Chop Off This Worm’s Head and It Can Still Detect Light – New York Times

Photo Scientists have discovered that even a decapitated planarian flatworm can detect light before it grows back its head and eyes. Credit Kent Wood/Science Source

The planarian flatworm is a smooshed noodle of an organism that can be found all over the planet. It has a triangular head occupied by a rather primitive version of a brain and two black dots for eyes. You can chop off this head, and it will grow back in about a week eyes, brain and all. And you can hack away at the critter until all thats left is a tiny speck of worm dust and the thing will still grow back.

But now this peculiar creature, famous for its regenerative abilities (like when some grew two heads in space), may have another unforeseen idiosyncrasy: It not only reacts to light after decapitation, but it gradually recoups an ability to see finer aspects of light as its eyes and brain grow back. And despite lacking the machinery to see colors, the worm somehow creates a workaround, essentially converting this rainbow colored world to a grayscale, said Akash Gulyani a multidisciplinary scientist at the Institute for Stem Cell Biology and Regenerative Medicine in Bangalore, India, who led the study.

His teams findings, published last week in Science Advances, could offer new opportunities for studying how animals recover after injuries and reveal additional details about function to the story of how animal eyes evolved.

Planarians, like many other organisms across the animal kingdom, have fairly basic eyes, unable to detect color and lacking a lens to focus. The eyes are shaped like cups and lined with cells that detect the presence of light and the direction from which it comes. They send signals to two blobs of cells that constitute a pretty basic brain (some argue the first one). Their view of the world is probably limited to moving shadows, not the clear picture production of a humans cones, rods and lenses.

In the wild, the worms avoid sunlight and the predators and other dangers lurking within it.

And when scientists shined bright lights on the animals in the lab both UV and white, which contains a rainbow of colors or hues they swim away, flapping the sides of their little noodle bodies like wiggly linguine. When given a choice between hues, the worms preferred green over blue and red over green, the scientists found. They werent detecting wavelengths or truly sensing colors, but perceiving one as darker than another, as if representing a deeper, safer depth in the wild, the scientists reasoned.

But decapitated worms still responded to UV light. You wont believe what happened, Dr. Gulyani recalled some students reporting, the samples ran away from the light even though they didnt have heads.

This reflex-like response had been observed by other scientists, but not fully explained. It made sense, because the worms split in half to reproduce, and a blind, brainless tailpiece is vulnerable as its body develops. But as the body parts regenerated, function gradually recovered. With eyes, the worms could detect white light again. And after the brain was fully developed and the connections became strong, the worms regained their fine-tuning abilities. But just how they do this and why they developed the greater complexity is still a mystery.

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Chop Off This Worm's Head and It Can Still Detect Light - New York Times