by Octopuses are iconic for their eight arms. But how many hearts does one make? octopus I have?
It turns out that an octopus has three hearts, Kirt Onthanks (opens in new tab), an octopus biologist at Walla Walla University in Washington, told Live Science. The same applies to 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 throughout 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 branchial hearts, each of which is attached to one of the octopus’s two gills, “so they’re often called ‘gill hearts,'” Onthank said.
The job of each branched heart is to pump blood through the gills to which it is attached. “These hearts are relatively small and not particularly 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,” Onthank explained.
Animals need enough blood pressure to efficiently transport blood throughout their bodies. If a person suffers from low Blood Pressure, “they can get dizzy or even pass out if they get up too quickly or exercise,” Onthank noted. “That’s because the low pressure isn’t enough to get blood to the brain.”
The octopus’s gills help draw vital oxygen from the water, and branched hearts help draw oxygen-poor blood through the gills. However, the oxygen-rich blood emerging from 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 re-pressurize the blood so it can be sent to the body efficiently,” he explained.
People have a similar problem. The heart’s two right chambers—the right atrium and right ventricle—pump oxygen-poor blood from veins in the lungs. When oxygen-rich blood leaves the lungs, it comes out at low pressure, Onthank said.
However, people then send this oxygen-rich blood back to the heart—specifically, to the two left chambers: the left atrium and the left ventricle. These chambers constrict the blood and send it through the arteries to the rest of the body.
In other words, octopuses and humans solve the same problem in two very different ways: octopuses having multiple hearts and humans having a multi-chambered heart.
“At the end of the day, those three hearts do the same job that your four-chambered heart does,” Onthank said. “Octopuses are a great example of how a complex, intelligent organism could evolve into a completely separate lineage from vertebrates. They have the same problems but have found different solutions.”
Quaintly, 1962 study (opens in new tab) suggested that the systemic heart of the giant Pacific octopus (Enteroctopus dofleini) can stop completely “for long periods of time when they’re resting, when they don’t need such high blood pressure,” Onthank said. Instead, “gill hearts do all the work.”
In addition, octopuses’ hearts stop for a few moments when they swim, and no one is sure why, Onthank said.
“I think the best explanation is that swimming puts such a high strain on their hearts that it’s better to stop them for a few moments while they’re swimming than to try to cope with that strain,” Onthank said.
Octopuses swim by shooting jets of water from their bodies.
“It’s a little bit like filling up a balloon and letting it fly around,” Onthank said. This puts a lot of stress on their body, which can prevent their heart from pumping properly. “So instead of fighting that pressure, they might just hit the pause button on their heart for a moment or two,” he added.
Octopuses generally prefer crawling to swimming. “Really, swimming for octopuses is a mess,” Onthank said. “They are blown forward with the same current of water that they breathe with, so swimming also mixes up their breathing. With swimming stopping their hearts for a few moments and messing up their breathing, it’s no wonder they don’t swim much.”
Copper based blue blood
Another way the octopus’s circulatory system differs from that of humans is how their blood is blue. This is because octopuses and their cephalopod Relatives use copper-based proteins called hemocyanins to carry oxygen in their blood, instead of the iron-based protein called hemoglobin that humans make.
Hemocyanins are less efficient than hemoglobin at binding to 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 and cold-temperature environments, which makes them more useful in the sea, Onthank said.
Additionally, when octopus hemocyanin binds to one oxygen molecule, it makes it more likely to attach to another. This property, called cooperativity, makes it much better at transporting oxygen than most hemocyanins, Onthank said.
Overall, in the sea, octopus hemocyanin “is at least a comparable, if not better, oxygen-carrying pigment than hemoglobin,” Onthank said. “Now if we think about whether octopuses could conquer the earth, then hemocyanin will probably hold them back.”
