Why lobster hemolymph is blue

The blood-like fluid of many arthropods, including lobsters, is called hemolymph. It carries oxygen with hemocyanin, a copper-containing protein rather than an iron-based one. When hemocyanin binds oxygen, it turns blue, giving oxygenated hemolymph a bluish tint. Hemocyanin is found in many aquatic invertebrates and functions in cold conditions too, although its performance depends on the species and environment.