Marine worm with giant eyes 20 times size of its head leaves scientists perplexed

If humans had eyes proportionally as big they would add an extra 100 kg to our heads, researchers say

Scientists are perplexed by the discovery of a marine worm with giant eyes that are 20 times heavier than its head and that give it vision as sharp as a mammal.

The eyes of the Vanadis bristle worm, found around the Italian island of Ponza, also enable the species to have a secretive language that is only seen by their own kind in the dark depths of the sea, researchers suspect.

The nocturnal worm’s eyes are so large that if humans had eyes proportionally as big, they would add an extra 100 kg to our heads.

Its eyes provide the worm “outstanding vision” on a par with that of mice or rats, enabling the species to see small objects and track their movements in the sea at night, scientists say.

“It’s really interesting because an ability like this is typically reserved for us vertebrates, along with arthropods (insects, spiders) and cephalopods (octopus, squid),” Dr Anders Garm from the University of Copenhagen said.

“This is the first time that such an advanced and detailed view has been demonstrated beyond these groups,” Dr Garm, who co-authored the study, said.

Scientists are seeking to understand how the otherwise simple nervous systems of the worm performs very complex functions.

The worm has a transparent body, except for its eyes, meaning the evolutionarily benefits of the eyes must outweigh the consequences of it making the Vanadis visible to its predators.

Researchers are also unsure of the exact functions of the large eyes, particularly since the creatures are nocturnal and tuck away during the day, when eyes usually work best for all animals.

“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,” Dr Garm said.

“What we do know is that its most important activities, like finding food and mating, occur at night. So, it is likely that this is when its eyes are important,” he added.

Scientists also found that the worm’s eyes are tuned to see ultraviolet light, invisible to the humans, indicating that the Vanadis sees bioluminescent signals in the otherwise pitch-black nighttime sea.

In the new study, scientists speculate that the worms themselves could be bioluminescent, communicating with each other via UV light for mating and hunting prey.

“If you use normal blue or green light as bioluminescence, you also risk attracting predators. 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,” Dr Garm explained.

“It makes things truly exciting as UV bioluminescence has yet to be witnessed in any other animal. So, we hope to be able to present this as the first example,” he added.