The European eel (Anguilla anguilla) is a migratory, catadromous fish species, consisting of a single stock, distributed throughout the European continent as well as in the Mediterranean basin, which reproduces in the Atlantic Ocean and for which the panmixia hypothesis is currently accepted. Eel fishing is carried out throughout the distribution area of the species and concerns the juvenile and preadult stages, but the conservation of the stock depends on the recruitment and emigration of breeding animals at sea. Eel farming is practiced in many countries, for a European level of around 8000 tons; the production completely depends on the wild seed, since the reproduction artificial, although implemented experimentally, does not go beyond the larval stage, at least in the European eel. Adult eels can survive in both air and water thanks to the fact that respiratory exchanges can occur both through the gills and through the skin. Eels are present in a wide range of aquatic habitats (rivers, canals, estuaries, lakes, ponds, and lagoons), in relation to their great adaptability to different environmental conditions; moreover, being a euryhaline species, eels can adapt to both fresh and sea water, and well tolerate variations in oxygen concentrations. As a primarily bottom-dwelling fish, the eels rely in their feeding on the prey population that is present there and prefer to eat at night [22,23].

The biological cycle of the eel is considered unique in relation to the nature and extent of reproductive migration. This life cycle takes place mainly in continental fresh waters, for most of the life of the eel, and in open ocean during the reproductive phase. Moreover, the life cycle involves a succession of metamorphic stages: leptocephalus larvae, hatching from eggs in the Sargasso Sea and heading for continental waters; glass eels that move into continental waters; yellow eels that remain in continental waters until sexual maturity is reached; silver eels that migrate back to reproductive areas [22,23].

This complex life cycle exposes the eels to different and numerous stressors, and many studies have shown that overfishing, habitat loss, pest attack and water contamination pose a serious threat to eel survival. In particular, the presence of fat in the eels, their long stay in the same area and their ability to accumulate contaminants especially lipophilic, make this species particularly susceptible to aquatic contamination. Indeed, to date, the European eel is included, according to IUCN, among the endangered species and considered at risk of extinction [22,24,25,26]. In addition to the different types of contaminants present in the water, illicit drugs are proving to be a new, widespread class of contaminants with remarkable pharmacological properties, that are likely to have effects on aquatic fauna [3,4,5,6,7].

Therefore, researchers' research group has begun to study the effects of one of these illicit drugs, cocaine, on European eel, to understand how this substance could affect the physiology and reproductive capacity of this species. The experiments were carried out on silver eels, which, after a month’s acclimatization, were exposed to an environmental concentration of cocaine (20 ng/L⁻¹), among those measured in surface water. The eels were housed in aquariums with a capacity of 300 L, in dechlorinated and well-aerated tap water, exposed to natural photoperiod and not fed, as in the silver stage eels do not normally feed. Moreover, the following parameters of the water were established and monitored: dissolved oxygen 8.1 ± 0.5 mg/L; salinity 0, temperature 15 ± 1 °C, pH 7.3 ± 0.2, ammonia < 0.1 mg/L. For the exposure, a stock solution of 3 mg/500 mL of cocaine free-base in ethanol was prepared and kept in the refrigerator. During the exposure, 1 mL of the stock solution was administered every day, after the change of the water, directly in each aquarium for 30–50 days. At the same time control groups, exposed to tap water only, and carrier groups, exposed to ethanol only, in the same concentrations as eels receiving cocaine, were set up. At the end of exposure, some of the exposed eels were deprived of cocaine and exposed to tap water only, for 3–10 days, to verify their recovery ability. At the end of the exposure, or the recovery period, histological, histochemical, biochemical, and molecular biology analyses were performed [27]. All these studies were carried out in accordance with EU Directive 2010/63/EU for animal experimentation and institutional guidelines for care and use of laboratory animals and were authorized by the Italian Ministry of Health’s General Directorate of Animal Health and Veterinary Drugs.