacts only at critically high average temperatures. The short-term assay demonstrated that during the warming phase, animals stopped feeding between 24 and 30celcius while gradually suppressing respiration. In the subsequent cooling phase, feeding and respiration partially and fully recovered to pre-heating rates respectively. Furthermore, nonlinear averaging of short-term feeding responses (upscaling) well-predicted longer term growth responses to fluctuations. Our findings suggest that fluctuations can be beneficial to or detrimental for the long-term performance of ectothermic animals, depending on the fluctuations' average and amplitude. Furthermore, the observed effects can be linked to fluctuation-mediated metabolic suppression and recovery. In a general framework, we propose various hypothetical scenarios of fluctuation impacts on ectotherm performance considering inter- or intra-species variability in heat sensitivity. Our research highlights the need for studying metabolic performance in relation to cyclic abiotic fluctuations to advance the understanding of climate change impacts on aquatic systems. A free Plain Language Summary can be found within the Supporting Information of this article.

The Baltic Sea has a salinity gradient decreasing from fully marine (> 25) in the west to below 7 in the central Baltic Proper. Habitat-forming and ecologically dominant mytilid mussels exhibit decreasing growth when salinity < 11; however, the mechanisms underlying reduced calcification rates in dilute seawater are not fully understood. Both [HCO3-] and [Ca2+] also decrease with salinity, challenging calcifying organisms through CaCO3 undersaturation (Ω≤1) and unfavourable ratios of calcification substrates ([Ca2+] and [HCO3-]) to the inhibitor (H+), expressed as the extended substrate–inhibitor ratio (ESIR). This study combined in situ monitoring of three southwest Baltic mussel reefs with two laboratory experiments to assess how various environmental conditions and isolated abiotic factors (salinity, [Ca2+], [HCO3-] and pH) impact calcification in mytilid mussels along the Baltic salinity gradient. Laboratory experiments rearing juvenile Baltic Mytilus at a range of salinities (6, 11 and 16), HCO3- concentrations (300–2100 µmol kg−1) and Ca2+ concentrations (0.5–4 mmol kg−1) reveal that as individual factors, low [HCO3-], pH and salinity cannot explain low calcification rates in the Baltic Sea. Calcification rates are impeded when Ωaragonite ≤ 1 or ESIR ≤ 0.7 primarily due to [Ca2+] limitation which becomes relevant at a salinity of ca. 11 in the Baltic Sea. Field monitoring of carbonate chemistry and calcification rates suggest increased food availability may be able to mask the negative impacts of periodic sub-optimal carbonate chemistry, but not when seawater conditions are permanently adverse, as observed in two Baltic reefs at salinities < 11. Regional climate models predict a rapid desalination of the southwest and central Baltic over the next century and potentially a reduction in [Ca2+] which may shift the distribution of marine calcifiers westward. It is therefore vital to understand the mechanisms by which the ionic composition of seawater impacts bivalve calcification for better predicting the future of benthic Baltic ecosystems.

Perspective: An ongoing loss of expertise on the biochemistry and physiology of marine organisms hampers our understanding of biological mechanisms upon rapidly growing “-omics” approaches reply -ultimately affecting our ability to predict organismal responses to climate change.

Climate change imposes unusual long‐term trends in environmental conditions, plus some tremendous shifts in short‐term environmental variability, exerting additional stress on marine ecosystems. This paper describes an empirical method that aims to improve our understanding of the performance of benthic filter feeders experiencing changes in environmental conditions, such as temperature, on time scales of minutes to hours, especially during daily cycles or extreme events such as marine heatwaves or hypoxic upwelling. We describe the Fluorometer and Oximeter equipped Flow‐through Setup (FOFS), experimental design, and methodological protocols to evaluate the flood of data, enabling researchers to monitor important energy budget traits, including filtration and respiration of benthic filter‐feeders in response to fine‐tuned environmental variability. FOFS allows online recording of deviations in chlorophyll and dissolved oxygen concentrations induced by the study organism. Transparent data processing through Python scripts provides the possibility to adjust procedures to needs when working in different environmental contexts (e.g., temperature vs. pH, salinity, oxygen, biological cues) and with different filter‐feeding species. We successfully demonstrate the functionality of the method through recording responses of Baltic Sea blue mussels (Mytilus) during one‐day thermal cycles. This method practically provides a tool to help researchers exposing organisms to environmental variability for some weeks or months, to relate the observed long‐term performance responses to short‐term energy budget responses, and to explain their findings with the potential to generalize patterns. The method, therefore, allows a more detailed description of stress‐response relationships and the detection of species' tolerance limits.

