Animals
Three Hearts, Blue Blood and Arms That Think: A Guide to the Octopus
The octopus looks like something a science-fiction writer invented: three hearts, blue blood, eight arms that each behave a little like a small brain, and the ability to change colour and texture in a fraction of a second. Yet every one of these details has been studied and documented. In this article we go through them one by one, and explain not only what is strange about the octopus, but why its body is built this way.
Three hearts, each with its own job
Humans have one heart that sends blood both to the lungs and to the rest of the body. The octopus splits that work between three hearts. Two of them, the branchial hearts, sit next to the gills and push blood through them so it can pick up oxygen. The third, the systemic heart, takes that oxygen-rich blood and sends it to the organs and muscles.
Here is the surprising part: the systemic heart stops beating while the octopus swims. That is one reason octopuses usually prefer to crawl along the sea floor. Swimming tires them quickly, because their body gets less oxygenated blood exactly when it is working hardest.
Why is octopus blood blue?
Our blood is red because of haemoglobin, an iron-based protein that carries oxygen. In octopuses the job is done by a different protein, haemocyanin, which is built around copper instead of iron. When haemocyanin binds oxygen it takes on a bluish tint, so octopus blood is blue.
Haemocyanin works well in cold water that is low in oxygen, which is why it is common in molluscs and crustaceans that live in such conditions. It also has a weakness: it is very sensitive to changes in water acidity. That is one of the reasons scientists worry about how ocean acidification may affect octopuses and their relatives.
A brain spread through the body
Octopuses have a very large nervous system for an invertebrate. The surprising part is where it sits: about two-thirds of their neurons are not in the head but in the arms. Each arm has its own cluster of nerves and can sense, touch, taste and carry out complex movements with little instruction from the central brain.
That is where the popular line "an octopus has nine brains" comes from: one central brain plus eight nerve centres in the arms. It is an exaggeration, since the arm centres are not true brains, but it captures the idea well: much of an octopus's thinking happens outside its head.
This split lets an octopus do several things at once. One arm can search a crack for food while another explores the surroundings. The suckers carry chemical receptors, so an octopus effectively tastes whatever it touches.
Camouflage without rivals
Octopus skin holds thousands of small pigment cells that the animal can expand and contract very quickly. That lets it change colour, add spots or stripes, and even change the texture of its skin to look like rock, coral or seaweed, all within a fraction of a second.
When camouflage is not enough, the octopus has another trick: ink. The dark cloud hides it from predators and gives it time to escape. The ink also contains an enzyme that irritates a predator's eyes and temporarily dulls its sense of smell and taste.
Short lives, total devotion
Despite their impressive intelligence, most octopuses live short lives, often just one or two years. Males die soon after mating. The female guards her eggs, cleans them and keeps water flowing over them, and usually stops eating for the whole period. She dies around the time the eggs hatch.
What the octopus teaches us
The octopus is a great example of how intelligence can evolve along a path completely different from ours. Its evolutionary line split from ours hundreds of millions of years ago, and yet it developed abilities such as problem-solving and learning from experience. That is why many researchers see it as a "natural experiment" that helps us understand what it really takes to think.
Test yourself
In Bullshit, our bluffing game, one of the questions is exactly about this: how many hearts does an octopus have? Players have to invent a believable fake answer and then spot the truth. After reading this article, you have an edge.