The Hearts and Blood of the Octopus

All octopuses have three hearts. There are two 'accessory' hearts, known as branchial hearts, which pump blood through the gills, while the third is a systemic ('normal') heart that pumps blood through the body.

The octopus's blood is bluish in colour because it uses the copper–rich protein haemocyanin to transport oxygen around the body. Most creatures use haemoglobin, which is iron–rich and makes the blood red.

In cold conditions, with low oxygen levels, haemocyanin transports oxygen more efficiently than haemoglobin. It makes the blood very viscous, and it requires considerable pressure to pump it round the body. An octopus's blood pressure can exceed 75 mm of mercury; this is comparable with that of humans, which might be expected to be higher as the human is a much bigger creature.

The octopus is not the only creature that uses haemocyanin to transport oxygen around its body. Haemocyanins were first discovered in the edible snail (a mollusc, like the octopus) and the horseshoe crab (an arthropod). They were subsequently found to be common among crustaceans and are also used by some land arthropods such as the tarantula, the emperor scorpion, and the house centipede.

© Haydn Thompson 2018