The octopus has three hearts

The octopus carries three hearts inside. Two pump blood to the gills to oxygenate it; the third drives it to the rest of the body.

The odd part comes when the octopus swims. The central heart stops beating under the effort, which is why swimming exhausts these animals so much. Whenever they can, they would rather walk along the seabed on their arms. It is not laziness: it is cardiology.

As if that were not enough, their blood is not red but blue. Instead of haemoglobin with iron they use haemocyanin with copper, far more efficient at carrying oxygen in the cold, dark water of the sea floor.

Three hearts, blue blood and a clear preference for going on foot. The octopus is like nothing else we know.

Source: Natural History Museum (London)