Baltic Sea Blue Mussels Research Articles

Long-term decrease in Baltic Sea blue mussel shell length

Multi-decadal monitoring data and archived biological samples make the rapidly changing Baltic Sea an ideal system for quantifying alterations in ecosystem function. The Baltic Sea blue mussel (Mytilus edulis trossulus) acts as a keystone species by providing food and habitat for other species, and through its filtering activity clearing the water and promoting organic matter and nutrient cycling between benthic and pelagic ecosystems. A decrease in biomass of blue mussels along with altered environmental conditions has been observed in the northern Baltic Sea over a 24-year period (1993–2016), but the size distribution of the mussels was unknown. The present study focuses on retrospective measurements of the individual shell lengths of these archived blue mussels. The mean shell length was found to be significantly lower at the end of the time series than in the beginning (11.1–11.5 mm for the years 1993–1996 compared to 10.3–10.9 mm during 2012–2015), with the proportion of larger mussels decreasing in the population. This 6% decrease in shell length translated into a significantly lower (16%) mean filtration capacity at the end of the time series compared to the beginning, influencing the ecosystem services blue mussels provide.

Simultaneous recording of filtration and respiration in marine organisms in response to short‐term environmental variability

AbstractClimate 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.

Metabolite profiling of Baltic blue mussel (Mytilus sp.) hybrids and their associated microbes

The blue mussel genus Mytilus is a popular food source and an important model organism for many scientific disciplines [1,2]. The morphologically similar species M. edulis (Me) and M. trossulus (Mt) form an unprecedented and unique hybridisation pattern in the Baltic Sea [3, 4]. Recently, it was shown that metabolite profiling of Mytilus species provides significant potential for biomonitoring of aquatic environments but also for informing natural product research [1,2]. However, the metabolome of Mytilus sp. is still little investigated and no metabolome data from the Baltic Sea blue mussels are available so far. In order to compare the metabolite profiles of Baltic Mytilus sp. at an individual level, 30 Mytilus specimens were sampled at a blue mussel farm in Kiel (Baltic Sea) and genotyped with two commonly utilized nuclear markers, EFbis and Glu-5' [4]. Genotypic assessment revealed a dominance of Mt alleles at Kiel Fjord with 90% of the samples being intermediate (Me/Mt) or Mt-like hybrids. Secondary metabolites of the whole tissue extracts (EtOAc) were explored by HPLC-DAD-MS. Metabolite profiles were found to be similar but not identical, differing mainly in minor nonpolar secondary metabolites. Considering that secondary metabolites are often produced by associated microbes and not by the macroorganism itself, the microbiome of the mussel specimens was also investigated. A culture-based approach yielded 259 bacterial and fungal strains, including Actinomycetes. Bacteria belonging to the orders Alteromonadales and Pseudomonadales (Acinetobacter sp., Pseudomonas sp., Pseudoalteromonas sp., Psychrobacter sp., Shewanella sp.) dominated the microbial community (57%). In addition, an RFLP based culture-independent approach was developed for Mytilus sp. individuals. Both the culture-dependent and culture-independent approaches showed inter-individual differences with respect to the associated microbes, but no genotypic dependence.

Sewage treatment plant associated genetic differentiation in the blue mussel from the Baltic Sea and Swedish west coast.

Human-derived environmental pollutants and nutrients that reach the aquatic environment through sewage effluents, agricultural and industrial processes are constantly contributing to environmental changes that serve as drivers for adaptive responses and evolutionary changes in many taxa. In this study, we examined how two types of point sources of aquatic environmental pollution, harbors and sewage treatment plants, affect gene diversity and genetic differentiation in the blue mussel in the Baltic Sea area and off the Swedish west coast (Skagerrak). Reference sites (REF) were geographically paired with sites from sewage treatments plant (STP) and harbors (HAR) with a nested sampling scheme, and genetic differentiation was evaluated using a high-resolution marker amplified fragment length polymorphism (AFLP). This study showed that genetic composition in the Baltic Sea blue mussel was associated with exposure to sewage treatment plant effluents. In addition, mussel populations from harbors were genetically divergent, in contrast to the sewage treatment plant populations, suggesting that there is an effect of pollution from harbors but that the direction is divergent and site specific, while the pollution effect from sewage treatment plants on the genetic composition of blue mussel populations acts in the same direction in the investigated sites.

