## 1 Introduction  
The aquatic environment is the ultimate container for pollutants. These include metal-elemental inorganic fertilizers that enter the aquatic environment, often through atmospheric deposition, the erosion of the geological matrix, or as a result of anthropogenic activities (e.g., the discharge of industrial, domestic, and mining wastes) and the use of pesticides and pesticides [1]. Metal source pollution is one of the greatest threats facing the world today. In fact, heavy metals pose serious ecological problems due to their toxicity and potential to accumulate in a variety of aquatic organisms, thereby causing damaging effects on the ecological balance of the aquatic environment. Therefore, this situation requires awareness of the vulnerability of these environments, regular monitoring, and the implementation of preventive measures. However, the sound and scientific use of biological indicators, especially in environmental biological assessments (monitoring the state of the environment or the effectiveness of compensation or restoration measures) has not been recognized until recently. This discipline was originally based on the use of biological indicators, which can provide information about the contamination of the environment by chemicals. On the other hand, the identification of the bioavailability of contaminants and their accumulation in the tissues of these organisms requires the use of certain ecotoxicological tools such as biomarkers. The purpose of these tools is to provide information on the harmful effects of environmental exposures manifested at the individual level. However, due to the abundance of pollutants, the complexity of their effects, and the many interactions between them, environmental diagnosis must be based on the use of a multi-biomarker approach [2; 3; 4]. Like polluted aquatic environments, the Mediterranean marine environment is one of the most polluted semi-enclosed basins in the world, especially with respect to metallic pollutants. According to previous studies, about 49-82% of copper, 68-76% of zinc, 21-65% of lead, and 75-92% of cadmium are deposited in the Mediterranean Sea in soluble forms and can thus be easily accumulated by organisms [5]. One of the nations in the Mediterranean region in which the importance of metals to aquatic environments is growing is Tunisia, particularly in coastal regions where chemical manufacturing is most significant. Numerous studies revealed that across these coastal areas, the Gulf of Gabes constitutes an ecosystem under intense industrial stress and is therefore likely to be polluted with various contaminants such as metallic trace elements [6; 7]. Numerous aquatic species have been applied in diverse ecotoxicological investigations to suggest the status of the pollution of ecosystems. Crustaceans, which are noted for their high accumulative potential, are among them [8]. The blue crab _Portunus segnis_ ([PERSON], 1775) was among the first lessepsian intruders to be identified in Egyptian waters 30 years after the Suez Canal was completed in 1868 [9]. It then established itself in coastal environments across the Levantine Basin, from Turkey to Egypt [10]. This crab species has been found in various Mediterranean locations such as the Aegean Sea, Sicily, and the northern Tyrrhenian Sea [11], as well as in Tunisian coastal waters and the surrounding region in recent years [10], such as Malta [12]. _P. segnis_ was initially discovered in Tunisia in the Gulf of Gabes in 2014 [13]; it quickly spread throughout the Gulf, comprising Djerba Island and the northern beaches, namely, the Gulf of Hammamet, and it has also been found in Libya on occasion [14; 15]. In this context, this work aims to use an invasive species of the Gulf of Gabes, _P. segnis_, as a bioindicator of the quality of the marine environment. This study aims to mirror the state of contamination of the aquatic surroundings in the Gulf of Gabes by pollutants through the determination of the heavy metal contents of the tissues of _P.