Hemocyanins are copper-containing oxygen carriers occurring freely dissolved in the hemolymph of many mollusks and arthropods. Binds 2 copper ions per heterodimer. Decamers of large identical subunits (350 kDa), each containing 7 globular oxygen-binding functional units: ODA, ODB, ODC, ODD, ODE, ODF, and ODG. Decamer formation requires the presence of magnesium ions. Belongs to the tyrosinase family. Hemocyanin subfamily.