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Mercury (Hg) is one of the most hazardous environmental pollutants, negatively affecting the ecosystem. The pathways of Hg elimination are well recognized in organisms from higher trophic levels compared to invertebrates such as clams. The aim of this study was to identify pathways of Hg accumulation in an alien species clams: Rangia cuneata, which represented an unrecognized source of Hg into the trophic chain of the southern Baltic Sea. An important aspect of this study was to determine Hg detoxification processes based on physiological state and biometric parameters of the atlatntic rangia. Special consideration was given to the role of shell in this process and the form of Hg in which it occurred. The study was also considered in terms of geographical changes in the Hg concentration in clams and the factors involved. Sex did not determine the concentration of Hg and its fraction in clams soft tissue and shell. Clams detoxified xenobiotic effectively in summer when their metabolism was accelerated. As a result, clams grew faster in warmer water than they accumulated Hg. In addition, this process was intensified by their reproduction. The mass of accumulated mercury was higher in the shell mass than in the body mass in summer. Transfer of Hg from the body to the shell depended on the forms Hg, mostly HgS. Geographical changes in the mercury concentration in clams was related to the form of Hg in the sediment. In areas where were more fines sediment fraction and organic matter accumulated in the sediment, mercury was present in a less bioavailable form, which caused that clams had lower Hg concentrations in their body. With assumption that in the future, due to its increasingly frequent occurrence, atlatntic rangia will become more common component of fish diet, a smaller load of toxic mercury will be introduced to the marine trophic chain.

We studied the behavioural (burrowing and valves gaping) and metabolic responses of Atlantic rangia Rangia cuneata (G.B. Sowerby I, 1832) at different salinities and temperatures. Three salinities (S = 0.5, 7.0, 10.0) were tested at 20 °C, with an additional control of 2.4. The control salinity (S = 2.4) was also tested at 10 °C to examine responses specific to colder temperatures. Clams exposed to the lowest (S = 0.5, T = 20 °C) and the highest salinity (S = 10.0, T = 20 °C), as well as lower temperature (T = 10 °C, S = 2.4) significantly (p < .05) reduced the rate of routine metabolism. At the lowest salinity (S = 0.5, T = 20 °C) and temperature (T = 10 °C, S = 2.4), they were also more abundant on the surface of the sediment, however the differences in relation to control conditions (S = 2.4, T = 20 °C) were not significant (p > .05). Clams kept their shells open in the lowest salinity, but closed in the highest salinity and at lower temperature. The closure time was longer in salinity of 10.0 than at temperature of 10 °C, where the highest reduction in the metabolic rate occurred in relation to the control conditions. This latter fact, in combination with shell closure, may indicate the activation of an anaerobic pathway. Thus, in the long term, lower temperatures may adversely affect R. cuneata, especially at very low salinity (S = 0.5, 2.4), in which this hyper-osmoregulating species has to strongly pump water with ions and oxygen to compensate for energy demand. In salinity of 10.0, where the species is isosmotic to the environment, changes in behaviour (moving to the sediment surface and closing the shell) with a simultaneous reduction in the metabolic rate may be due to lower costs of osmoregulation. Our results may help explain the strong inter-annual fluctuations in the population size of this subtropical species in the Baltic coastal waters resulting from the mass mortality of adults after long winters.

The clam Rangia cuneata, originating from the Gulf of Mexico, was recorded in the Vistula Lagoon for the first time in the early 2010s, and quickly became the dominant component of the zoobenthic biomass. To assess mortality as a factor potentially controlling the growth of Rangia population, a year-long field experiment involving marked bivalves placed in sediment-filled trays deployed on the bottom was conducted in 2014 and 2015. Predator-induced mortality of the clams was low in summer, and very high in the winter–spring period. It was inversely proportional to the size of the clams. Such changes can be partially attributed to predation from at least five fish and three duck species, which contained clams in their digestive tracts. Non-predatory mortality particularly affected large individuals, and was highest in spring, several weeks after the end of winter. We hypothesize that it could be caused by persistent low temperatures over several winter months which led to considerable weakening of the condition of clams. A long winter could also reduce their resistance to environmental stress and potential effect of epibionts, as well as increase susceptibility to predation.

This study aimed to investigate the sequestration of phenolic endocrine disrupting compounds (EDCs) such as bisphenol A (BPA), 4-t-octylphenol (4-t-OP), and 4-nonylphenol (4-NP) in the shells of the mature clam Rangia cuneata from the Vistula Lagoon (southern Baltic Sea) and to determine the influence of sex and shell length on bioaccumulation of these contaminants. Even though there is broad interest in EDCs influences on aquatic organisms, these basic parameters are poorly understood, yet necessary for assessing environmental risks for clams. Average proportions of the total body burden (ng/individual) deposited in shells of R. cuneata were more than 70% for BPA and 4-NP and up to 32% for 4-t-OP. These results indicate that shell storage can be an important route for elimination of specific EDCs. Relationships between EDCs concentrations and the size and sex of R. cuneata indicate that females and large individuals experience greater exposures to the adverse effects of these pollutants than males and smaller clams. This effect could have significant impacts on population ecology and ultimately affect the entire ecosystem, in which bivalves play an important role. In the context of using clams to assess water pollution, the co-variation of EDCs concentrations with the size and sex of bivalves could influence the quality of monitoring data, unless accounted for in sampling design and data analysis.

The present work reports the first record of Limnodrilus cervix in the south-eastern part of the Baltic Sea. A specimen of this North American species was found in 2010 in the Vistula Lagoon, near the village of Piaski (Poland). The distribution of other Nearctic Limnodrilus species in Europe is also presented.

Summary The aim of this work was to find out whether the difference between the central Vistula Lagoon (the southern Baltic Sea) and the western lagoon was reflected in the relationships between nutrients and phytoplankton during one-day hot summer conditions when the water temperature exceeded 20°C. Significant differences in Soluble Reactive Phosphorus (SRP) and Dissolved Inorganic Nitrogen (DIN) concentrations, and also in the biomass of the dominant phytoplankton assemblage of Cyanoprokaryota, were noted in the studied parts of the lagoon. No such differences were found for the nitrogen to phosphorus ratio (N:P) or for the biomasses of Bacillariophyta and Chlorophyta. The very low values of N:P (on average 2.8 and 3.4) indicated strong nitrogen limitation. The Correspondence Canonical Analysis (CCA) showed that the central part of the lagoon could be defined as positively related to DIN and to N:P, and western part could be characterized by correlation with temperature, dissolved oxygen and SRP concentrations. Competition for the limited resources of Dissolved Inorganic Nitrogen in the western, shallower part of the lagoon was in favour of Cyanoprokaryota, to the detriment of other phytoplankton assemblages. In contrast, the Cyanoprokaryota biomass in the central part of the lagoon, where DIN concentrations were increased, was lower, and Bacillariophyta in particular prospered at their expense. Here, the competition for Soluble Reactive Phosphorus was not so clear-cut.