Mercury- Selenium concentrations were determined in 136 samples of Carcharhinus falciformis (Silky shark) sampled from the Chiapas coast, Gulf of Tehuantepec, Mexico during August 2014 – January 2015. Average Hg concentrations in neonates and juveniles (all values in μg g⁻¹ w.w.) were found to be 0.115 and 0.129 respectively, below the maximum permissible limit of 1 μg g⁻¹ set by the Mexican government. However, excess Se values (all values in μg g⁻¹ d.w.) observed in neonates (5.366) and juveniles (2.815) prove to maintain antioxidant ability by inducing Hg excretion and reducing its toxicity. Calculated Biomagnification Factor (BMF_TL) denoted high values for the prey C.hippurus, signifying absolute magnification of Hg and Se along the food chain. This study provides key toxicological evidences of Hg- Se interaction and their effects in marine systems and human health.