Why do octopuses have three hearts?

Are three hearts better than one? And is it possible to sleep underwater? The answer to both questions is yes — if you’re an octopus.

Octopuses need all three hearts to circulate blood through their bodies. Two of the hearts are called branchial hearts. They sit right behind the eyes and are connected to an octopus’s gills, where oxygen gets into its blood. From there, blood goes to the systemic heart, in the octopuses’ center, which pumps the oxygen-rich blood to the rest of its body. With all that blood flow, plus a big, smart brain, a squishy body, and fine-tuned senses — especially eyesight — octopuses can act fast to get in and out of tight spots — sometimes not much bigger than a key-hole.

But octopuses can get sleepy every now and then. And Paul Smith of Fresno, California, wants to know, “How do octopuses sleep?” Paul, octopuses don’t have eyelids. When octopuses in captivity need sleep, they slump into corners of their tanks and catch a few winks by narrowing their pupils. Their bodies turn whitish in color and their breathing slows down. In experiments, octopuses that are kept awake end up needing more rest later on, which shows how important it is that they get enough shut-eye.

They might need it because octopuses have the biggest brains of any invertebrate.


Octopuses have so many cool features, it is hard to know where to begin.

There are hundreds of species, ranging in size from less than an inch long, in the case of the tiny Californian Octopus (Octopus micropyrsus), to more than 30 feet, in the case of the North Pacific Octopus (Octopus dofleini), which can weigh 100 pounds. Even within species, octopuses are constantly changing their image. With special cells called chromatophores, they can change colors instantaneously to reflect their moods or blend in to their surroundings. Many species have toxic venom in their tentacles to kill shrimp, lobsters, crabs, snails, and other prey. The Australian Blue-Ringed Octopus has venom so potent, it can kill people. And in 1999, scientists discovered a type of octopus whose tentacles actually glow in the dark.

Though common in almost every type of ocean environment, octopuses are hard to see in the wild. They hide during the day, and usually only come out at night and on gloomy days to. Their eight tentacles have an amazing sense of touch that can distinguish between objects. Their eyes are also highly developed and similar to humans. Octopuses are deaf.

When threatened or frightened, octopuses can squirt out a blob of dark ink, which often buys them time to escape. To move, they propel water like a jet engine, which moves them rapidly in any direction.

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