Octopuses Have Three Hearts

An octopus has three hearts, and its blood is blue.

Two hearts push blood through the gills and a third sends it around the body. The blue color comes from hemocyanin, a copper-based protein that carries oxygen where our blood uses iron-based hemoglobin.

Check it yourself

Look up hemocyanin in any zoology reference, or read the octopus pages from a major aquarium. Monterey Bay Aquarium is a good start.