As millions of species roam the planet, there can sometimes be years or even decades between sightings.

Some species go "missing" because they live in incredibly remote reaches of the Earth, while others face habitat destruction that causes their numbers to dwindle.

In other cases, animals can appear to have simply disappeared -- unless you know where to look.

In 1956, researchers discovered a new species of marine worm that lives in coral burrows off the southern coast of Japan, according to a study published Nov. 13 in the peer-reviewed journal Proceedings of the Royal Society B.

The species was named Haplosyllis anthogorgicola, and it is incredibly small. After the first description of the species, the worm was never seen again in the scientific record.

Then, Chloé Fourreau, a Ph.D. student at the University of the Ryukyus in Japan, got a visit from her labmate, Ai Takahata.

Takahata had recently collected a chunk of coral during a SCUBA trip off Kochi, Japan, in their study of pygmy seahorses, according to the study and a Nov. 13 post by Fourreau on X.

When the coral was collected and brought back to the lab, the researchers realized it was filled with tiny worms, each smaller than 0.2 inches long, Fourreau said.

The worms were identified as H. anthogorgicola, the first record of the species in nearly seven decades. Because they create burrows in the coral itself, Fourreau said they were likely incredibly hard to find.

Fourreau and her colleagues then realized that they could see the worm's burrows when they looked at pictures of the coral, she said in posts on X. When they pulled up the galleries of coral photos, they could identify which bumps and nodes were actually little worm homes.

The photos were taken to study the pygmy seahorses that also live among the coral, researchers said, a species that mimics the coral in appearance.

"Look at this tiny, pink, camouflaged, CUTE seahorse! And what (do) humans like to do with cute animals? Take their picture, of course!" Fourreau said on X. "So, we looked into the background of pictures of pygmy seahorses taken by divers on (iNaturalist)."

The citizen science site iNaturalist allows wildlife and nature enthusiasts to share images of species along with the location where it was found and some facts about their ecology.

Dozens of photos of pygmy seahorses had been posted on the site, including from southern Japan, and instead of looking at the bumpy creatures in the foreground, the researchers took a closer look behind the animals.

"And lo and behold, the burrows and worms galleries were visible there, in ¾ of the pictures!" Fourreau said. "But there's more! Sometimes we could not only see the burrows and galleries, but also the worms themselves, photobombing the seahorse!"

"Most of the time, the worms were 'chilling' in their burrows, with only their antennae outside, probably sensing the movement of surrounding water and nearby coral polyps," Fourreau said. "But sometimes, they partially exited the burrows to interact with things such as debris on the coral. This hints that they may be helping the coral by cleaning it."

Photos of the coral and pygmy seahorses, and worm burrows by association, go back years on the iNaturalist website, highlighting the benefit of citizen science in supplementing the scientific record.

Researchers said the photos not only showed that the species was not missing, but that it is also plentiful and widely distributed in southeastern Asia waters, according to the study.

"This goes to show that charismatic and popular species can sometimes help us learn about the ones that are lesser known or nearly forgotten," Fourreau said. "And we will probably be able to better understand all of them when considering their interactions together."

The coral containing the worm burrows was collected off the coast of Kochi, a city on the southeastern Shikoku island of Japan.

The research team includes Fourreau, Takahata, Laura Macrina, Jue Alef A. Lalas, Tatsuki Koido and James Davis Reimer.