Aakancha Shaw, St. Xavier’s College, Kolkata
Scientists recently discovered that worms can perceive light without their eyes or rather without their head.
Planarians are one of the most special types of flatworm. They are soft-bodied creatures that often lack complex organs. They simply have two eyes that connect to a centralized bundle of ganglia in their heads. This bundle of ganglia acts as the brain, and their eyes are sensitive to ultraviolet (UV) light. It is seen that in the presence of UV light, the worms use their cilia that are tiny hair-like structures on their bodies to wriggle away.
However, it was seen that the wee worms that measure just a few millimeters in length don’t need their eyes or for that matter their brains for light detection.
When researchers chopped off planarians’ heads, surprisingly the worms were still capable of sensing UV light. Amazing, isn’t it?
Removing Or chopping off an animal’s head might seem unethical and odd to conduct experiments with. But the planarians are known for surviving decapitation and not only that, but they also have the ability to regenerate missing body parts. Hence, amputation is not a big deal for planarians as even if it is cut into multiple pieces and each piece will regenerate into a new worm. The planarians with decapitated heads easily grow new heads and scientists could even tweak the genetic instructions of the worm to coax them into growing the heads of different species.
Researchers found that planarians contain cells that are known to produce a type of light-sensitive protein called an “Opsin”. The cells that produced only one opsin were pigment cells. 2 opsins in the periphery first detected UV light and then triggered movement in the headless worm. This caused the worm to wriggle away in response to that light. Also, only mature worms possessed this light-sensing superpower, the newly born worms could not detect UV light after their heads were chopped off. This indicated that the light-sensing ability in the worm’s bodies developed after hatching.
It was also found out that when the planarians were in an inactive state, they would perk up in the presence of UV light even when no response was seen in the case of visual stimuli. This suggested that whole-body light sensing helps protect these worms from harmful UV rays by activating movement even when the worm is at rest.
This discovery showcases the fascinating complexity of function of an eye-brain-independent light sensing network.
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