How a Fish Steals Its Ability to Glow

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How a Fish Steals Its Ability to Glow

Bioluminescence might seem uncommon, even alien. But biologists think organisms evolved the ability to light up the dark as many as 50 different times, sending tendrils of self-powered luminosity coursing through the tree of life, from fireflies and vampire squids to lantern sharks and foxfire, a fungus found in rotting wood.

Despite all this diversity, the general rules stay the same. Glowing in the dark or the deep takes two ingredients. You need some sort of luciferin, a molecule that can emit light. And you need an enzyme, luciferase, to trigger that reaction like the snapping of a glowstick.

Some creatures delegate this chemistry to symbiotic bacteria. Others possess the genes to make their own versions of luciferin and luciferase. But then there’s the golden sweeper, a reef fish that evolved a trick that hasn’t been seen anywhere else, according to a study published Wednesday in Science Advances: It just gobbles up bioluminescent prey and borrows the entire kit.

“If you can steal an already established, sophisticated system by eating somebody else, that’s way easier,” said Manabu Bessho-Uehara, a postdoctoral scholar at the Monterey Bay Aquarium Research Institute.

Biologists previously thought that the luciferase, at least, had to come from an animal’s own body. Animals do steal ready-made molecular machinery from each other: Sea slugs absorb chloroplasts from the algae they eat and use them to feed on sunlight. Nudibranchs, a related group of species, take stinging cells from anemones and repurpose them for self-defense.

But enzymes like luciferase are big, fragile proteins. Digestive systems typically break proteins into bits and absorb them as nutrients. “People just assumed it couldn’t be done,” said Edith Widder, a MacArthur-winning marine biologist who has used bioluminescence for conservation work and to hunt for giant squid.

But somehow the golden sweeper plucks luciferase intact from its own gut, a team led by Dr. Bessho-Uehara found. The researchers have proposed a new name for the target of this kind of molecular thievery: a kleptoprotein.

“I was initially quite skeptical when I heard what they were claiming,” Dr. Widder said. “But they’ve really done a very good job of convincing me.”

Dr. Bessho-Uehara began studying the golden sweeper as an undergraduate at Nagoya University in Japan.

At night, the pinky-size fish comes out to forage through reefs and shallow waters with a built-in cloaking device: a soft glow emitted in its underbelly. When predators below look upward, that glow helps the golden sweeper blend in against the moonlit surface.

Those same waters teem with bioluminescent crustaceans known in Japanese as “umi-hotaru,” or sea fireflies. In the 1950s, the future Nobel Prize-winning chemist Osamu Shimomura, also working at Nagoya, discovered a new version of luciferin that gave these and related seed shrimp their signature blue glow. Another 1958 study showed that the golden sweeper used the same version of luciferin, too, and hypothesized that it came from snacking on seed shrimp.

But the fish don’t produce their own luciferase, either. Now, through a series of experiments, Dr. Bessho-Uehara and his collaborators have filled in the missing piece of the puzzle. It turns out that the luciferase in the golden sweeper’s light-producing organs also comes entirely from those snackable, bluish seed shrimp.

“I couldn’t believe it,” he said. “It seemed impossible.”

Somehow, Dr. Bessho-Uehara said, the golden sweeper manages to funnel the shrimp’s luciferase into light organs adjacent to the fish’s digestive system, rather than merely digesting it. He hopes to eventually uncover how this system works.

Fish with similar organs, like the flag-in cardinal-fish, another reef-dweller, might use the same trick. So may other bioluminescent creatures.

“I think there are other systems, including crustaceans, that possibly also acquire luciferase from their diet,” said Heather Bracken-Grissom, an evolutionary biologist at Florida International University who was not involved in the study.

For now, the golden sweeper adds to a growing list of known glow-up strategies.

“It’s just a new example of the extreme convergent evolution we find in bioluminescence,” Dr. Widder said. “Animals keep figuring out different ways to make light.”

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