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Octopuses are iconic for their eight arms. But how many hearts does one need octopus have?
It turns out that an octopus has three hearts, Kirt Onthank (opens in new tab), a squid biologist at Walla Walla University in Washington, told Live Science. The same applies to their closest relatives, octopuses and cuttlefish.
The largest heart in squids, the systemic heart, is located in the center of the mollusc’s body. It pumps oxygen-rich blood throughout the body, but not to the gills. “It’s the largest and most muscular of the three hearts,” Onthank said.
The other two hearts are called gill hearts, each attached to one of the octopus’s two gills, “that’s why they’re often called ‘gill hearts,'” Onthank said.
The job of each gill 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 squid change color?
So why does an octopus need three hearts? “The same reason humans and other mammals need four chambers in their hearts — to solve the problem of low blood pressure,” Onthank explained.
Animals need adequate blood pressure to effectively move blood throughout their bodies. When a person suffers from low blood pressure“They can become light-headed or even pass out if they stand up too quickly or exert themselves,” Onthank noted. “That’s because the low pressure isn’t enough to supply blood to the brain.”
Octopus gills help draw vital oxygen from the water, and the branching hearts help pump deoxygenated blood through the gills. However, the oxygen-rich blood that exits the gills comes out at low pressure, “which isn’t good for sending blood to the body,” Onthank said. Octopuses “have another heart after the gills to repressurize the blood so it can be efficiently sent to the body,” he explained.
people have a similar problem. The two right chambers of the heart – the right atrium and right ventricle – pump deoxygenated blood out of the heart veins into the lungs. When oxygenated blood leaves the lungs, it comes out at low pressure, Onthank said.
However, humans then send this oxygen-rich blood back to the heart – more precisely to the two left chambers: the left atrium and the left ventricle. These chambers repressurize the blood and send it through the arteries to the rest of the body.
In other words, squid and humans solve the same problem in two very different ways: squid by having multiple hearts, and humans by having a multi-chambered heart.
“In the end, those three hearts do the same job as your four-chamber heart,” Onthank said. “Octopuses are a great example of how a complex, intelligent organism might evolve in a completely separate lineage from that of vertebrates. They have the same problems but encountered different solutions.”
Fascinating, a 1962 study (opens in new tab) proposed that the systemic heart of the giant Pacific octopus (Enteroctopus dofleini) could stop completely “for long periods of time when they’re resting, when they don’t need as much high blood pressure,” Onthank said. Instead, “the gill hearts do all the work.”
Also, octopus hearts stop for a few moments while swimming, and nobody’s sure why, Onthank said.
“I think the best explanation is that swimming puts such high pressure on her heart that it’s better to stop her swimming for just a few moments than to try to pump against that pressure,” Onthank said.
Octopuses swim by spouting jets of water from their bodies.
“It’s a bit like filling up a balloon and releasing it to let it fly around,” Onthank said. This puts a lot of pressure on her body, which can cause her heart to not pump properly. “Rather than fight that pressure, they can just press the pause button on their heart for a moment or two,” he added.
Octopuses generally prefer to crawl to swim. “Really, swimming for squid is kind of a mess,” Onthank said. “They blow themselves forward with the same stream of water they breathe with, so swimming messes with their breathing too. Since swimming stops their heart for a few moments and upsets their breathing, it’s not surprising that they don’t swim that much.”
Copper-based blue blood
Another way in which the octopus’ circulatory system differs from that of humans is how its blood is blue. This is because squid 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 use.
Hemocyanins are less efficient than hemoglobin in 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 environments and at low temperatures, making them more useful at sea, Onthank said.
Additionally, when octopus hemocyanin binds to one oxygen molecule, it is more likely to cross over to another. This property, called cooperativity, makes it much better at carrying oxygen than most hemocyanins, Onthank said.
All in all, marine octopus hemocyanin “is at least a comparable, if not better, oxygen-transporting pigment than hemoglobin,” Onthank said. “Now if we think about whether squid could conquer land, hemocyanin would probably hold them back.”
