Seafood contains iodine. Iodine, when dissolved in extracellular
fluid, will form iodide, which acts as a reducing agent which
reduces Calcium ions to form Calcium metal. The decrease in
concentration of calcium ions, which are needed as co-factors of
thrombokinase which converts prothrombin to thrombin, will cause
blood clotting to slow down due to the decrease in rate of
formation of fibrin from fibrinogen.
O Level biochemistry. 
