The researchers cut off the heads of a group of planarian worms but this didn’t blind them – let alone kill them. With photoreceptors along the body, they could sense UV light and moved away from it.
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
Cut planarians into small pieces and within days, each becomes a functional whole. Even headless pieces can self-reconstruct the brain and head. Besides, planarians’ unique ability to repair cells damaged by ageing makes them almost immortal. Most of these remarkable flatworms, related to flukes and tapeworms, live in the sea, while some live in freshwater and on land. The planarian brain is no more than a blob of nerve cells, a prototype of the vertebrate brain. Its eyes, called ocelli, have no lens or cornea and are so simple in structure that scientists thought they could only tell the direction of light.
Their eyes, which appear cross-eyed, are actually much more sophisticated, according to new research.
Researchers from the National Centre for Biological Sciences (NCBS), Bengaluru, experimented with the vision of a freshwater species, Schmidtea mediterranea. Around the world, this is a popular species in research laboratories that study stem cell biology and regeneration.
Few studies connected the planarians’ ability to regenerate with function. “So we set out to understand what the worms actually do. What are they capable of?” Akash Gulyani, the main author of the study, told The Wire. The Institute for Stem Cell Biology and Regenerative Medicine within NCBS is an imaging and sensing lab, and it was only natural, Gulyani says, that he chose to examine how these creatures sense their environment.
“Initially, we were just curious about many aspects and kept doing exploratory experiments,” he says. “My student Nishan [Shettigar] and a few undergraduate trainees, who are co-authors on this paper, were very creative and came up with new ways to probe light-sensing in planarians. We decided to venture into new areas, and while there were challenges, it was always a lot of fun.”
Often called smeds, the two-centimetre-long mediterranea dislikes light and lives in darkness.
Visible light spectrum ranges from 400 to 700 nanometers (nm). Blue light has a wavelength of 450 nm, green 545 nm, and red 625 nm. The researchers gave some of their laboratory’s planarians a choice of two lights of equal intensity but different wavelengths – blue and green, red and green, and blue and red. Although they have no colour vision, the smeds sought refuge in the higher wavelengths, discriminating between lights a mere 25 nm apart in wavelength.
However, when given a choice between UV (395 nm) and visible light, they chose the former. “It was very surprising to see the worms choose UV,” Wendy Scott Beane, an assistant professor of biological sciences at Western Michigan University, told The Wire. “This is counterintuitive to the known data, although binary choices have not been previously tested to my knowledge.” Beane wasn’t involved in this study.
Beane and her colleagues also investigated the smeds’ responses to light of different wavelengths. In the US, the worms fled faster from UV light than any other, seeking out the farthest corner of the container.
“For aquatic organisms being able to distinguish between different wavelengths, both visible and UV, can provide important information,” explains Beane. “Light waves have different abilities to travel through water, with red wavelengths only travelling a little way into water (approx. 20-30 feet), green wavelengths penetrate farther (maybe 70 feet), while at lower depths mostly only UV wavelengths penetrate. So being able to distinguish between visible and UV light could provide planaria information about where they are. It’s important to remember that most freshwater planarians really have few defences – no exoskeleton, no teeth or venom. So the ability to determine if they are hidden from above could be an important defence mechanism.”
Differences in the experimental setups explain the reason for the varied results. In the US study, the planarians reacted to a single source of light. Here, set in the middle, they made their preference clear when given a choice of two: visible light was more unpleasant than UV.
No one knew until now that planarians’ simple cup-shaped eyes could detect wavelength. Do they have wavelength receptors? How do they compare two wavelengths? By turning up the intensity of light, the researchers manipulated the planarians to avoid their preferred wavelengths. This indicated they don’t have receptors to sense wavelength, but instead, they compare two lights and choose the least unpleasant one.
“Planarians show surprisingly complex light sensing and processing,” says Gulyani.
The researchers became executioners, cutting off the heads of a group of smeds, but this didn’t blind the worms, let alone kill them. With photoreceptors along the body, they could sense UV light and moved away from it. “This response is a reflex-like response, very different from the brain mediated sensing,” says Gulyani.
But these receptors couldn’t detect visible light since the worms moved toward it to escape from the UV. This is contrary to their preference as intact planarians. Why would headless smeds avoid UV?
They can reproduce asexually by dividing themselves. “In this scenario, if a tail piece (after fission), without eyes and brain, is subject to direct sunlight, it would show an escape response,” says Gulyani. “This response would likely be beneficial for it to avoid predators or other elemental exposure during this period when it is regenerating its head. This is speculation.”
In a paper published recently, Beane and her colleagues identified the mechanism: an ion channel called TRPA1 that detects irritants in the environment responsible for this non-visual (or extraocular) perception of UV light. Four days after decapitation, the planarian eye regrows. On the fifth day, nerves connect the eye to the brain, but they continue to recoil from UV light. After the seventh day, they start to distinguish between intensities and wavelengths. On day 12, they are as good as whole, with the ability to tell a difference of 25 nm.
Since whole smeds have two light detecting mechanisms, how do they reconcile the contradictory signals? After all, the extraocular perception drives them away from UV while their eyes lead them away from visible light. In entire worms, the brain’s response to what the eye senses supersedes the body’s photoreceptors. Extraocular receptors dictate how headless ones with no eye or brain react.
“We were amazed by how breathtakingly complex the sensory and processing capabilities of these so called ‘simple worms’ are,” says Gulyani.
Beane said the study provides “a unique look into a poorly understood area of neuroethology”. Neuroethology is the study of the evolutionary conservation of neural mechanisms controlling animal behaviour.
Many animals can sense light without eyes. Humans produce melanin in response to sunlight using the same mechanism (TRPA1) that smeds use to detect UV light. “However, the mechanisms underlying extraocular (non-visual) light sensing are poorly understood,” says Beane. In a few investigations, creatures discerned light in various ways, complicating researchers’ ability to zero in on an overriding mechanism that has withstood evolutionary changes. “Thus this study’s findings on how visual vs. non-visual light cues are integrated into an organism’s behavioural choices have the ability to help inform how other organisms, including humans, integrate different light cues,” says Beane.
Researchers have to rely on planarians to understand how humans sense light non-visually, especially since the latter don’t do very well after decapitation.
The study was published in the journal Science Advances on July 28, 2017.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.