## 1 Background  
In the last century, the marine environment has been severely affected by pollution associated with heavy industrialization and enormous population growth. Most of the world's population lives near the coast and therefore anthropogenic activities such as industry, urbanization, extensive agriculture, and tourism contribute to the pollution of the marine environment [1, 2]. Pollutants such as heavy metals, pesticides, oil-based products, plastics, particles from antifouling paint, and industrial and civil wastes pollute the marine environments [3, 4, 5, 6]. In the last two decades, nanotechnology has been one of the most innovative arguments for the scientific community. This is also because it can be a novel source of pollutants, especially for the marine environment [7, 8, 9, 10]. Nanomaterials (NMs)--which have at least one dimension in the range of 1-100 nm [11]--include nanoparticles (NP), which are normally found in nature and are formed by natural phenomena, mainly weathering and mineral formation processes in soil [12, 13, 14]. In addition, engineered nanomaterials (ENMs) and/or nanoparticles (ENPs) of human origin have become emerging contaminants nowadays. Among inorganic ENPs, the Organisation for Economic Co-operation and Development (OECD) has highlighted four metal or metal oxide ENPs that are of great interest due to their intrinsic properties, widespread use, and commercial importance, namely cerium oxide (CeO\({}_{2}\)), silver (Ag), zinc oxide (ZnO), and titanium dioxide (TiO\({}_{2}\)). In particular, silver nanoparticles (AgNPs) are used for their antibacterial properties [15, 16, 17, 18] and are included in over two hundred consumer medical products [19]. In addition, AgNPs are used in microelectronics due to their attractive properties such as high electrical and thermal conductivity, chemical stability, catalytic activity, and nonlinear optical behavior [20]. All these properties make AgNPs one of the most commercialized nanomaterials. AgNPs originating from wastewater generatedduring their synthesis and/or use in consumer products, as well as during their recycling and disposal, can enter the aquatic environment via industrial and municipal wastewaters. Their presence in the marine environment is a potential source of stress [7]. Oxidative stress is an important component of the stress response of marine organisms exposed to impairments from a variety of pollutants [3; 21; 22]. AgNPs can promote oxidative stress responses in aquatic organisms by impairing the capacity of antioxidant enzymes [23; 24; 25]. In zebrafish, [PERSON] et al. [26] showed increased levels of malondialdehydehyde and total glutathione (GSH), a decrease in catalase (CAT) and glutathione peroxidase activity (GPx), and DNA damage. It seems that the mechanism of AgNPs-mediated cytotoxicity in marine invertebrates is also mainly due to the action of reactive oxygen species (ROS) [8; 27; 28]. Bivalves are sensitive and ecologically relevant organisms that have a sessile filter-feeding diet and can accumulate organic and inorganic pollutants due to their low metabolic detoxification rate. Therefore, they are good bioindicators for monitoring chemical pollution of the marine environment [29; 30; 31]. The _Mytilus_ congeners are not only valuable indicators of pollution, but extensive background information is also available on their biological responses to a wide range of inorganic and organic chemicals [32; 33; 34; 35; 36; 37]. Indeed, [PERSON] et al. [30] proposed to use them as sentinel species for studying the specific effects of nanoparticles. The aim of this work is to develop electrophoretic techniques to evaluate the activity of antioxidant enzymes--catalase, superoxide dismutase, and glutathione peroxidase--and then apply the same techniques in an ecotoxicological study of the effects of AgNPs in the tissues of _Mytilus galloprovincialis_ (Lam. ), i.e., in the digestive gland and gills.