Octopuses, those whip-smart, bizarre cephalopods, embody everything creepy and mysterious about the sea. Their soft, squishy bodies, lurking in the oceans’ dark reaches, have inspired monsters from the sailor-eating Kraken of Nordic legend to the Caribbean sea demon Lusca. The creatures’ otherworldly forms and unfurling tentacles have inspired modern monsters and villains, too—think Disney’s sea witch Ursula or Spider-Man’s enemy Doctor Octopus. (And don’t forget the many octopus-themed horror movies.)

As science journalist Katherine Harmon Courage writes in her 2013 book, Octopus! The Most Mysterious Creature in the Sea, octopuses are more than culinary delicacies or ogle-worthy curiosities. They’re intelligent, anatomically unique creatures that have inspired both ancient Greek philosophy and Japanese erotic art. Here, we highlight a few of the animals’ fascinating traits, from their ability to play to their tragic sex lives.

1. Octopuses have been around for a long time.

The oldest known fossil of an octopus ancestor belongs to an animal that lived some 330 million years ago, long before the dinosaurs. Discovered in Montana’s Bear Gulch limestone formation and described in 2022, the specimen has ten limbs, whereas modern octopuses have eight. Previously, the oldest octopus fossil found was an approximately 296-million-year-old, soft-bodied invertebrate called Pohlsepia mazonensis. Harmon Courage described it as a “cow patty—flattened out into a globular splat,” but a close examination revealed the telltale eight arms, two eyes and possibly an ink sac. Long before life on land had progressed beyond puny reptiles, octopuses had already established their shape for millions of years to come.

2. Octopuses have three hearts.

Two of the hearts work exclusively to move blood past the animal’s gills, where it releases carbon dioxide and gains oxygen. Then, the third heart circulates that oxygen-rich blood to the organs and muscles, giving them energy. But the latter heart actually stops beating when the octopus swims, explaining the species’ penchant for crawling rather than swimming, which exhausts them.

3. The plural of octopus is octopuses.

The word “octopus” comes from the Greek oktopus, meaning “eight foot,” and its plural form has long sparked debate. The first attempt at making the word plural was “octopi,” using a common pattern in Latin words, such as how “alumnus” becomes “alumni” when pluralized. Others have argued that, as a word that stems from Greek, “octopus” as a plural should incorporate a Greek word ending, becoming “octopodes.” But with the English adoption of the word, a classic English “-es” has become the generally accepted pluralization of octopus. Sorry, “octopi” and “octopodes.”

4. Octopuses aren’t as dumb as Aristotle thought they were.

In History of Animals, written around 350 B.C.E., the Greek philosopher claimed, “The octopus is a stupid creature, for it will approach a man’s hand if it be lowered in the water; but it is neat and thrifty in its habits: that is, it lays up stores in its nest, and, after eating up all that is eatable, it ejects the shells and sheaths of crabs and shellfish, and the skeletons of little fishes.” After describing a few more quirks of octopus life—it releases ink for self-defense, it’s slimy, it can crawl on land—he signs off with a flippant burn on the octopus’ phylum: “So much for the mollusca.”

Aristotle’s dismissal has since been proved wrong. Octopuses have big brains for their size, and they’re able to figure things out, like how to open a clamshell that’s been wired shut. They can navigate mazes, solve problems, remember solutions and take things apart for fun. Octopuses even have distinct personalities, as explored in the Oscar-winning documentary My Octopus Teacher. Finally, the cephalopods play—in one instance, by conducting a pseudo-game of catch with a floating bottle.

5. Octopus’ arms have a mind of their own.

Two-thirds of an octopus’ neurons reside in its arms, not its head. As a result, some of the arms can figure out how to crack open a shellfish while the rest of the animal is busy doing something else, like checking out a cave for more edible goodies. Octopus tentacles can even react after they’ve been completely severed from a dead animal. In one experiment, amputated tentacles jerked and curled up when researchers pinched them.

6. Octopus ink doesn’t just hide the animal.

The ink also physically harms enemies. It contains a compound called tyrosinase, which, in humans, helps to control the production of the natural pigment melanin. But when sprayed in a predator’s eyes, tyrosinase causes irritation. It also garbles creatures’ senses of smell and taste.

octopus shooting ink!

7. Octopuses have blue blood.

To survive in the deep ocean, octopuses’ blood is powered by a copper-containing protein called hemocyanin, instead of the more common, iron-based hemoglobin in human blood. Whereas our iron-rich blood is red when it meets oxygen, the copper in octopus blood makes it appear blue. Hemocyanin, a bigger protein, transports oxygen more efficiently in the extreme environments inhabited by octopuses: At the bottom of the ocean, water temperatures are very low, and there isn’t much oxygen around. Unfortunately, octopuses’ blood also causes them to be extremely sensitive to changes in acidity. If the surrounding water’s pH dips too low, the animals can’t circulate enough oxygen to survive. Accordingly, researchers worry about what will happen to octopuses as climate change increases the ocean’s acidity.

8. To some, octopuses are erotic muses.

“Tentacle erotica” goes as far back as a sensual 1814 woodblock print, published in Katsushika Hokusai’s Kinoe no Komatsu collection of erotic books. According to Harmon Courage, the image takes inspiration from a legend about a female shell diver who is chased by sea creatures, including octopuses, after attracting the eye of a sea dragon god. As Japanese art curator Ann Yonemura wrote for the Pulverer Collection, the 1814 print referred to a longtime erotic association with abalone divers. “Nonetheless,” she notes, “the unease that many viewers initially experience arises in part from the startling realism of the octopus.”

9. After mating, it’s game over for octopuses.

Octopus Mating | National Geographic

Mating and parenthood are brief affairs for octopuses, which die shortly after. The species practices external fertilization. A male inserts his spermatophores directly into the female’s mantle cavity, using his hectocotylus, a special, longer arm. Afterward, the male’s “sex arm” falls off, and the animal dies. As for the females, they can lay up to 400,000 eggs, which they obsessively guard and tend to. To prioritize their motherly duties, females stop eating. By the time their eggs hatch, female octopuses are either dying or dead. Their optic glands rapidly produce self-destructive chemicals, causing a rapid change in cholesterol metabolism and ultimately killing them. Some captive octopus mothers have been known to intentionally speed up their own deaths by mutilating themselves.

10. Most of the octopus we eat comes from North and West Africa.

Octopus meat has been a popular food item in East Asia, Spain, Greece and other countries for centuries, and recently, its demand extended to the United States and beyond.

But such culinary prevalence has had an impact on octopus populations around the world. The international demand for octopus inspired North and West African fisheries to start targeting the animals in the 1980s, and overfishing in those waters has shifted the industry from Morocco to Mauritania and, more recently, Senegal. According to the Sustainable Fisheries Partnership, worldwide production of octopus has now surpassed 550,000 tons per year.