Background Biomineralization by molluscs involves regulated deposition of calcium carbonate crystals within a protein framework to produce complex biocomposite structures. Effective biomineralization is a key trait for aquaculture, and animal resilience under future climate change. While many enzymes and structural proteins have been identified from the shell and in mantle tissue, understanding biomieralization is impeded by a lack of fundamental knowledge of the genes and pathways involved. In adult bivalves, shells are secreted by the mantle tissue during growth, maintenance and repair, with the repair process, in particular, amenable to experimental dissection at the transcriptomic level in individual animals. Results Gene expression dynamics were explored in the adult blue mussel, Mytilus edulis, during experimentally induced shell repair, using the two valves of each animal as a matched treatment-control pair. Gene expression was assessed using high-resolution RNA-Seq against a de novo assembled database of functionally annotated transcripts. A large number of differentially expressed transcripts were identified in the repair process. Analysis focused on genes encoding proteins and domains identified in shell biology, using a new database of proteins and domains previously implicated in biomineralization in mussels and other molluscs. The genes implicated in repair included many otherwise novel transcripts that encoded proteins with domains found in other shell matrix proteins, as well as genes previously associated with primary shell formation in larvae. Genes with roles in intracellular signalling and maintenance of membrane resting potential were among the loci implicated in the repair process. While haemocytes have been proposed to be actively involved in repair, no evidence was found for this in the M. edulis data. Conclusions The shell repair experimental model and a newly developed shell protein domain database efficiently identified transcripts involved in M. edulis shell production. In particular, the matched pair analysis allowed factoring out of much of the inherent high level of variability between individual mussels. This snapshot of the damage repair process identified a large number of genes putatively involved in biomineralization from initial signalling, through calcium mobilization to shell construction, providing many novel transcripts for future in-depth functional analyses.

Baltic blue mussels can colonise and dominate habitats with far lower salinity (<10 psu) than other Mytilus congeners. Pervasive gene flow was observed between Western Baltic Mytilus edulis living at high salinity conditions and Eastern Baltic M. trossulus living at lower salinites, with highest admixture proportions within a genetic transition zone located at intermediate salinities (Darss Sill area). Yet, we do not understand the impacts of low salinity on larval performance, and how salinity may act as an early selective pressure during passive larval drift across salinity gradients. This study tested whether larvae originating from two different populations along the natural salinity cline in the Baltic Sea have highest fitness at their native salinities. Our results suggest that Eastern Baltic M. trossulus (Usedom, 7 psu) and Western Baltic M. edulis (Kiel, 16 psu) larvae display better performance (fitness components: growth, mortality, settlement success) when reared at their respective native salinities. This suggests that these populations are adapted to their local environment. Additionally, species diagnostic markers were used for genetic analyses of transition zone (Ahrenshoop, 11 psu) mussel larvae exposed to low salinity. This revealed that low salinity selection resulted in a shift towards allele frequencies more typical for Eastern Baltic M. trossulus. Thus, salinity acts as a selective pressure during the pre-settlement phase and can shape the genetic composition of Baltic mussel populations driving local adaptation to low salinity. Future climate change driven desalination, therefore, has the potential to shift the Baltic Sea hybrid gradient westward with consequences for benthic ecosystem structure.