Natures wonders: headless planaria avoid light (Introduction)

by David Turell , Sunday, July 30, 2017, 20:08 (2671 days ago) @ David Turell

The flatworm normally avoids light, but still does it when the head is removed, which indicates it senses light elsewhere on its body:

https://www.newscientist.com/article/2142167-flatworms-can-still-see-even-after-they-ar...

"Off with their heads. Light-averse planarian flatworms, known for their incredible ability to regenerate lost body parts, shy away from light even after they have been decapitated. "This suggests they have evolved a second way to respond to light that doesn’t involve eyes.

"Planarian flatworms, which often live in dark, watery environments shielded from direct light, don’t have complex eyes like we do. But many do have two lensless, primitive “eyespots” on their heads that can detect the intensity of light.

***

"Unexpectedly, it turned out that S. mediterranea actually has colour vision of sorts. Even though its eyespots lack wavelength-specific photoreceptors, Gulyani’s team found that the animal was more likely to move away from blue than red light.

"The researchers think the worms are distinguishing between different colours by comparing the amount of light being absorbed by the two eyespots, rather than seeing the colour of the light itself: for instance, they could override the flatworm’s preference for red over blue light by increasing the intensity of the former.

"But there’s much more to flatworm vision than this. Gulyani and his colleagues next exploited the fact that their planarian flatworms can survive decapitation – and regrow their heads – to explore how they respond to light when headless.

"It turned out that the worms still reacted to light, but in the ultraviolet rather than the visible part of the spectrum. This suggests that the worms have evolved two completely independent ways to respond to light, say the researchers – one mediated through the eyespots and brain, and one a body-wide reflex that doesn’t involve the eyes, the exact mechanism for which still needs to be identified.

"Over the week-long period it took for the flatworms to regenerate their heads, the team monitored how quickly their brains and eyespots regrew, and when they began responding to visible light again.

"After four days, the eyespots had grown back, but the worms continued to react more strongly to UV than to visible light. Only after seven days did they regain their stronger preference to slither away from visible light – suggesting that their eyespots and brains were retaking control. It was not until the 12th day that their sensitivity to such light increased to the point that they reacted more strongly to light at the bluer end of the visible spectrum.

"Gulyani’s team speculates that the “gut instinct” response to UV light may be an ancient mechanism, with the eyespot and brain-controlled response to visible light a later evolutionary acquisition. As such, the researchers wonder whether their experiments might “replay” evolution in fast forward, showing how flatworms went from responding to ultraviolet light as an unthinking reflex to responding to visible light through a brain-controlled pathway.

“'It’s a fascinating coincidence that decapitation-regeneration experiments appear to copy – chronologically, at least – what may have occurred in evolution,” says Gulyani. It’s an idea that might be worth exploring in future experiments."

Comment: The planaria seem to have two light sensing areas. What is fascinating to me is how very simple life forms can regenerate parts. There obviously is a level of complexity beyond which this cannot be done.