First evidence of arsenic and mercury bioaccumulation and associated genotoxic effects in Caiman crocodilus in a mining-affected river in the Colombian Pacific.

As a keystone species in tropical freshwater ecosystems, Caiman crocodilus (Linnaeus, 1758) serves as a valuable bioindicator for assessing genetic damage in polluted environments. This study examined mercury (Hg) and arsenic (As) bioaccumulation in caudal scutes and blood across various age groups, alongside the evaluation of genotoxic effects using the micronucleus (MN) assay. Among adults, subadults and juveniles (n = 16), Hg concentrations in the scutes ranged from 41.8 to 535 μg/kg with a median of 145.0 μg/kg, while in blood they ranged from 32.5 to 472.9 μg/L and a median of 131.7 μg/L. The median concentrations of As in blood were 1.0 μg/L, whereas in scutes they were below the limit of quantification (LOQ) established in the analytical methods. Females exhibited slightly higher Hg levels in both scutes (162.0 μg/kg) and blood (131.7 μg/L) compared to males (scutes: 145.0 μg/kg; blood: 118.4 μg/L), although these differences were not statistically significant (p > 0.05). Subadult individuals had significantly higher blood Hg concentrations than juveniles (U = 9; p = 0.03; n = 9). Neonates (n = 6), the median Hg concentrations were 303.1 μg/kg and 109.6 μg/L in scutes and blood, respectively. The MN assay revealed evidence of genotoxic damage. Although the mean MN frequency in large individuals (excluding neonates) was low (0.3), nuclear buds (NB, 9.8) and binucleated cells (BC, 1.4) were more prominent. A negative trend was observed between Hg concentrations and the frequency of MN, NB, and BC, whereas As showed a positive correlation with BC (r = 0.38, p = 0.28). Additionally, 37.5% of the individuals exhibited poor body condition (Elsey condition factor < 1). These findings support the potential of C. crocodilus as an effective sentinel species for assessing genotoxic effects linked to environmental pollution. Moreover, this study contributes valuable data to pollution monitoring efforts in the Colombian Pacific region, which have largely focused on toxic metal(loid) analysis in fish to date.