The blood of a human is red as it contains haemoglobin. The haemoglobin, which carries oxygen around our body, contains iron. However, the blood of an octopus is blue as it contains the protein, haemocyanin. Haemocyanin, which carries oxygen around the octopus's body, contains copper.
The copper-based protein is more efficient at transporting oxygen molecules in cold and low-oxygen conditions so is ideal for life in the ocean.
If the blood (called haemolymph in invertebrates) becomes deoxygenated - when the animal dies, for example - it loses its blue colour and turns clear instead.