Impact of Chinese mitten crab Eriocheir sinensis on blue mussel Mytilus edulis trossulus – laboratory studies of claw strength, handling behavior, consumption rate, and size selective predation

Summary We examined the claw strength, handling behavior, consumption rate, and size selective predation of the invasive Chinese mitten crab Eriocheir sinensis feeding on native Baltic Sea blue mussels Mytilus edulis trossulus during 24 h laboratory experiments. Single starved crabs were offered 15 mussels (five mussels in three length classes) at a time. The total number of mussels consumed by a single crab increased significantly ( P −1 h −1 after 4 h to 0.2 ± 0.7 # mussels crab −1 h −1 after 24 h. The highest consumption rate was observed within the first 4 h, and it decreased significantly ( P E. sinensis can also harm blue mussel shells by crushing them without further consumption. The mean daily damage, and not consumption, by a single crab was 0.9 ± 1.4 of 11–40 mm mussels. The claw strength of E. sinensis ranged from 1.50 to 20.43 N (mean 8.51 ± 5.93 N) and was significantly correlated ( P E. sinensis may be able to impact the native M. edulis trossulus population abundance in the coastal Baltic waters either through direct predation or indirect mortality by damaging (crushing) the shell.

Response and recovery of Baltic Sea blue mussels from exposure to pharmaceuticals

Physiological responses to, and recovery from, exposure to 3 concentrations of a pharmaceutical mixture (diclofenac and propranolol) were examined experimentally in Baltic Sea blue mussels Mytilus ...

Scale‐dependent influence of environmental variables on species distribution: a case study on five coastal benthic species in the Baltic Sea

Statistical modelling can be used to relate biological survey data to environmental factors, thereby providing a basis for predictive mapping of species or communities. Although environmental variables vary and influence biota at different scales, models are often fitted without discussion of how scale dependency influences the results. In this study, we analysed the relative importance of environmental factors for the distribution of aquatic species as a function of extent, using data on the cover of five common benthic species (four macrophytes and one animal), from 1731 sites along the Swedish Baltic Sea coast. We modelled the cover and distribution of the five species in relation to salinity, depth, slope, wave exposure and substrate in scale steps from 25 to 1500 km, and analysed the relative contribution of the environmental variables to each species model. The average total deviance explained by the models was generally quite high, and decreased with increasing scale for all macrophyte species, while it increased for the animal, the Baltic Sea blue mussel Mytilus edulis. The relative contribution of different environmental variables changed with scale, and responses also differed between species. The average contribution of salinity increased for all species when moving from local to Baltic Sea scale, and for the Baltic Sea blue mussel it was the single most important factor at the Baltic Sea scale. The average contribution of depth decreased with increasing scale for all species. However, regardless of scale, depth was the most important environmental factor to explain the distribution of all but one of the investigated macrophyte species. This shows that fine scale predictor variables can be of major importance also for species distribution models covering large areas.

Biomarker responses and accumulation of hazardous substances in mussels (Mytilus trossulus) transplanted along a pollution gradient close to an oil terminal in the Gulf of Finland (Baltic Sea)

Baltic Sea blue mussels (Mytilus trossulus) were used as sentinel organisms to detect the biological effects of chemical contamination in the low salinity environment. Mussels naturally adapted to a salinity of ca. 6.0PSU were caged for 30days at four sites along an assumed pollution gradient (salinity ca. 4.5PSU) in the vicinity of Finland's largest oil refinery and harbor Kilpilahti in the Gulf of Finland. Tissue concentrations and accumulation rates of especially organic contaminants (PAHs, PCBs and organotins) were clearly elevated at the innermost coastal stations near the harbor area. Biological effects of contaminant exposure on caged mussels were evaluated by measuring a suite of biomarkers including catalase, glutathione S-transferase, superoxide dismutase, glutathione reductase, lipid peroxidation, acetylcholinesterase activity and lysosomal membrane stability. Mussels transplanted near the harbor area were able to elevate their antioxidant defense in response to environmental contamination. Reduced morphometric condition index and soft tissue growth rate together with increased lipid peroxidation and low lysosomal membrane stability were also observed at the most contaminated site. The results suggest that caging of M. trossulus for four weeks at lower salinity is a feasible method for the detection of environmental pollution also in low salinity areas of the Baltic Sea.

