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Salinity fluctuations promote adverse effects on the immunity of marine bivalves; however, sometimes experimental data are controversial. In the present work, we evaluated in vivo effects of short-time salinity changes on hemocyte functions of Mediterranean mussel. Mytilus gailoprovincialis. Mussels were acclimated to hyposaline (6. 10, 14) and hypersaline (24.30) conditions and exposed to altered salinity for 2 days. Then hemolymph cellular composition, hemocyte immune (intracellular reactive oxygen species (ROS) production, phagocytic activity), and functional parameters (morphology, mitochondrial membrane potential) were measured using flow cytometry and fluorescent microscopy. Salinity fluctuations led to an increase of ROS production by hemocytes in all experimental groups. Increased ROS levels were accompanied with elevated mitochondrial membrane potential for all treatments except salinity 30, where the parameter decreased significantly. Salinity stress (all experimental groups) inhibited phagocytosis in agranulocytes, whereas stimulated phagocytic activity of granulocytes at salinity 14 and 24 indicating that moderate salinity fluctuations may cause stimulating effect on immunocytes of bivalves. Hypersaline and hyposaline treatments did not affect hemocyte morphology. These results indicate that hemocytes likely possess physiological mechanisms that restore initial cellular volume following hypoosmotic swelling/hyperosmotic shrinkage. Salinity stress affects hemocyte functionality of bivalves with varying intensity. KEY WORDS: salinity adaptation, intracellular ROS, mitochondrial membrane potential, INTRODUCTION Bivalve molluscs are major invertebrate organisms for fisheries and aquaculture and also play a crucial role in functioning of aquatic ecosystems. Bivalves are filter feeders; by filtering large amount [...]

Bivalve mollusks that inhabit low-depth coastal and estuarine areas frequently experience osmotic stress that may be also associated with alterations of antioxidant enzyme activities and markers of oxidative stress. Mitochondria are a major source of reactive oxygen species (ROS) in eucaryotic cells. Overpoduction of ROS induces oxidative stress leading to a damage of intracellular compartments and cell death. In euryhaline bivalves, information concerning cellular ROS production upon osmotic stress and changes in mitochondrial membrane potential is scarce. The present study investigates osmotic stability and hemocytes` regulatory volume decrease (RVD) of Mediterranean mussel (Mytilus galloprovincialis) and the Pacific oyster (Magallana gigas). We also studied dynamic changes in intracellular ROS levels and mitochondrial membrane potential in hemocytes undergoing the RVD response following hypoosmotic swelling. Our data revealed that osmotic stability of mussel and oyster hemocytes did not significantly differ. Loss of environmental osmolarity from 460.0 ± 2.0 mOsm l -1 to 216.0 ± 4.0 mOsm l -1 resulted in an increase of hemocyte volume by 60% of the initial cellular volume in mussels and by 28% in oysters. After rapid hypoosmotic swelling hemocytes of both species demonstrated the RVD response. At the end of 60 min exposure to hypoosmotic environment, hemocyte volume significantly decreased in both species by 10-12% compared to the maximal hemocyte volume. Hypoosmotic shock induced an increase of mitochondrial membrane potential in hemocytes of mussels and oysters. In mussels, increased mitochondrial membrane potential was accompanied with decreased ROS levels in hemocytes, whereas oyster hemocytes showed enhanced ROS production., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Mussels were exposed to di-(2-ethylhexyl) phthalate (DEHP) (0.4 mg L -1 and 4.0 mg L -1 ) for 24 h and 48 h and its effect on hemocyte cellular composition and spontaneous reactive oxygen production (ROS) levels in hemocytes were evaluated. Exposure to DEHP induced a loss in spontaneous ROS production levels in hemocytes and decreased agranulocyte number in hemolymph. DEHP was found to accumulate in hepatopancreas of mussels and this process was associated with an increase of catalase (CAT) activity after 24 h incubation. At the end of the experimental period (48 h) CAT activity recovered up to control levels. Superoxide dismutase (SOD) activity in hepatopancreas increased following 48 h exposure to DEHP. The results indicated that DEHP could affect hemocyte immune properties, and also cause non-specific general stress-response of the antioxidant complex, which, in turn, was not associated with pronounced oxidative stress., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Ekaterina Kladchenko reports financial support was provided by A.O. Kovalevsky Institute of Biology of the Southern Seas of RAS., (Copyright © 2023. Published by Elsevier Ltd.)

This study describes the analysis of antioxidant enzymatic activities (catalase and superoxide dismutase) in gills and functional state of hemocytes (osmotic stability, mitochondrial membrane potential) of ark clams (Anadara kagoshimensis) from the Black Sea basin exposed to salinity stress. For this, the effects of 48 h periods of exposure to low (8 ‰, 14 ‰) and high (35 ‰, 45 ‰) salinity were assessed. Our results showed that ark clams, A. kagoshimensis, possessed pronounced tolerance to hypersalinity stress and are sensitive to a short-time hyposalinity treatment. Salinity 35 ‰ inhibited production of reactive oxygen species (ROS) by hemocytes and did not affect their levels of mitochondrial membrane potential. Acclimation to 45 ‰ salinity caused significant increase in mitochondrial membrane potential accompanied with recovery of intracellular ROS levels up to controls levels. Acclimation to low salinity (8 ‰) induced an increase in both ROS and mitochondrial membrane potential levels in hemocytes. Catalase and superoxide dismutase activity in gills decreased following acclimation to low (8 ‰) and high (35 ‰) salinity. Exposure to the highest salinity levels (45 ‰) led to a decrease of superoxide dismutase activity levels, but did not influence the levels of catalase activity. Acclimation to low and high salinity was not accompanied with changes in osmotic fragility of hemocytes despite osmotic fragility curve according to changes in hemolymph osmolarity. Based upon these results, we postulate the involvement of cellular osmoregulatory mechanisms in the adaptation of the ark clam to short-term fluctuations of environmental salinity., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Inc. All rights reserved.)