## 4 Discussion  
The present study is the first report on oxidative stress of RBCs of captive loggerhead turtles (_Caretta caretta_) that were exposed to methylmercury in bioassays. RBCs of sea turtles have not been used to perform in vitro bioassays until today. This study showed RBCs to be a sensitive, simple, and inexpensive model for performing bioassays for ecotoxicological evaluation of heavy metals, such as mercury and, potentially, other environmental pollutants. Furthermore, SOD, GST, and MDA were sensitive biomarkers of environmental pollution and oxidative stress in loggerhead turtles. The blood count was conducted prior to the MeHg bioassay, in order to show the health status of the turtles. Plasma and hematological biochemistry analytes vary significantly between loggerhead turtle populations in the wild, due to environmental factors, temperature, type of diet, sex, size, pH, salinity, water quality, population density, development stage, maturity, weight, and season [50; 51]. Additionally, the hematological response to man-made influences must be taken in consideration when reviewing previously published data. For these reasons, it is important to establish population-specific reference information [52]. In this study, all of the specimens presented an apparently good health showed by the blood parameters obtained (Table 2). The percentages of all white blood cells were within the previously described ranges for loggerhead turtles (Figure 7, [47]). Additionally, mean corpuscular volume, albumin, and plasma protein were similar to those reported previously [47; 48; 51; 52; 53]. The variations in concentration of the different parameters were within the accepted levels [53; 54]. A simple analysis that is presented in Figure 7 shows that the percentages of white cells: heterophile, eosinophils, limphocytes, monocytes, and azurophiles are within the reference ranges [47] that are present in loggerhead turtles with an apparent good health. However, as we have seen, the size and weight of turtles are important variables that produce large variations in the concentrations of the different hematological analytes. In the study that was carried out by [PERSON] et al. (2019) [47], the 191 analyzed individuals are 2.5 times larger than the individuals in our study according to the SCL range (54.8-100.8 cm vs. 25-32 cm). For this reason, the percentages of the heterophile and limphocytes in this study are approximately half of the maximum values (91.6% and 90%, respectively). In the same way, the eosinophils, monocytes, and azurophiles found are four, five, and three times under the maximum value (13.7%, 16.1%, and 8%, respectively). All of the cell types exceed the minimum range to present them within the normal ranges. These differences are apparently a result of the weight and size of the turtles, but, as described above, it might be due to other variables, such as captivity, feeding, and/or stress [50; 51]. Figure 6: [PERSON]’s correlation between the variables MeHg concentration, exposure time and MDA levels. This study used captive loggerhead turtles. For this reason, the background Hg-T in water and in turtle RBCs were determined prior to bioassays (0.0032 \(\pm\) 0.001 mg L\({}^{-1}\) and 0.031 \(\pm\) 0.011 mg L\({}^{-1}\), respectively). The 10-fold concentration of baseline Hg-T in RBCs demonstrates how feeding is an important factor for the bioaccumulation and biomagnification of this metal. The diet of loggerhead turtles in their natural habitat is composed of cnidarians, gastropods, copepods, and other crustaceans, polychaetes, cephalopods, fish, algae, plant remains, fish eggs, and hydrozoans [55]. In this study, captive donor loggerhead turtles from Santa Martha Aquarium were fed with fish of species as machuelo (_Optishonema oglinum_), macabi (_Elops saurus_) and picuda (_Sphyraen ensis_). It is presumed that fish diet had Hg concentrations like those that were previously reported for machuelo (_Optishonema oglinum_) (0.11 \(\upmu\)g L\({}^{-1}\)) [56].