Long-lost ocean worms photobomb tiny seahorses, surprising scientists

A species of marine worm that scientists lost sight of for almost 70 years has finally resurfaced, thanks to some eagle-eyed sleuthing and a seahorse less than an inch long. When researchers analyzed images of tiny seahorses taken by scuba divers, they found evidence of photobombing worms in the hundreds, living alongside the seahorses in coral colonies from Japan to Australia.
The long-lost worm is Haplosyllis anthogorgicola, a species of bristle worm, or polychaete. It typically measures no more than 0.24 inches (6 millimetres), and it burrows inside branching gorgonian corals at a density of up to 15 worms per cubic centimeter. But the creature hasn’t been directly observed in the wild since 1956, when Kyoto University marine biologist Huzio Utinomo first identified it, scientists reported Wednesday in the journal Proceedings of the Royal Society B: Biological Sciences.
Finding these worms is extremely challenging because their small size and near transparency make them almost impossible to see underwater, said lead study author Chloé Fourreau, a doctoral student in the Molecular Invertebrate Systematics and Ecology, or MISE, Laboratory at the University of the Ryukyus in Okinawa, Japan.
“I love that this paper crowdsources imaging from the public to learn more about where and how these animals occur and what they are doing,” said Karen Osborn, a researcher and curator in the department of invertebrate zoology at the Smithsonian National Museum of Natural History in Washington, DC.
With about 10,000 named species of bristle worms and perhaps twice that number yet to be discovered, there are many open questions about polychaete biology, interactions with other species, “and the impacts they have on the areas they inhabit,” said Osborn, who was not involved in the study, in an email. “This paper beautifully shows how they are just below our noses, but virtually unnoticed.”
Study coauthor Ai Takahata, an undergraduate student at MISE and Forreau’s lab partner, was researching the camouflage of pygmy seahorses (Hippocampus bargibanti) when she unexpectedly found several H. anthogorgicola worms in coral samples collected in waters near Japan.
“When she cut a branch of the coral, she noticed some worms came out of it,” Forreau said. “She gave them to me as she knew I was interested in polychaetes, but prior to looking at the worms, I didn’t even know about this species.”
Forreau suspected that pygmy seahorses’ gorgonian coral colonies might yield more of the worms, she told CNN. In 2023, during an unrelated survey in southern Sukumo Bay in Kochi, Japan, she asked the boat captain to make a detour to sample corals, and she found H. anthogorgicola bristle worms inside them.
Shown above is a close-up of a coral branch, with visible burrows and protruding worms. (Chloé J.L. Fourreau / Proceedings of the Royal Society B via CNN Newsource)
But as Forreau was sorting her underwater photos of seahorses and corals, she made another unexpected discovery: The worms’ coral burrows were visible in the images. Perhaps, she thought, that might also be the case in other photos of pygmy seahorses. She and her study coauthors turned to iNaturalist, a website where people share nature images and information on biodiversity, to find photos of pygmy seahorses (and possibly of their worm neighbors, too).
Because pygmy seahorses are so small — about 1 inch (2.5 centimetres) long — divers tend to photograph them in extreme close-ups, which include detailed views of nearby corals. On iNaturalist, 489 photos of the seahorses also included evidence of worms, the researchers reported.
Images of pygmy seahorses taken by scuba divers were crawling with worm photobombs. Worms’ limbs, heads and tails poked out of coral burrows in the hundreds; the scientists even counted seven examples of worms crawling on seahorses’ bodies. Worms’ tunnels snaked through coral branches and into polyps, and approximately 84 percent of the photographed corals were hosting worm infestations, the researchers estimated.
Prior to this study, very little was known about H. anthogorgicola’s range and habits; piecing together the lifestyles of hard-to-find animals is especially challenging when published research about them is nearly nonexistent, Forreau said.
“But our paper shows that we can repurpose the large information available on better known species to learn about understudied ones like worms,” Forreau added in an email.
GPS coordinates on the iNaturalist photos expand the worms’ potential distribution much farther to the south than previously thought, encompassing Australia, East Timor, Indonesia, Japan, Malaysia, New Caledonia, Papua New Guinea, the Philippines and Taiwan, according to the study.
Utinomo’s 1956 paper linked the worms to just one coral species — Anthogorgia bocki — but analysis of the photos suggests that these worms inhabit other corals in the genus. The images also provided the first evidence of the worms interacting directly with seahorses.
Galleries and burrows that H. anthogorgicola worms reside in are visible on the branch shown here. (Chloé J.L. Fourreau/Proceedings of the Royal Society B via CNN Newsource)
Polychaetes live in diverse habitats “including polar ice, methane seeps and deep-sea hydrothermal vents, the spaces between corals, and the vast open ocean,” Osborn said. “They are critical to food webs and build habitat for other animals.”
The iNaturalist photos answered some long-standing questions about H. anthogorgicola’s lifestyle and habits, Forreau added.
“They confirm that the worms are highly reliant on their burrows; most of their time is spent inside,” she said. In many of the images, the only visible part of a worm was its antennae and other trailing appendages poking out of the tunnel. “This position suggests that worms spend a lot of time sensing their environment around them, perhaps waiting for something to eat in the water, or to detect the coral polyp’s movement to steal its food.”
However, many of the burrows were not close to polyps, hinting that the worms could be coral cleaners rather than food thieves.
“There is still much to be learned about the relationship between the worms and the corals where they live,” Forreau said.
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.