by 
Octopuses are iconic for their eight arms. But how many hearts does one have octopus to have?
It turns out that an octopus has three hearts, Kirt Onthank (opens in new tab), an octopus biologist at Walla Walla University in Washington told Live Science. The same goes for their closest relatives, squid and cuttlefish.
The largest heart of octopuses, the systemic heart, is located in the middle of the mollusk’s body. It pumps oxygenated blood around the body but not to the gills. “It’s the biggest and most muscular of the three hearts,” Onthank said.
The other two hearts are called gill hearts, each of which is attached to one of the octopus’s two gills, “so they’re often called ‘gill hearts,'” Onthank said.
The function of each gill heart is to pump blood through the gill to which it is attached. “These hearts are relatively small and not especially strong,” Onthank said.
Related: How do octopuses change color?
So why does an octopus need three hearts? “The same reason humans and other mammals need four chambers in their hearts – solving the problem of low blood pressure,” explained Onthank.
Animals need sufficient blood pressure to deliver blood throughout the body effectively. If a person suffers from low blood pressure, “they can get dizzy or even pass out if they stand up too quickly or exert themselves,” Onthank noted. “This is because the low pressure is not enough to supply blood to the brain.”
Octopus gills help extract vital oxygen from the water, and gill hearts help pump oxygen-poor blood through the gills. However, the oxygen-rich blood coming out of the gills comes out at low pressure, “which is not good for sending blood around the body,” Onthank said. So octopuses “have another heart after the gills to pressurize the blood again so that it can be sent to the body efficiently,” he explained.
Humans have a similar problem. The two right chambers of the heart – the right atrium and the right ventricle – pump oxygen-poor blood from the veins in the lungs. When oxygen-rich blood leaves the lungs, it leaves at low pressure, Onthank said.
However, humans send this oxygen-rich blood back to the heart – specifically, to the two left chambers: the left atrium and the left ventricle. These chambers repressurize the blood and send it through arteries to the rest of the body.
In other words, octopuses and humans solve the same problem in two very different ways: octopuses by having multiple hearts and humans by having a heart with multiple chambers.
“Ultimately, these three hearts are doing the same task that your four-chambered heart does,” Onthank said. “Octopuses are a great example of how a complex, intelligent organism can evolve into a completely separate lineage from vertebrates. They have the same problems, but they found different solutions.”
Curiously, a 1962 study (opens in new tab) suggested that the systemic heart of the giant Pacific octopus (enteroctopus dofleini) can stop altogether “for long periods of time when they’re resting, when they don’t need high blood pressure as much,” Onthank said. Rather, “the hearts of gills do all the work”.
Also, octopuses’ hearts stop for a few moments when they swim, and no one is quite sure why, Onthank said.
“I think the best explanation is that swimming puts such high pressure on their hearts that it’s better to stop them for a few moments while swimming rather than trying to pump against that pressure,” Onthank said.
Octopuses swim by squirting jets of water from their bodies.
“It’s a bit like blowing up a balloon and letting it fly,” Onthank said. This puts a lot of pressure on their bodies, which can prevent their hearts from pumping properly. “So instead of fighting that pressure, they can just hit the pause button in their hearts for a moment or two,” he added.
Octopuses generally prefer crawling to swimming. “Really, swimming for octopuses is a mess,” Onthank said. “They launch themselves forward with the same flow of water they breathe with, so swimming also disrupts breathing. With swimming stopping the heart for a few moments and disrupting breathing, it’s not surprising that they don’t swim as much. “
Blue blood, copper based
Another way in which the octopus circulatory system differs from that of humans is how blue its blood is. This is because octopuses and their cephalopod relatives use copper-based proteins called hemocyanins to carry oxygen in their blood, rather than the iron-based protein called hemoglobin that humans use.
Hemocyanins are less effective than hemoglobin at binding oxygen at room temperature. One might then naively think that this might be a reason why the octopus needs three hearts. However, hemocyanins carry more oxygen than hemoglobin in low-oxygen environments and cold temperatures, which makes them more useful at sea, Onthank said.
Furthermore, when octopus hemocyanin binds to one oxygen molecule, it is more likely to bind to another. That property, called cooperativity, makes it much better at transporting oxygen than most hemocyanins, Onthank said.
In short, in the sea, octopus hemocyanin “is at least a comparable, if not better, oxygen-carrying pigment than haemoglobin,” Onthank said. “Now, if we’re wondering if octopuses could conquer land, then hemocyanin would probably stop them.”