Impacts of seawater acidification on mantle gene expression patterns of the Baltic Sea blue mussel: implications for shell formation and energy metabolism

Marine organisms have to cope with increasing CO2 partial pressures and decreasing pH in the oceans. We elucidated the impacts of an 8-week acclimation period to four seawater pCO2 treatments (39, 113, 243 and 405 Pa/385, 1,120, 2,400 and 4,000 µatm) on mantle gene expression patterns in the blue mussel Mytilus edulis from the Baltic Sea. Based on the M. edulis mantle tissue transcriptome, the expression of several genes involved in metabolism, calcification and stress responses was assessed in the outer (marginal and pallial zone) and the inner mantle tissues (central zone) using quantitative real-time PCR. The expression of genes involved in energy and protein metabolism (F-ATPase, hexokinase and elongation factor alpha) was strongly affected by acclimation to moderately elevated CO2 partial pressures. Expression of a chitinase, potentially important for the calcification process, was strongly depressed (maximum ninefold), correlating with a linear decrease in shell growth observed in the experimental animals. Interestingly, shell matrix protein candidate genes were less affected by CO2 in both tissues. A compensatory process toward enhanced shell protection is indicated by a massive increase in the expression of tyrosinase, a gene involved in periostracum formation (maximum 220-fold). Using correlation matrices and a force-directed layout network graph, we were able to uncover possible underlying regulatory networks and the connections between different pathways, thereby providing a molecular basis of observed changes in animal physiology in response to ocean acidification.

Physiological effects of diclofenac, ibuprofen and propranolol on Baltic Sea blue mussels

Pharmaceuticals are constantly dispersed into the environment and little is known of the effects on non-target organisms. This is an issue of growing concern. In this study, Baltic Sea blue mussels, Mytilus edulis trossulus, were exposed to diclofenac, ibuprofen and propranolol, three pharmaceuticals that are produced and sold in large quantities and have a widespread occurrence in aquatic environments. The mussels were exposed to pharmaceuticals in concentrations ranging from 1 to 10,000 μg l −1. The pharmaceuticals were added both separately and in combination. Mussels exposed to high concentrations of pharmaceuticals showed a clear response compared to controls. Firstly, they had a significantly lower scope for growth, which indicates that the organisms had a smaller part of their energy available for normal metabolism, and secondly, they had lower byssus strength and lower abundance of byssus threads, resulting in reduced ability to attach to the underlying substrate. Mussels exposed to lower concentrations showed tendencies of the same results. The concentration of diclofenac and propranolol was quantified in the mussels using both liquid chromatography coupled to mass spectrometry (LC–MS). The measurements showed a significantly higher concentration in the organisms as compared to the water the mussels were exposed to; the uptake reached concentrations two orders of magnitudes higher than found in sewage treatment plant effluents. This study showed that common pharmaceuticals are taken up and negatively affect the physiology of a non-target species at levels of two to three orders of magnitudes higher than found in sewage treatment plant effluents.

Physiological performance of Baltic Sea blue mussels, Mytilus edulis, under CO2 induced stress: Cardiac performance, activity and filtration rate

Physiological performance of Baltic Sea blue mussels, Mytilus edulis, under CO2 induced stress: Cardiac performance, activity and filtration rate

SALDI-MS Signal Enhancement Using Oxidized Graphitized Carbon Black Nanoparticles

The signal intensity of low-molecular-weight compounds analyzed using surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF-MS) was significantly enhanced when oxidized graphitized carbon black (GCB) particles were used as the desorption/ionization surface. The surface of oxidized GCB contains more carboxylic acid groups than non-oxidized GCB. Carboxylic acid groups enhance the efficiency of the ionization process and the desorption of more hydrophobic compounds. A common pharmaceutical compound, propranolol, was successfully extracted from Baltic Sea blue mussels and quantified using oxidized GCB as the SALDI surface, whereas deuterated propranolol was used as the internal standard. The calibration curve showed a wide linear dynamic range of response (0.1-20 microg/mL) and good reproducibility (RSD < 10%). It was not possible to detect propranolol in Baltic Sea blue mussels when non-oxidized GCB was used as the SALDI surface.

