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And it stops one of them to swim

An Octopus Has Three Hearts and Blue Blood

It carries oxygen with copper instead of iron, which turns its blood blue and forces a strange trade-off.

Two branchial hearts,one for each gillOne systemic heart,for the whole bodyBlood coloured blueby copper, not red byironAnd the systemicheart stops while itswims
Three pumps, because one would not be enough.

  1. Step 1 of 5

    Our blood carries oxygen with iron. Theirs uses copper.

    Your blood Haemoglobin, builtaround iron — redOctopus blood Haemocyanin, builtaround copper — blue

    Haemoglobin holds an iron atom, and iron plus oxygen is red — the same chemistry as rust. Octopus blood uses haemocyanin, which holds copper instead. Copper plus oxygen is blue. Same job, different metal, different colour.

    Drained of oxygen, haemocyanin is nearly colourless. The blue only appears when it is carrying.

  2. Step 2 of 5

    Copper wins in cold water with little oxygen.

    Copper-based bloodBetter in cold, oxygen-poor waterCarries less oxygen per unit of blood
    A good deal on the sea floor, a poor one anywhere warm and active.

    Haemocyanin holds on to oxygen better in the cold, and works well when there is not much of it dissolved in the water. For an animal living on a cold sea floor, that is exactly the right trade — which is why crabs, lobsters, spiders and squid use it too.

  3. Step 3 of 5

    The cost is that it is inefficient, so it needs more pumping.

    Hearts anoctopus hasHearts youhaveSame job, donelessefficiently

    Haemocyanin floats loose in the blood rather than being packed into cells, and it simply carries less oxygen than haemoglobin does. To keep up, an octopus has to push a lot more blood around — and that is what the extra hearts are for.

  4. Step 4 of 5

    Two hearts serve the gills, one serves the body.

    Branchial heart pushes blood into a gill Blood picksup oxygen Systemic heart drives it round the body Oxygendelivered,blood returnsoctopus

    The two branchial hearts sit beside the gills and push blood through them to pick up oxygen. The systemic heart then takes that oxygenated blood and drives it round the rest of the animal. It is a two-stage pump, and the staging is what makes the low-capacity blood workable.

  5. Step 5 of 5

    And swimming stops the main heart, so it prefers to walk.

    JettingThe body squeezeswater outPressureThe squeeze hitsthe heartStallThe heart stopswhile it jetsDebtIt must stop andrecoverChoiceSo it walks onits arms instead
    A rare case of an animal whose fastest gear is also its most costly.

    When an octopus jets through the water, the pressure of that movement stops the systemic heart beating. Swimming is therefore exhausting and it has to recover afterwards. Given a choice, an octopus crawls across the sea floor on its arms instead.

    This is one reason octopuses are usually found near the bottom rather than out in open water.

The short version

An octopus carries oxygen with copper-based haemocyanin, which is blue and low-capacity, so it needs two gill hearts plus a body heart — and jetting stops the body heart, which is why it would rather walk.

Try it yourself

Look at a cut copper pipe left outdoors. The blue-green film is copper joined to oxygen — the same pairing that colours an octopus's blood.

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