Octopuses have three hearts: one pumps blood around the body; the other two pump blood to the gills. The reason for this impressive cardiac hardware probably comes down to the unusual composition of their blood.
Unlike vertebrates that have iron-rich haemoglobin packed into red blood cells, octopuses – along with some tarantulas, scorpions and horseshoe crabs – have copper-rich haemocyanin dissolved directly in their blood (this means their blood is blue!).
Haemocyanin is less efficient than haemoglobin as an oxygen transporter. The three hearts help to compensate for this by pumping blood at higher pressure around the body to supply the octopuses’ active lifestyle.