The blood of fish contains numerous cells and molecules that are able to respond to the many dangers that the marine environment can suffer. The presence of heavy metals, various pollutants and microplastics in the sea interact with aquatic species and tend to accumulate through the food chain, representing a threat to higher species (including man). Although there are detoxifying mechanisms of defense by fish, when the environmental concentrations of these harmful substances increase, the fish species respond by modifying important parameters of the blood that are an indication of the state of health of this species. It is therefore possible to carry out a simple blood sample from the fish to understand, through laboratory investigations, how these organisms respond to the insults that the sea undergoes and this in the future will represent an additional weapon to protect the sea and its beauties.