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WITH THEIR EIGHT arms and giant egg-shaped head, octopuses (sadly not octopi) are one of the most alien-looking creatures on the planet.

Yet scientists have an extremely difficult time studying them in the wild because these highly intelligent invertebrates are the ultimate masters of disguise.

In her book, Octopus!, Katherine Harmon Courage travels the globe to swim, observe, and even taste some of the many octopuses of the world. Here are some mind-blowing facts she learned about this squishy predator of the sea

Over 95% of all animals on Earth are invertebrates. The octopus is the smartest of them all and has approximately 300 million neurons throughout its body. That’s not much compared to the 100 billion in humans, but it’s a giant leap from the 16 million in frogs.

Polbo / Octopus N. Feans N. Feans

Octopuses are solitary creatures who spend most of their lives swimming alone, even when it comes time to mate.

Octopus NOAA Ocean Explorer NOAA Ocean Explorer

Some, but not all, types of male octopus will steer clear from a female mate. Instead of getting close, he’ll send a package of his sperm to her from a distance, which she’ll grab and store for later.

Coconut Octopus prilfish prilfish

A male will sometimes send a gift along with his package — one of his eight arms, which he severs himself.

Dumbo Octopus NOAA Ocean Explorer NOAA Ocean Explorer

Luckily, octopuses can regrow lost limbs, just like starfish. They can even close off the severed artery to reduce blood loss.

Giant Pacific Octopus Being Playful

When they do bleed, octopuses bleed blue blood, not red. That’s because their blood is high in copper instead of iron.

Meet Fifi laszlo-photo laszlo-photo

Octopuses will sometimes deliberately sever an arm to distract a predator long enough to jet away at top speeds of 25 mph.

Ocean Bodhi / YouTube

An octopus’s brain is located in its egg-shaped head, called the mantle. But the brain does not have complete control over its body. Each arm contains enough neurons to operate semi-independently.

Octopus at Poipu 3 Makuahine Pa'i Ki'i Makuahine Pa'i Ki'i

That means each sucker on the arm can decide when it wants to extend, release, and pinch without requiring a command from the brain to do so. That also means the arms will continue to function for a short time after the arm is severed from the body.

Giant pacific octopus. Three Tree Point, Burien, WA. papertygre papertygre

A special Korean dish called “live” octopus takes advantage of this fact. The chef will take a live octopus, kill it, chop up the arms and serve them while they’re still wriggling on the plate. The dish is often garnished with raw garlic, green onion, and jalapeno.

20101221 fresh octopus schizoform schizoform

Octopus is actually highly nutritious: A 3-ounce serving has 139 calories, 2 grams of fat, 25 grams of protein, 45% of your daily iron value, and 510% of your daily B12 value. As Katherine Courage puts it, octopus “totally trumps any chicken”.

Sizzling squid and baby octopus at Oishii Kitchen in Prahran ultrakml ultrakml

Octopuses have three hearts. The main heart will actually stop beating when the animal is swimming, so it can’t swim very far before tiring out.

day octopus (octopus cyanea), waikiki aquarium brx0 brx0

And sometimes their hearts will skip a beat. In a 1970s experiment, a male octopus’s main heart actually skipped a few beats when a potential female mate was introduced into its tank.

BXP47978 Angell Williams Angell Williams

Because swimming is so tiring, an octopus’s favorite way of getting around is “walking” along the seafloor. They use their back four arms for walking and their front four to probe for food.

expl0792 NOAA Photo Library NOAA Photo Library

But walking is slow and makes the octopus vulnerable to predators. As a result, some octopuses carry protection with them. Here, an octopus is transporting two empty coconut halves it can hide inside of if necessary.

Poussin Diver / YouTube

Octopuses’ most impressive survival tactic is their skin: They can change the color and texture of their skin to blend in with their surroundings and avoid detection. Can you spot the octopus in this photo?

octopuses-most-impressive-survival-tactic-is-their-skin-they-can-change-the-color-and-texture-of-their-skin-to-blend-in-with-their-surroundings-and-avoid-detection-can-you-spot-the-octopus-in-this-photo Screenshot Screenshot

Despite their miraculous color-changing abilities, octopuses are colorblind. Scientists aren’t sure if the octopus is even aware what its skin is doing when it transforms from one colour and texture to the next.

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In general, octopuses are not picky eaters: Some of the things found in the stomachs of captured octopuses are shocking: clams, crabs, hermit crabs, lobster, and even seagulls. They’ll even venture on land to capture their prey.

Porsche Indrisie / YouTube

Scientists have tested octopuses’ intelligence by placing food inside of jars with screwed caps. The eight-armed creatures easily unscrew the jar for the prize inside. One scientist even discovered one could unscrew childproof caps. Now that’s impressive!

Beata Svengt / YouTube

Octopuses live in small dens such as natural rock outcroppings or confined corners in sunken ships. They enjoy decorating these homes with stones and shiny objects. This behavior inspired the popular Beatles song Octopus’s Garden.

Octopus and his snorkel out of the Den amanderson2 amanderson2

Because of their intelligence, octopuses in captivity will get extremely bored and stressed if they’re not stimulated. In one study, scientists found that octopuses who spent time in bare tanks began eating their own arms from stress but ceased this behavior when placed in tanks with hiding spots and decorative knickknacks.