Octopuses have three hearts and blue blood that pumps through their bodies.

The longer version

Octopuses rely on copper-based haemocyanin to carry oxygen, which gives their blood a distinctly blue tint when it is oxygenated. The pigment works less efficiently than the iron-based haemoglobin used by vertebrates, so the circulatory system compensates with extra pumping power.

Two smaller branchial hearts sit near the gills and push blood through them to pick up oxygen. The larger systemic heart then sends that blood out to the organs and muscles. Because the main heart pauses while the animal swims, vigorous movement can leave an octopus short of breath, which is one reason crawling is often preferred over jetting away.

Where this entry sits

This collection gathers small surprises about mammals, reptiles, amphibians, birds, fish, insects, and the pets living alongside people. Expect odd details about senses, anatomy, and everyday behaviour that many people never notice in the animals around them. The full list sits on the Animals page, where every entry is listed in order.