Hemocyanin is a copper-containing protein found in the blood of some invertebrates, such as arthropods and mollusks. It serves a similar function to hemoglobin in vertebrates, transporting oxygen throughout the organism's body. Hemocyanin is blue when oxygenated due to the presence of copper ions, which distinguishes it from the red color of oxygenated hemoglobin. This protein is crucial for the survival of many marine and terrestrial species, allowing them to thrive in various environments. Hemocyanin's ability to bind oxygen efficiently makes it an important subject of study in fields like biochemistry and evolutionary biology.