Worm bodies might not seem very interesting. But looking closer, we can see how some worms use extra body parts to move through the water effortlessly, like 'magic carpets,'” while others shed their rear ends to reproduce. Scientists have found that a type of bristle worm has a complex vision system dominated by two very large eyes.
The Vanadis bristle worm's eyes may be able to communicate and locate mates and/or food using ultraviolet (UV) light, a phenomenon not well documented in nature. These worms could be among the few bioluminescent animals that use UV light to glow. The findings are detailed in a research paper released on April 8 in the Current Biology.
Meet Vanadis bristle worms
The Vanadis bristle worms in this study are found around the island of Ponza, in the Mediterranean Sea west of Naples, Italy. They belong to a family of large-eyed bristle worms called polychaeta. They are approximately six inches long and primarily feed on plankton, algae, and organic matter from dead organisms. Together, the worm's eyes weigh about 20 times more than the rest of its head, and look like two large red orbs attached to its body. If human eyes were proportionally as large, we would need to carry around about 220 extra pounds. Since these worms are nocturnal and disappear when the sun is up, scientists are curious about the purpose of their eyes and their activities.
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In the study, a team from the University of Copenhagen in Denmark, Lund University in Sweden, and Tuscia University in Italy studied three species of bristle worms that they gathered by hand in shallow water. They brought them to a laboratory, where they closely examined their eyes. The team discovered that Vanadis' vision is more advanced and better than previously thought. Its eyes can perceive very small objects and track their movements, despite having a relatively simple nervous system.
A 'secret language'–for mating
The team is still trying to understand how they developed such keen eyesight. The worms' bodies are see-through, except for their eyes, which need to detect light to function properly. This suggests that they cannot be naturally transparent, so their visible eyes must come with certain evolutionary trade-offs. Having a see-through body with visible eyes must have had evolutionary advantages that outweigh the drawbacks.
The advantages that the worms gain remain unclear partly because they do not appear during the day, when eyes typically work best.
“No one has ever seen the worm during the day, so we don’t know where it hides. So, we cannot rule out that its eyes are used during the day as well,” said University of Copenhagen marine biologist and neuroscientist Anders Garm in a statement said in a statement. “What we do know is that its most important activities, like finding food and mating, occur at night. Therefore, it is likely that this is when its eyes are important.”
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The team suggests that part of the reason for this is that these worms can see different light wavelengths from those visible to humans, similar to many birds, reindeer, and other more complex creatures, the worm's vision can detect UV light that is undetectable to the human eye. This may suggest that the purpose of the eyes is to see glowing signals in the dark nighttime ocean. Bioluminescence happens when organisms can generate their own light. Glow-worms are a well-known example that use specific chemicals to create light inside their bodies.
“We have a theory that the worms themselves are bioluminescent and communicate with each other via light. If you use normal blue or green light as bioluminescence, you also risk attracting predators,” said Garm. “But if instead, the worm uses UV light, it will remain invisible to animals other than those of its own species. Therefore, our hypothesis is that they’ve developed sharp UV vision so as to have a secret language related to mating.”
The worms also may need to be on the lookout for UV bioluminescent prey. Regardless of what it is used for, the Vanadis worm could become the first creature proven to naturally create UV bioluminescence to communicate, according to Garm.
Robotics research and evolutionary debates
The team has begun working with robotics researchers from the University of Southern Denmark to explore how to better understand the mechanism behind these eyes well enough to translate it into technology.
“In collaboration with the robotics researchers, we are working to understand how animals with brains as simple as these can process all of the information that such large eyes are likely able to collect,” said Garm. “This suggests that there are super smart ways to process information in their nervous system. And if we can detect these mechanisms mathematically, they could be integrated into computer chips and used to control robots.”
Aside from robotics, their eyes could also help settle a intense debate around evolutionary theory. Did eyes only evolve once into every form we know today or have they arisen several times in evolutionary history?
Vanadis has eyes that are built relatively simply, but have very advanced functions. They have simultaneously evolved in only a few million years–a relatively short span of time in terms of evolution. These worm eyes likely developed independently of more complex eyes like humans, and could help prove that the development of vision is possible over a relatively short time.
