Christmas Tree Worms and Gobies Reveal a Hidden Coral Reef Relationship

A Christmas tree worm spends much of its life concealed in full view among coral reefs.

At even the smallest disruption, its vividly coloured, spiral crown withdraws into its tube almost instantly.

That instinctive response makes another aspect of its behaviour difficult to explain. Tiny fish can settle among the arms while the worm shows almost no reaction.

Research location

The study emerged from the Boston University Marine Program (BUMP), a practical course that takes undergraduate students onto working reefs.

During one week in November 2025, its divers surveyed reefs and seagrass beds around Turneffe Atoll.

Much of the fieldwork was carried out through the Calabash Caye Field Station, a marine research base off Belize’s coast.

The facility is operated by the University of Belize Environmental Research Institute (UB ERI).

Concealed coral reef relationships

Morgan Bennett-Smith, a PhD candidate at Boston University, headed the research. He had previously observed the same behaviour on reefs in Papua New Guinea, on the other side of the world.

“It may represent a widespread ecological phenomenon that scientists have simply overlooked and suggest that there are potentially many micro interactions that remain undocumented,” said Bennett-Smith.

“Coral reefs are host to various forms of life and observing these small interactions could unveil new discoveries of how coral reef ecosystems function.”

Christmas tree worms remained open

Christmas tree worms are embedded within living coral, with only their radioles visible. These branching crowns are used both to breathe and to collect food drifting through the water.

The crown also functions as a sensitive alarm system. A passing shadow or movement nearby can make the worm retract swiftly, and it may remain closed for a minute or longer.

A worm allowing a fish to sit in its crown is therefore unexpected. This apparent contradiction was what caught the researchers’ attention.

Fish stay still

The fish involved are cryptobenthic species: small bottom-dwellers that most divers pass without noticing. In Belize, the main species was the Caribbean neon goby, a tiny fish marked with blue stripes.

Repeatedly, it perched within a worm’s crown without causing the worm to withdraw. It commonly rested its tail against the radioles, a behaviour the researchers called tailing.

In roughly one quarter of observations, the fish made direct contact with the crown. Even then, the worm remained open.

Earlier photographs from Bonaire documented a yellownose goby and a glass blenny behaving in the same way. Gobies were also tolerated by another species, the star horseshoe worm.

Recording the species

To establish whether the pattern was merely coincidental, the divers recorded every coral colony they encountered.

They classified each colony according to whether it contained gobies, worms, both species, or neither. Of more than 1,600 colonies, just 12 contained both animals.

Even so, the pairing appeared around eleven times more frequently than would be expected by chance.

Coral preferences are important

Coral species had a greater influence than habitat. Gobies preferred large brain corals and star corals, whereas the worms were more concentrated on mustard hill coral.

Colony size created another division. Gobies were more frequently found on larger coral heads, while worms tended to favour smaller colonies.

When a large colony supported both species, the likelihood of finding a goby nestled within a worm increased markedly. The two animals were not simply occupying the same reef by chance.

Possible benefits for both species

It is not yet clear which animal, if either, gains from the arrangement. The nature of the relationship remains unresolved, but the paper puts forward three explanations.

The fish could be acting as a thief, taking food particles gathered from the water by the worm’s crown.

Alternatively, it may be an unharmful resident, using the crown as shelter and as an elevated perch above the reef.

The more positive interpretation is that the pair exchange services. Neon gobies are recognised cleaners, so the worm’s fine, hair-like arms might remove parasites and mucus from the fish, while the fish clears debris from the crown.

A natural alarm system

One observation points towards a genuine partnership. Whenever a worm abruptly retracted, the nearby goby fled at exactly the same moment.

This synchronisation indicates that the fish may interpret the worm’s retreat as a warning. The worm detects a disturbance and closes up, while its small companion receives an early alert to danger.

The opposite may also be true. A goby’s keen vision could detect a predator long before the worm can, allowing the crown time to shut.

The students behind the discovery

The finding came from a teaching class rather than a large laboratory. Undergraduate students learned to dive, completed the surveys and recognised the pattern themselves.

“Participating in projects like this is all I’ve ever wanted to do,” said Boston University undergraduate Mason Murchison.

“From poring over marine science books as a kid to now having the opportunity to work with researchers in the program and explore marine ecosystems, that’s what I spent every night dreaming about. It was the opportunity of a lifetime for me.”

The value of careful observation

“Modern biology often focuses on genomics, big datasets, and advanced technology. Those tools are incredibly important, but discoveries still begin with people paying attention to the natural world,” said Bennett-Smith.

“This study started because divers noticed something unusual and asked questions.”

“The fact that we were able to connect observations from Papua New Guinea, Belize, and archival photographs from Bonaire highlights that many hidden interactions on coral reefs are waiting to be discovered.”

The class project contributed directly to the published dataset. The students’ surveys in the field became a significant component of the published research.

Future tests

The straightforward answer is that this relationship is still unexplained. Field observations can establish that two animals occupy the same space, but cannot show why they do so.

The team plans to conduct controlled aquarium experiments, comparing worms kept with gobies against worms kept without them. Dyed food will then be used to determine whether the fish steals meals or contributes in return.

For now, the small fish in a worm’s crown remains an unresolved reef mystery. It is the sort of finding revealed only through close observation.