Benzo[ a]pyrene up-regulates the expression of the proliferating cell nuclear antigen (PCNA) and multixenobiotic resistance polyglycoprotein (P-gp) in Baltic Sea blue mussels ( Mytilus edulis L.)

The expression of protein biomarkers in Baltic Sea blue mussels was analyzed after three days exposure to low (2.8 μg/animal/day), intermediate (28 μg/animal/day), or high (280 μg/animal/day) nominal doses of benzo[ a]pyrene (B aP). Significant expression changes were found in the animals exposed to the low dose, the lowest reported dose for DNA adduct formation in the gills of Baltic Sea blue mussels. Up-regulated expression of the proliferating cell nuclear antigen (PCNA), quantified from Western blots, and no change in the 5-bromo-deoxyuridine (BrdU) staining pattern, determined by immunocytochemistry, indicated that the observed PCNA response was mainly non-proliferative, and thus possibly due to DNA damage. The expression of the multixenobiotic resistance polyglycoprotein (P-gp) was also up-regulated, proving its usefulness as an exposure marker to planar organic compounds. No effect of the B aP treatment with respect to the retinoblastoma 110 protein or heat shock proteins 60 and 70 was found. The variance in the medium and high dose data was too large to allow for the detection of significant expression changes. We suggest PCNA to be a marker for genotoxic stress derived from the polycyclic aromatic hydrocarbon B aP, irrespective of whether the stress leads to DNA repair or to cell proliferation.

Abstracts from Thirteenth International Symposium on Pollutant Responses in Marine Organisms (PRIMO 13) – Heavy metals: Mechanisms of detoxification and mechanisms of toxicity

Effects of copper and cadmium on protein expression in Baltic sea blue mussels (Mytilus edulis sp.)

Tissue differences, dose–response relationship and persistence of DNA adducts in blue mussels ( Mytilus edulis L.) exposed to benzo[ a]pyrene

Baltic Sea blue mussels ( Mytilus edulis) were experimentally exposed to the genotoxic model substance benzo[ a]pyrene (B[ a]P) to study DNA adduct formation. The specific aims were (a) to examine where in the mussels the DNA adducts were formed, in gills or digestive gland; (b) to study the dose–response relationship between B[ a]P exposure and DNA adduct formation; and (c) to examine the persistence of the formed adducts. A Scope for growth (SFG) study was also run to compare physiological responses of the mussels with the degree of DNA adduct formation. In an initial dose–response experiment, the mussels were exposed to 0, 5, 50, and 100 μg/l of tritium labelled B[ a]P under semi-static conditions for 4 days, and thereafter the bioaccumulation of B[ a]P and DNA adduct formation in different tissues was determined using liquid scintillation counting and 32P-postlabelling analysis, respectively. In a following exposure–depuration experiment, mussels were exposed to 17 μg/l of radiolabelled B[ a]P under semi-static conditions for 6 days. B[ a]P accumulation and DNA adduct formation were determined during the exposure, and B[ a]P elimination and persistence of DNA adducts were studied during 28 days of depuration in uncontaminated water. The results revealed large tissue differences in DNA adduct formation. DNA adduct levels were not elevated in the digestive gland of the mussels at any exposure concentration (0–100 μg/l), even though the highest B[ a]P tissue concentrations were found in the digestive gland (1.0±0.1 mg B[ a]P/g tissue dry wt at 100 μg/l, mean±SE, n=12). DNA adducts were on the other hand formed in the gills, with the highest levels found in mussels exposed to 50 and 100 μg B[ a]P/l, and a dose dependent increase in adduct levels (from 1.6 to 5.9 nmol adducts/mol nucleotides) from 0 to 50 μg B[ a]P/l. In gills, DNA adduct levels increased with time during the 6-day exposure period in the exposure–depuration experiment, and then persisted for at least 2 weeks after exposure cessation while B[ a]P tissue levels exhibited a rapid decrease (half-life of 8 days). No significant differences were observed in SFG between the control and exposed groups. Since DNA adducts exhibited a relatively high persistence in gills compared to B[ a]P tissue concentrations, they seem to be a more integrated measure of genotoxic exposure than only chemical analysis of the contaminant bioaccumulation. The results also suggest that if using analysis of DNA adducts in M. edulis for monitoring purposes, analysis of gills in addition to the more commonly used digestive gland should be taken into consideration.

