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That reflex is what makes its behavior so puzzling. Small fish<\/a> settle right inside those arms while the worm barely reacts.<\/p>\n Location of the research<\/p>\n The project grew out of the\u00a0Boston University Marine Program\u00a0(BUMP<\/a>) a hands-on course that puts undergraduates on real reefs.<\/p>\n Its divers spent a week in November 2025 working the reefs and seagrass beds of Turneffe Atoll.<\/p>\n Much of that time ran through the Calabash Caye Field Station, a marine base off the coast of Belize.<\/p>\n The station is run by the\u00a0University of Belize Environmental Research Institute\u00a0(UB ERI<\/a>).<\/p>\n Hidden reef relationships<\/p>\n Morgan Bennett-Smith, a PhD candidate at Boston University, led the work. He had already seen the same behavior an ocean away, on reefs in Papua New Guinea.<\/p>\n \u201cIt may represent a widespread ecological phenomenon that scientists have simply overlooked and suggest that there are potentially many micro interactions<\/a> that remain undocumented,\u201d said Bennett-Smith.<\/p>\n \u201cCoral reefs are host to various forms of life and observing these small interactions could unveil new discoveries of how coral reef ecosystems function.\u201d<\/p>\n Tree worms stayed open<\/p>\n Christmas tree worms live embedded in living coral, showing only their radioles. Those branched crowns handle both breathing and catching food that drifts past.<\/p>\n The crown is also a tripwire. A shadow or a nearby movement, and it retracts fast, sometimes staying shut for a minute or more.<\/p>\n So a worm that tolerates a fish sitting in its crown is doing something unexpected. That small mismatch is what pulled the team in.<\/p>\n Fish remain stationary<\/p>\n The fish here are cryptobenthic species, the tiny bottom-dwellers most divers glide right past. In Belize, the primary species involved was the Caribbean neon goby, a blue-striped sliver of a fish.<\/p>\n Again and again it perched inside the worm\u2019s crown without triggering the retreat. It often laid its tail against the radioles, a move the team named tailing.<\/p>\n In about a quarter of the encounters the fish touched the crown directly. The worm still stayed open.<\/p>\n Older photos from Bonaire caught a yellownose goby and a glass blenny doing the same. A second worm, the star horseshoe worm, tolerated the gobies too.<\/p>\n Counting the species<\/p>\n To test whether this was chance, the divers logged every coral<\/a> colony they crossed. <\/p>\n Each one was marked for gobies, worms, both, or neither. Across more than 1,600 colonies, only 12 held the two animals together.<\/p>\n Yet that pairing turned up far more often than random chance allows, by roughly eleven times.<\/p>\n
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<\/a>Small coral reef fishes and Christmas Tree worms interacting. Credit: Morgan F. Bennett-Smith. Click image to enlarge.Coral preferences matter<\/p>\n