## 1 Introduction  
The loggerhead turtle (_Caretta caretta_) is a marine reptile that is distributed throughout the world; however, its population is in decline and in danger of extinction due to anthropic and environmental causes. The loggerhead turtle is considered to be a sentinel species of environmental pollution due to its longevity and high rate of biomagnification of contaminants [1]. These conditions allow for loggerhead turtles to be used as monitoring units for environmental pollutants. Loggerhead turtles reside in the Mediterranean Sea, and the Atlantic, Indian, and Pacific oceans [2]. In Colombia, the loggerhead turtle is distributed in the departments of Atlantico, San Andres Archipelago, Providencia and Santa Catalina, La Guajira, Magdalena, and Sucre. It has been reported that some juvenile individuals of this species use the Colombian Caribbean as foraging area, where they feed on algae, vascular plants, and invertebrates, such as crabs, mollusks, shrimps, starfish, and jellyfish [3]. It has been estimated that sea turtles can survive for very long periods (more than 50 years), time during which they store tremendously contaminating heavy metals in their tissues. Among them, mercury enters through the digestive tract and it is accumulated [4]. These sea turtles are sensitive to mercury storage due to their life history (prolonged anoxia, longevity, omnivore feeding, aquatic habitat) [4]. In Colombia, the loggerhead turtle is categorized as critically endangered according to the criteria of the IUCN. A 97% reduction of individuals was reported, from 200 nesting females that arrived per season to the Colombian Caribbean (in 1972) to six per year in the most recent assessment [5]. For this reason, it is important to preserve this specie in order to carry out studies that increase the survival, birth rates, reproductive success, and allow for collecting samples to monitor their health, and support future research [6]. Mercury produces lethal and sublethal effects in wildlife, causing different alterations (immunopathological, carcinogenic, teratogenic, neural, cardiovascular, renal, reproductive, and endocrine) [7; 8; 9; 10]. To date, it is known that mercury, interacting with the sulfhydryl groups of cells, generates oxidative stress and alters calcium levels. However, the reaction that it experiences in its methylated state (MeHg), with hundreds of proteins and the changes that they cause, need to be investigated in greater depth in order to understand the adverse effects on embryonic and fetal development, and how it interferes with the normal functioning of the neurological, cardiovascular, and immune systems [11]. Mercury (Hg) has potential for biomagnification in the environment. Its high ecotoxicological risk is due to its biogeochemical behavior, allowing for it to associate with solids, suspended matter, and sediments [10]. In oceans and seas, mercury is present in its inorganic and organic forms, with the methylated form, being methylmercury, the most toxic one (CH\({}_{3}\)-Hg\({}^{1}\)+) (MeHg) [12]. MeHg is more toxic than inorganic mercury, and it prevails in the marine food chain. It typically comprises 75-99% of the total mercury (Hg-T) in organisms [13]. In the Colombian Caribbean, sediment Hg-T concentrations range between 20.6-552.7 \(\mu\)g L\({}^{-1}\), with averages around 180-300 \(\mu\)g L\({}^{-1}\) and peaks of up to 1300 \(\mu\)g L\({}^{-1}\)[14; 15]. The threshold effect level of Hg-T is 130 \(\mu\)g L\({}^{-1}\)[15]. It is estimated that the global release of mercury from natural sources, anthropogenic sources, and reemission from past activities is 6000-11,000 tons/year [11]. In particular, anthropogenic sources are believed to account for approximately 80% of the annual Hg emission to the environment [16; 17]. Methylmercury induces the production of reactive oxygen species (ROS), generating oxidative stress and altering the antioxidant defense system of the cells when it joins the sulfhydryl and selenotid groups [18; 19].