Three Hearts and Blue Blood: Meet the Octopus

Octopuses have three hearts, blue copper-based blood and most of their neurons in their arms. Read that sentence again and it starts to sound like a description of a creature from another planet. Yet octopuses live in oceans all over Earth, from shallow tide pools to the deep sea, and some species are common along the coasts of the United States. They are clever, curious, short-lived and endlessly surprising. Spend a little time learning about them and you may never look at a seafood menu the same way again.

An unusual body plan

Octopuses belong to a group of animals called cephalopods, which also includes squid, cuttlefish and the nautilus. They are mollusks, which makes them distant relatives of snails and clams. Over hundreds of millions of years, their ancestors lost the heavy outer shell and became soft, flexible, fast-moving predators.

Three hearts and blue blood

Two of an octopus’s three hearts pump blood through the gills, where it picks up oxygen. The third pumps that oxygen-rich blood through the rest of the body. Curiously, the main heart pauses when an octopus swims, which may be one reason many octopuses prefer to crawl rather than swim long distances.

Their blood is blue because it uses a copper-based molecule, called hemocyanin, to carry oxygen. Our blood uses iron-based hemoglobin, which makes it red. Hemocyanin works well in cold water with little oxygen, conditions many octopuses face.

A brain spread through the body

An octopus has a central brain wrapped around its throat, but most of its neurons are in its arms. Each arm can sense, grab and react on its own to some extent, while still taking direction from the brain. Scientists are still working out exactly how this shared control works, and it is one of the reasons octopuses are so interesting to researchers who study intelligence.

Clever escape artists

Octopuses can open jars, solve puzzles and squeeze through any gap bigger than their beak. Because they have no bones, the only truly hard part of an octopus’s body is its parrot-like beak. If the beak fits through a gap, the rest of the animal can usually follow.

Aquarium staff around the world tell stories about octopuses slipping out of tanks overnight, wandering across floors or quietly raiding neighboring tanks before returning home. Some have learned to unscrew lids from inside a jar. Others have been seen squirting water at lights they seemed to find annoying, or at particular caretakers.

In the wild, scientists have watched some octopuses carry coconut shell halves along the seafloor and assemble them into a portable shelter when danger appears, a behavior that looks a lot like tool use. Others stack stones at the entrance of their dens like a front door.

Studying octopuses is like meeting an alien intelligence that evolved right here.

Dr. Priya Raman, biologist

Dr. Priya Raman, a marine biologist, explains why that comparison resonates with so many scientists. “Our closest smart relatives, like apes and dolphins, share a lot of our history,” she said. “Octopuses don’t. The last ancestor we share with them was a simple worm-like animal. Their intelligence evolved on a completely separate path, so they show us a very different way to be clever.”

Inside an octopus lab

Researchers who keep octopuses for study say the animals demand creativity from their caretakers too. Bored octopuses can become stressed, so labs and aquariums provide enrichment: puzzle boxes with food inside, toys to explore, new objects to investigate and changes in their surroundings. Some keepers report that individual octopuses seem to have distinct personalities, with one bold and eager to grab anything new and another shy and cautious. Studies of these differences are still developing, but they hint at how much variety exists within a single species.

Masters of disguise

Special skin cells let them change color and texture in a fraction of a second. Octopus skin contains thousands of tiny, elastic sacs of pigment called chromatophores, each one surrounded by muscles. By squeezing or relaxing those muscles, the octopus can expand or shrink the colored dots, creating patterns of brown, red, yellow and black. Other cell layers reflect light to add whites, shimmers and subtle blues and greens.

Many species can also raise small bumps and spikes on their skin, called papillae, to match the texture of rocks, coral or seaweed. Combine color and texture, and an octopus can practically vanish against the seafloor in moments. Some species go further, imitating the shape and movement of other animals, such as venomous fish or sea snakes, to scare off predators.

Oddly, most octopuses appear to be color-blind in the way we understand color. Scientists are still investigating how they match their surroundings so well. One idea is that light-sensitive proteins in their skin may help them sense light directly.

A short, busy life

For such smart animals, octopuses live surprisingly brief lives. Most species live only one to two years. A few deep-sea and cold-water species live longer, and the giant Pacific octopus, found along the West Coast of the United States, may live three to five years.

Most octopuses reproduce only once. After mating, a female lays her eggs, often tens of thousands of them in some species, in a protected den. She then guards them constantly, cleaning them with her arms and blowing fresh water over them. She usually stops eating during this time, and she typically dies soon after the eggs hatch. One deep-sea octopus was observed guarding her eggs for more than four years, the longest known brooding period of any animal.

Where to see one

If you would like to meet an octopus, you have a few good options:

• Public aquariums: many large aquariums keep octopuses and offer talks about their care and enrichment.
• Tide pools: on the Pacific Coast, small octopuses sometimes hide in rocky pools at low tide. Look, but never remove animals or turn over rocks without putting them back carefully.
• Snorkeling and diving: in Hawaii and Florida, patient snorkelers occasionally spot octopuses blending into reefs.
• Night walks: some coastal parks host guided evening tide-pool walks, when octopuses are more active.

Always follow local rules, watch the tides and give wildlife plenty of space.

Why octopuses matter to the ocean

Octopuses are not just curiosities. They play an important role in their ecosystems, both as predators and as prey. They keep populations of crabs, clams and other shellfish in check, and in turn they are an important food for seals, sea lions, large fish and seabirds. Scientists sometimes study the health of octopus populations as one clue to the health of a stretch of coastline. When rocky reefs, kelp forests and coral reefs are healthy, octopuses tend to do well too.

Key takeaway: octopuses are smart, strange and short-lived, with bodies and brains that work very differently from ours.

Quick facts

• Two hearts pump blood to the gills, one to the body
• Most octopuses live only one to two years
• Each arm can taste what it touches

That last fact is a fitting place to end. An octopus’s suckers are lined with sensors that let it taste and feel at the same time, so every arm is constantly exploring the world around it. In a way, that is the octopus in a nutshell: endlessly curious, always investigating, and quietly reminding us that the ocean still holds minds we are only beginning to understand.