No Head, No Problem: Decapitated Flatworms Still Sense Light

Decapitated flatworms can still detect light with the help of an eye-independent light-sensing system, find researchers from India.

AsianScientist (May. 6, 2021) – Who says you need eyes to see? Certainly not flatworms. In an eye-opening discovery, researchers from India have found that flatworms can respond to light even after decapitation thanks to an eye-independent light-sensing system that coordinates movement. Their findings were published in the Proceedings of the National Academy of Sciences.

While eyes are said to be the windows to the soul, it turns out that they’re not as essential as we think. From tiny jellyfish-like hydra to nematodes that live in near-complete darkness, various organisms have evolved their own unique means to detect light even without the sense of sight.

Now, a group led by Assistant Professor Akash Gulyani from the University of Hyderabad, has shown that is possible for both eye-dependent and independent light-sensing systems to coexist in a single, tiny organism: the flatworm.

Named for their heavily flattened bodies, flatworms typically possess two sensitive eyes connected to a primitive brain. Though primitive, the flatworm’s brain plays a crucial role in controlling movement and behavior. Upon decapitation, worms not only lose their eyes and brains, but also their ability to feed and respond to chemical cues as well as temperature changes, among other things.

Yet, when exposed to low doses of ultraviolet light, decapitated flatworms still retain the ability to move away from the light source—a behavior known as phototaxy. This surprising discovery prompted Gulyani and his team to investigate if there was another light-sensing system at hand.

After decapitating microscopic flatworms—all for the sake of science—the team discovered an eye-independent system lining the periphery of the worm’s body. Cells within this eye-independent system expressed light-sensitive proteins called opsins, explaining the worm’s ability to respond to light even in the absence of vision.

While the flatworms’ eyes can detect a broad wavelength of visible light (~365 to 625 nm), the eye-independent system only responds to a limited range of ultraviolet light at 365 to 395 nm. Moreover, low doses of ultraviolet light can awaken resting flatworms through the aforementioned system.

Interestingly, though the worm develops its standard set of eyes as an embryo, the eye-independent system only arises in their adult counterparts. According to the authors, this suggests that specific evolutionary pressures like a low-light environment may trigger flatworms to develop the eye-independent light-sensing system. Alternatively, the same system also be an ancient trait that unexpectedly develops after the eyes are in place.

“Overall, these findings offer a unique and stunning insight into coexistence, development and evolution of independent light-sensing systems in a single organism,” conclude the authors. “Our work showcases the fascinating complexity of form and function of an eye-brain-independent light-sensing network to regulate the physiology of the organism in multiple, unique ways.”

The article can be found at: Shettigar et al. (2021) Discovery of a body-wide photosensory array that matures in an adult-like animal and mediates eye-brain-independent movement and arousal.


Source: University of Hyderabad; Photo: Shutterstock.
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