Which ocean animal has three hearts and blue blood?

An octopus has three hearts and blue blood. Two branchial hearts pump blood through the gills, while one systemic heart sends oxygenated blood around the body.

Octopus blood appears blue because it uses hemocyanin, a copper-containing protein, to transport oxygen. Hemocyanin works especially well in the cold, low-oxygen conditions where many octopuses live, although it is less efficient than human haemoglobin under ordinary conditions.

The systemic heart stops beating while an octopus swims, which helps explain why many octopuses prefer crawling along the seafloor. Octopuses are cephalopod molluscs, related to squid, cuttlefish, and nautiluses. Unlike nautiluses, modern octopuses lack an external shell.