Pop science
Tiny worms can ‘see’ light without their eyes, or their heads.
Planarians are a type of flatworm, soft-bodied creatures that lack complex organs. They have two eyes that connect to a centralised bundle of ganglia in their heads that acts as a brain, and those eyes are sensitive to ultraviolet (UV) light. In the presence of UV light, the worms use their cilia, tiny hairlike structures on their bodies, to wriggle away. However, it turns out the worms, measuring just a few millimetres long, don’t need their eyes or their brains for light detection. When researchers lopped off planarians’ heads, the worms were still capable of sensing UV light.
Removing an animal’s head might seem like an odd way to conduct behaviour experiments. But planarians are known for not only surviving decapitation, but also for easily regenerating missing body parts. Amputation is no big deal for a planarian; cut one into multiple pieces and each piece will regenerate into a new worm. Decapitated planarians easily grow new heads, and scientists can even tweak the worms’ genetic instructions to coax them into growing the heads of different species.
Researchers found planarian bodies contain cells that produce a type of light-sensitive protein called an opsin. Arrays of these cells found around the periphery of the planarians’ bodies expressed two opsins: NC R-opn 1 and NC R-opn 2, while the more centrally located cell populations only expressed NC R-opn 1.
The cells producing just one opsin were pigment cells. In the peripherally located cells, the two opsins first detected UV light and then triggered movement in the headless worm, which wriggled away in response to that light. Only mature worms possessed this light-sensing superpower; newly hatched worms couldn’t detect UV light after their heads were snipped off, hinting that light-sensing cells in the worms’ bodies develop after hatching.
The scientists also found that when whole planarians were resting in an inactive sleep-like state, they would perk up in the presence of UV light, even when they didn’t respond to other visual stimuli. This suggests that whole-body light sensing helps to protect snoozing worms from harmful UV rays by activating movement when the worm is at rest and its vision is temporarily offline.
“Such a mechanism may be distinctly advantageous to a water-dwelling, light-aversive organism that is likely nocturnal and would rest during the day,” the researchers wrote. The discovery of a whole-body light-sensing infrastructure in planarians that doesn’t require a central brain or eyes represents “a major advance spanning virtually all facets of photosensory biology”.