Animal Loses Head But Remembers Everything

When I first saw this," says cell biologist Alejandro Sanchez Alvarado, "it was with total amazement."

Planaria
Robert Krulwich/NPR

This is a worm. It's called a planarian. It's about an inch long, and you'll find it gliding along the bottoms of rivers and ponds all over the world. It's very flat, like a moving bit of pasta — nothing special to look at, but it has a hidden talent that has made it famous. It can regrow its body parts better than almost any other animal on Earth. So if by some chance something bites its head off ...

The planarian's head cut off.
Robert Krulwich/NPR

... or removes its tail, or zaps its head and its tail simultaneously ...

The planarian's head and tail are cut off.
Robert Krulwich/NPR

... the dangling middle piece will, within a couple of weeks, grow both its front and its back to full size. Meanwhile, the severed head, if left alone, will also generate a full-sized new worm. The lonesome tail will do that, too, so where you started with one worm, you'll now have three. This worm likes to regenerate.

The planarian regenerates it's head and tail.
Robert Krulwich/NPR

You can even remove a single, special cell from an adult planarian (not an infant cell, but a cell taken from a mature adult), and from that one cell, scientists discovered a few years ago, the worm can regenerate a whole new creature — the skin, guts, nerves, muscle, eyes, mouth, everything. It will be genetically identical to the donor.

Tweezers holding a planarian cell.
Robert Krulwich/NPR

How do they do this? How does a random slice of worm (often with no brain, just a clump of meat, says Alvarado — "I mean, really, it's a piece of flesh") figure out where its front is, where its bottom is? Clearly, this is a genetically driven talent, but do we know which genes, how many genes are involved?

No, we don't. Intriguingly, we share a bunch of genes with these flatworms. If we ever figure out how they do it, we might be able to pick up a few of their tricks, which are astounding to see. Just click below, in a video from San Francisco's Exploratorium, to watch a severed head grow back a body, and — my favorite — see a worm sliced down the middle like a banana split create its other half. Professor Alvarado is the swooning narrator.

But we haven't gotten to the most remarkable part. Two biologists at Tufts University, curious to learn how this little worm remembers things, decided to do an experiment.

Michael Levin and Tal Shomrat know that planarians don't like light. In the dark, the worms can hide from predators, feel safe and skulk about gathering food. They have those two weird little eye cups — angled, so they seem to be cross-eyed — so these worms can detect light. And when they do, they normally scuttle off (or, in my imagination, cover their eyes).

Planarians don't like light. i i
Robert Krulwich/NPR
Planarians don't like light.
Robert Krulwich/NPR

But Levin and Shomrat wanted to implant a memory in their laboratory worms, and the memory they decided on was, "Light's OK. Good things happen when you go to lit spaces." To do that, they placed a delicious liver snack on a well-lit plate, and rewarded the worms who ventured close (and punished them if they sought the safety of the dark). So, gradually, their worms began to trust the light, and venture in more quickly.

Planarian looking into the light at a slice of liver.
Robert Krulwich/NPR

Then (sorry PETA), they chopped off their heads. That would be mean if they were chopping off, say, Marie Antoinette's head. But remember, these are planarians, so in a couple of weeks the beheaded worms had new heads. What Levin and Shomrat wondered was, did those new heads keep their old memories?

Yes they did!

According to National Geographic's account, after a short refresher course, these worms-with-new-heads "remembered where the light spot was, that it was safe, and that food could be found there. The worms' memories were just as accurate as those worms who had never lost their heads."

Planarian grows back a new head and remembers he is not afraid of light.
Robert Krulwich/NPR

So these worms grew new heads with old memories, a remarkable finding, particularly when you consider it took 14 days or so for the head to grow back. How'd the worms do it? Once again, "We have no idea," Michael Levin told National Geographic. Since these animals were briefly brainless after their decapitation, he says, "What we do know is that memory can be stored outside the brain — presumably in other body cells — so that [memories] can get imprinted onto the new brain as it regenerates."

The unanswered questions here are so fascinating. "When I see a planarian," says Professor Alvarado, "I just see a big void of mystery." How does an animal know how to build itself? Is there a body plan in the brain? In our cells? Which cells? What triggers those cells? Could we ever do this? Could we ever build what the worms have into ourselves? Questions, questions, questions and, so far, hardly any answers.

But the worms — they know.


The mysteries of the planarian are so cool, there's a "regeneration" T-shirt that celebrates these worms. I found it here.

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