Already possessing three hearts, nine brains (a central brain plus an autonomous neural ganglion inside each of its eight arms), and skin that changes color and texture in milliseconds, the octopus frequently feels like the closest thing to an alien intelligence on Earth. Adding to that otherworldly biology is the fluid pumping through its veins: instead of the bright crimson blood shared by humans, birds, fish, and reptiles, an octopus bleeds a striking, royal shade of cobalt blue. Far from a random quirk of nature, this blue blood is a brilliant chemical adaptation that allows octopuses to survive in the freezing, oxygen-starved depths of the world’s oceans.
Copper-Based Hemocyanin vs. Iron-Based Hemoglobin
The color of any animal’s blood is determined by the specific metalloprotein molecule it uses to capture oxygen from its lungs or gills and deliver it to its cells. In humans and nearly all vertebrates, that oxygen-carrying protein is hemoglobin, which is packed inside red blood cells and built around four atoms of iron. Just as an iron nail turns reddish-orange when exposed to oxygen and moisture (rust), the iron atoms inside human hemoglobin reflect red light when bound to oxygen, giving our blood its familiar scarlet color.
More than five hundred million years ago, when the evolutionary ancestors of cephalopods (octopuses, squids, and cuttlefish), along with horseshoe crabs, lobsters, and spiders, branched off from the lineage that led to vertebrates, they evolved a completely different oxygen-transport protein called hemocyanin. Instead of iron, hemocyanin binds oxygen using two atoms of copper. Just as the copper skin of the Statue of Liberty oxidizes to a bluish-green patina when exposed to air, when deoxygenated colorless octopus blood passes through the gills and binds with oxygen molecules, the copper atoms change their electronic configuration, absorbing red wavelengths of light and reflecting a vivid blue hue.
Why Copper Beats Iron in the Freezing Deep Sea
Chemically, at comfortable room temperatures on dry land, iron-based hemoglobin is actually a much more efficient oxygen carrier than copper-based hemocyanin. That is why an octopus needs three separate hearts—two branchial hearts to pump blood through each of its two gills, and one systemic central heart to pump the oxygenated blue blood out to the rest of its body—and why octopuses tire out quickly during prolonged high-speed chases.
Why, then, did octopuses keep copper blood? Because down on the dark ocean floor and in polar waters where temperatures plunge to near-freezing (sometimes minus one point eight degrees Celsius in the Southern Ocean) and dissolved oxygen levels drop dangerously low, iron-based hemoglobin hits a fatal wall: in extreme cold, hemoglobin holds onto its oxygen molecules so tightly that it refuses to release them into the animal’s starving muscles and organs. Giant, free-floating hemocyanin proteins dissolved directly in the octopus’s blood plasma (hemolymph) do the exact opposite—maintaining the delicate chemical balance required to both grab scarce oxygen at the gills and release it reliably into tissues even in icy, high-pressure abysses where a red-blooded animal would suffocate.
The Dangerous Vulnerability to Warming and Acidic Oceans
While copper-based hemocyanin makes the octopus a master of cold depths, research by marine physiologists at the Alfred Wegener Institute has revealed that blue blood also comes with a strict environmental Achilles’ heel: exquisite sensitivity to changes in water pH (acidity) and extreme heat.
Even more fascinatingly, studies comparing the Antarctic pareledone octopus with warm-water octopuses show that cold-water octopuses produce specialized genetic isoforms of hemocyanin tailored specifically to release oxygen at sub-zero temperatures. However, if ocean waters warm too rapidly or absorb excess carbon dioxide—lowering the pH of the blood—hemocyanin loses its ability to bind oxygen efficiently. Thus, the very copper chemistry that allowed octopuses to conquer the freezing depths for ratusan of millions of years makes them uniquely sensitive barometers of modern oceanic change.
