It looks weird, but this fish has a secret glow

What color is a sea hare? The answer is more complicated than expected.

Found in the North Atlantic and parts of the Arctic Ocean, these bumpy, bottom-dwelling fish come in a variety of colors that change as the fish age. However, scientists believe they have found the true color of the fish — fluorescent green.

In a study published this month in the Journal of Fish Biology, scientists discovered that lumpfish glow under UV light. They believe these fish use their biofluorescent radiation to identify and possibly communicate with each other.

In recent years, biofluorescence has been observed in catsharks, wombats and flying squirrels and many other types. And now add the lumpfish to nature’s secretly glowing beasts.

Lumpfish are solitary animals that spend most of their lives on the sea floor. These fun-looking fish cling to rocks and seaweed using a modified pelvic fin on their underside that acts like a suction cup to help them hang off until something tasty swims by.

They have also become pseudo-celebrities on TikTok, where an endless stream of videos posted by researchers and fishermen have amassed millions of views.

“It’s a pretty chunky, clumsy-looking fish,” said Nathaniel Spada, a research assistant at the Woods Hole Oceanographic Institution in Massachusetts and a part-time lumpfish influencer. He wasn’t involved in the study, but his Werner Herzog-inspired TikToks about the lumpfish in his lab got millions of views. “I didn’t expect him to be as popular as he is,” he said, “but I should have guessed because they really are a cool fish.”

Last year dr. Thomas Juhasz-Dora, a veterinarian and PhD student at University College Cork in Ireland, curious as he looked into the bulging eyes of a lumpfish in his lab – and had an idea. He had seen biofluorescence in other marine species and wanted to know if his lumpfish had this trait. He collected 11 young lumpfish and photographed them under different lighting conditions. Under normal light, they appeared sea foam green. But when exposed to UV light, their entire bodies would emit a bright, neon green glow.

“I was like ‘wow,'” said Dr. Juhasz-Dora, who was amazed by the intensity of the fish’s biofluorescence. This phenomenon occurs when an organism absorbs ultraviolet rays that are normally invisible to humans and re-emits them in colors visible to us, typically red, orange, or green. This is not to be confused with bioluminescence, where animals create their own light through a chemical reaction.

Many species have special filters in their corneas that allow them to see biofluorescence without the aid of UV light. dr Juhasz-Dora theorizes that sea hares are equipped with these filters, which would allow them to somehow signal their own kind while remaining hidden from predators.

“It’s certainly plausible,” said Elizabeth Fairchild, a researcher and associate professor at the University of New Hampshire who studies lumpfish and other commercially farmed aquatic species.

The fish can also use its biofluorescence to attract prey, but Dr. Fairchild’s money is in communication. “Communication is probably the most likely answer,” she said, “we just don’t know what they’re communicating.”

It’s also possible that the biofluorescence serves no purpose at all. dr But Fairchild, who wasn’t involved in the study, said that’s unlikely given the importance of coloration to lumpfish.

“Lapsticks have such crazy color plasticity,” said Dr. Fairchild. As hatchlings, they can be almost any color of the rainbow. As juveniles, their thick, knobbly skin changes color to blend in with their surroundings, which helps conceal them from predators. In adulthood, sea hares develop pale gray to light blue skin. However, this changes during the breeding season when males turn orange-red and females turn blue-green.

How and why biofluorescence evolved in sea hares is one of many questions raised by Dr. Juhasz-Dora to be raised. He and his colleagues are now investigating whether sea hares can control their biofluorescence. Their recent discovery “opened the door to new discoveries,” he said. “It allows us to really see the world from their perspective, rather than our own.”

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