Efficiencies of polychlorinated biphenyl assimilation from water and algal food by the blue mussel (Mytilus edulis)

A novel method was used to estimate assimilation efficiencies (AEs) of dissolved and food associated PCBs (IUPAC 31, 49, and 153) by the Baltic Sea blue mussel (Mytilus edulis). Mussels were exposed to radiolabeled PCBs in a series of short-term toxicokinetic experiments at different algal food concentrations, both at apparent steady-state (ASS) and non-steady-state (NSS) conditions in respect to PCB partitioning between water and algae. The PCB AEs were calculated using a physiologically based bioaccumulation model where experimentally determined uptake and exposure rates at ASS and NSS conditions were combined into linear equation systems, which were solved for PCB AE from water and food. A positive relationship between PCB uptake and algae clearance by the mussels was observed for all three PCBs. The PCB AEs from both water and food increased with congener hydrophobicity (octanol/water partition coefficient [Kow]), but AEs decreased with increases in water pumping and filtration rate of the mussels, respectively. The average contribution of food-associated PCB to the total uptake also increased with Kow from approximately 30% for PCB 31 and PCB 49 to 50% for PCB 153, mainly as a consequence of increased sorption to the algal food.

Bioaccumulation kinetics of PCB 31, 49 and 153 in the blue mussel, Mytilus edulis L. as a function of algal food concentration

Uptake and depuration kinetics of a tri, a tetra and a hexa chlorobiphenyl (IUPAC no. 31, 49 and 153) in Baltic Sea blue mussels ( Mytilus edulis L.) were determined at different algal food rations. Differences in food concentration markedly influenced the water pumping and filtration rates of the mussels. This indicates that physiologically-based (PB) rather than simple equilibrium partitioning bioaccumulation models are more appropriate to use when describing the influence of differences in food ration on PCB bioaccumulation in mussels. A PB bioaccumulation model was used to describe the gradual shift in PCB exposure with increasing food concentration from apparently dissolved to food-associated PCBs. This change in the route of PCB exposure was owing to the combined effect of PCB partitioning to algae and regulation of the rates of water pumping and algae filtration by the mussels. Differences in food ration significantly influenced the PCB uptake rate, whereas PCB depuration was mainly unaffected by differences in algal food concentration. The observed differences in PCB uptake were primarily caused by the influence of food ration on the physiological activity of the mussels and only to a lesser degree by changes in PCB bioavailability owing to PCB partitioning between water and algal food. Calculated bioaccumulation factors (BAFs) varied up to two orders of magnitude over the food ration interval studied. Maximum BAFs of 4.92 10 5, 1.10 10 6, and 6.91 10 6 ml g −1 DW were obtained for PCB 31, 49 and 153, respectively, at a food concentration of 0.076 mg particulate organic carbon per litre. Thus, differences in food ration affects both the route and the rate of contaminant exposure and uptake. This emphasises the importance of controlling particulate food concentrations when designing bioaccumulation and toxicity studies with suspension-feeding organisms. Long-term changes in food availability may also affect contaminant accumulation and cycling in field populations of suspension-feeding mussels.

