Researchers at Hyderabad University find out flatworms can sense light without eyes

Researchers from the University of Hyderabad (UoH) have found that planarian flatworms can sense light, even without eyes, with the help of an eye-independent system (extraocular) lining the periphery of the worm’s body.

Prior research has shown that planarians can survive decapitation (removal of the head) and retain the ability to move away from the light source when exposed to low doses of ultraviolet light, a UoH release said on Monday. The researchers wish to explore use of such sensitive natural light-sensing proteins for helping patients with vision disabilities to ”sense” light and to control inner working of cells/tissues with light (optogenetics), according to the release.

The researchers sought to ascertain as to how these organisms sense the light without eyes and is there any other light-sensing system that is helping them to sense the light.

“In a breakthrough discovery, a research team led by Dr Akash Gulyani from Department of Biochemistry, School of Life Sciences at UoH showed that an eye-independent system (extraocular) lining the periphery of the worm’s body allows even a headless flatworm to move like an intact worm with incredible coordination,” it said.

The researchers found that the worm body is dotted with a whole array of very unique light-sensing cells that are patterned all over the worm, especially at its periphery, it said.

Intriguingly, these newly discovered light-sensing cells appear unique as they do not resemble any neuron-like cells but are more similar to a distinct cellular class (parenchymal cells) that includes glia-like cells, which are generally thought to have a supporting rather than a sensory role.

This seems very distinct from all light sensing systems known so far across the animal kingdom, the release said.

Cells within this eye-independent system expressed light-sensitive proteins called opsins which help the flatworm to respond to light even in the absence of eyes.

However, the eye-independent system only responds to a limited range of ultraviolet light at 365 to 395 nm (nanometer), whereas the flatworm eyes can detect a broad wavelength of visible light (~365 to 625 nm), it said.

Interestingly, the eye-independent system arises only in the adult organisms, unlike the standard set of eyes that develop in the embryo.

This discovery of such an array of body-wide light-sensing cells that link to the body-wide nervous system and allow headless worms to move may constitute the uncovering of a new kind of body-wide organ system for sensing light.

The researchers think that the extraocular light sensing ability of headless worms may help tailpieces escape light, and avoid predators, it said.

The researchers also show that the system helps intact animals respond to sudden light exposure even when the animals have gone to a sleep-like state, helping them avoid predators and danger.

This is again a rare example in nature where an intact adult animal uses an eye independent system for locomotion/escape, despite possessing a sensitive visual system, the release said.

“Overall, these findings offer a unique and stunning insight into coexistence, development and evolution of independent light-sensing systems in a single organism,” the authors said.

The work is published in the Proceedings of the National Academy of Sciences (USA).