1. The plural of octopus is octopuses, not octopi.
That's right — your fourth-grade teacher steered you wrong. That's because the word "octopus" is Greek, not Latin, so it doesn't follow Latin rules. If you want to be really pedantic and drive away all your friends, the real Greek plural form of the word is "octopodes." But according to Merriam-Webster, octopodes, octopuses, and octopi are all acceptable in English.
2. Octopus "tentacles" are actually arms.
Speaking of being pedantic, octopuses don't have tentacles like fellow cephalopods squid and cuttlefish. Their eight limbs are called arms. What's the difference? In a nutshell, arms have suckers all the way down, while tentacles have suckers only on the ends.
3. Male octopuses insert sperm with their arms.
Underwater mating is endlessly weird. Ducks have corkscrew penises, and some sea slugs have regenerating penises. The octopus has the hectocotylus, aka a penis-arm, which it uses to reach into a female octopus and deposit sperm by hand, if you will.
4. Octopus arms can regenerate.
Yes, even the penis-arm, which in fact breaks off during mating and stays inside the female's body. And while some animals, like lizards, regrow body parts that aren't quite as good as the originals, regenerated octopus arms work just as well as the ones they replace.
5. There are around 300 species of octopus.
They range in size from the Octopus wolfi (or the star-sucker pygmy octopus), which is fingertip-sized at less than an inch across, to the giant Pacific octopus, which can measure up to 30 feet.
6. All octopuses are venomous.
But don't worry — only the toxic-looking blue-ringed octopus, found in Australia, is deadly to humans.
7. Female octopuses can lay up to 400,000 eggs at once.
They give all their attention to these eggs and even stop eating. Once the eggs begin to hatch, the mothers die by a horrible-sounding process Smithsonian calls "cellular suicide."
8. Octopuses are super smart.
9. Their brains have nothing in common anatomically with ours.
Octopuses are smart in a very different way than humans are, and even their brains reflect this major difference. Technically, octopuses have nine brains, because each of their arms has a collection of neurons that functions independently of the central brain.
10. They play, and they're not just motivated by food.
Some have even been seen using their siphons to spray water and bounce objects around in their tanks.