Enhanced accumulation of PCB congeners by baltic sea blue mussels,Mytilus edulis, with increased algae enrichment

The objective of this study was to examine if natural variations in the quantity of phytoplankton-derived particulate and dissolved organic carbon influences the accumulation of polychlorinated biphenyls (PCBs) in the tissues of Baltic Sea blue mussels (Mytilus edulis L.). In a laboratory flow-through experiment we exposed M. edulis to the technical PCB mixture Aroclor® 1248 for 21 d at three different enrichments of the unicellular green algae Chlamydomonas sp., 0.10, 0.16, and 0.32 mg particulate organic carbon (POC)/L. Tissue and water concentrations were determined for seven PCB congeners (International Union of Pure and Applied Chemistry [IUPAC] 52, 101, 105, 118, 138, 153, and 180), and 21-d bioaccumulation factors were calculated against total water concentrations. Contrary to what would be expected, an increase in algae enrichment from 0.10 to 0.32 mg POC/L resulted in an enhanced PCB accumulation by a factor of approx. 2. This increase in PCB accumulation was more pronounced for PCB congeners with lower hydrophobicity. These observations have implications for the design of laboratory accumulation studies and potentially for PCB accumulation and cycling in field populations of suspension-feeding mussels in response to changes in eutrophication status.

Influence of body size on the uptake, depuration, and bioaccumulation of polychlorinated biphenyl congeners by Baltic Sea blue mussels, Mytilus edulis

The present study was designed to examine the influence of body size on the bioaccumulation of polychlorinated biphenyl (PCB) congeners by Baltic Sea blue mussels, Mytilus edulis L. This was done, firstly, by establishing the relationship (as a power function: PCB tissue conc = a tissue dry wtb) between tissue concentration and body weight for seven PCB congeners (International Union of Pure and Applied Chemistry No. 52, 101, 105, 118, 138, 153, and 180) in field sampled mussels; and, secondly, by assessing the influence of body weight on the uptake clearance coefficients, the depuration rate coefficients and the calculated bioaccumulation factors (BAFs) of three 14C-labelled PCB congeners (IUPAC No. 31, 49, 153) in mechanistic kinetic experiments. Both the background tissue concentrations of PCB 138, PCB 153 and PCB 180 and the predicted BAF values in the kinetic experiments correlated negatively with body weight (b=-0.17 and-0.31, respectively). Of the two kinetic rate coefficients examined, only the uptake clearance rate showed weight dependency (b=-0.32, i.e. negative correlation with body weight), whereas depuration rates were unaffectd by body weight. Uptake clearance rates and BAFs adjusted for body weight increased with the octanol/water partition coefficient (Kow) of the congener, whereas depuration rates dectreased with Kow. These observations suggest that size-dependent bioaccumulation of hydrophobic organic contaminants (HOCs) in suspension-feeding bivalves is driven by size-related differences in uptake rate, although several other mechanisms may also affect bioaccumulation in natural mussel beds (e.g. sizerelated differences in lipid content, production, and contaminant exposure). From an ecotoxicological perspective size-dependent bioaccumulation implies not only that variability due to body size differences has to be dealt with in experimental designs, but also that several ecological factors such as size-specific predation and shifts in population structure may affect HOC cycling by dense populations of bivalve suspensionfeeders.

Comparative study of the physiology and its probable effect on size in Blue Mussels (Mytilus EdulisL.) from the North Sea and the Northern Baltic Proper

Abstract It is a well-known fact that the Baltic Sea blue mussels are very abundant but do not grow as large and fast as Mytilus edulis from the North Sea. This is often attributed to physiological differences due to the decline in salinities, although the mechanisms behind this are not yet fully understood. In order to elucidate this, the metabolism of mussels from the northern Baltic proper (Asko) and the North Sea (Tjarno) was compared in the laboratory at ambient salinities of about 6.5 and 28.8 ‰, respectively, together with the magnitude of response to acute changes in the salinity. At 13°C and ambient salinities, respiration was higher for North Sea mussels in a non-reproductive stage, while after spawning respiration was higher for Baltic mussels. NH4-N excretion was always about two times higher for Baltic than for North Sea mussels at ambient salinities, and very sensitive to changes in salinity due to changes in the metabolism of free amino acids. While lowered salinities increased excretion, t...