What color is the blood of an octopus?

An octopus has blue blood. Its color comes from hemocyanin, a copper-containing protein that transports oxygen through the bloodstream.

Humans and most other vertebrates use iron-rich hemoglobin, which appears red when oxygenated. In an octopus, oxygen binds to copper in hemocyanin instead. Oxygenated hemocyanin has a blue appearance, giving the animal’s blood its distinctive color.

Hemocyanin works particularly well in cold, oxygen-poor seawater, conditions common in many octopus habitats. It is less efficient than hemoglobin in some respects, so the octopus compensates with three hearts: two pump blood through the gills, and one circulates it through the rest of the body.

The blue color is not related to ink. Octopus ink is a separate defensive secretion, usually dark because it contains melanin. Another common mix-up is assuming all blue-looking sea creatures have blue blood; in octopuses, the color specifically reflects copper-based oxygen transport.