Why do octopuses have blue blood?

Human blood is red because of hemoglobin, which contains iron. Octopuses, squid and many other molluscs and arthropods use hemocyanin, which binds oxygen with copper atoms and turns blue when oxygenated. Hemocyanin works better than hemoglobin in cold, low-oxygen water, which suits many ocean habitats. Because it is less efficient at carrying oxygen overall, octopuses have three hearts: two pump blood through the gills and one pumps it around the body. The heart supplying the body actually stops beating when an octopus swims, which is one reason they prefer crawling.