Distribution of metals in tissues of captive and wild Morelet's crocodiles and the potential of metallothioneins in blood fractions as a biomarker of metal exposure.

The distribution of Hg, Cd, Cu, and Zn in keratinized tissues, blood fractions, and excretory organs, and MTs in blood fractions and excretory organs was determined in captive, semicaptive, and wild Morelet's crocodiles and they were compared to select the most useful non-destructive tissues for the monitoring of metal exposure and to assess the potential of MTs as a biomarker. Our results indicate blood plasma, claws, and caudal scutes altogether are suitable tissues for xenobiotic metals exposure, with concentrations in blood plasma being an indicator of recent exposure, whereas concentrations in claws and caudal scutes are indicators of chronic exposure. Results in keratinized tissues suggest they are an important detoxification strategy in crocodiles, and claws presented the highest concentrations of metals in both captive (Hg = 0.44 ± 0.23 µg g-1, Cd = 11.10 ± 5.89 µg g-1, Cu = 45.98 ± 23.18 µg g-1, Zn = 124.75 ± 75.84 µg g-1) and wild populations (Hg = 1.31 ± 0.32 µg g-1, Cd = 26.47 ± 21.15 µg g-1, Cu = 191.75 ± 165.91 µg g-1, Zn = 265.81 ± 90.62 µg g-1). Thus, they are an appropriate tool for assessing metal exposure in populations where scutes clipping as a marking technique is not allowed, and their collection is less complicated than with other tissues. MTs are a suitable biomarker in blood plasma, whereas in erythrocytes detoxification processes might depend on hemoglobin, rather than MTs. Future studies should consider the implementation of these tools for the monitoring of wild populations.

Mercury and metallothioneins in blood fractions and tissues of captive Morelet's crocodiles in Quintana Roo, Mexico.

Even though studies of heavy metals and their relation with metallothioneins (MTs) in reptile tissues have been conducted, research on heavy metals and MTs in organs and blood fractions of crocodylians is limited. To date there are no studies on the distribution of MTs in organs and their relation with mercury (Hg), or the concentration of MTs in plasma and erythrocytes of crocodylians. In order to understand the role of MTs in crocodiles, our aim was to assess the detoxification mechanisms for exposure to metals in Morelet's crocodile (Crocodylus moreletii) by using two biomarkers (Hg and MTs) in blood fractions and tissues, and comparing them with concentrations between two populations of crocodiles, one previously wild and currently captive (Theme Park) and another raised in a Wildlife Management Unit (WMU). The caudal scutes from the Theme Park showed higher concentrations of Hg than those from the skin in the WMU samples, and significant negative relationships were observed between the total length (TL) and Hg in the scutes. The significant negative relationship between Hg and hemoglobin (Hb) may be due to disorders in the oxidation process and even denature of this protein, while the positive trend observed between MTs and Hb is consistent with the detoxifying functions and the protection against oxidative damage. This study is the first to report Hg in the erythrocytes of crocodylians and the use of MTs for testing the potential of these biomarkers as a tool to assess Hg exposure in crocodile's habitats.

Metals and metallothioneins in Morelet's crocodile (Crocodylus moreletii) from a transboundary river between Mexico and Belize.

The aim of this study was to determine concentrations of heavy metals (cadmium [Cd] and mercury [Hg]) and metallothioneins (MTs) in blood plasma and caudal scutes of Morelet's crocodile (Crocodylus moreletii) from Rio Hondo, a river and natural border between Mexico and Belize. Three transects of the river (approximately 20 km each) were surveyed in September 2012 and April 2013, and samples were collected from 24 crocodiles from these areas. In blood plasma, Cd (7.6 ± 9.6 ng/ml) was detected in 69 % of samples (n = 9); Hg (12.2 ± 9.2 ng/ml) was detected in 46 % of samples (n = 6); and MTs (10,900 ± 9,400 ng/ml) were detected in 92 % of samples (n = 12). In caudal scutes samples, Cd (31.7 ± 39.4 ng/g) was detected in 84 % of samples (n = 12) and Hg (374.1 ± 429.4 ng/g) in 83 % of samples (n = 20). No MTs were detected in caudal scutes. Hg concentrations in scutes from the Rio Hondo were 2- to 5-fold greater than those previously reported in scutes from other localities in northern Belize. In blood plasma, a significant positive relationship between Hg and body size was observed. Mean concentrations of Cd and MTs in size classes suggest that MTs may be related to Cd exposure. This is the first report of MT presence in crocodile blood.