Film solves mystery of sleepwalking coral

Was coral ecologist John R. M. Chisholm losing his mind? Lumps of coral, with the ambulatory power of your average rock, somehow kept changing places in his aquarium overnight.

In false-color, infrared images (from top), a eunicid worm (yellow) writhes out from its rock and grasps a coral lump from 5 centimeters away, tenses its body, and in less than 2 seconds, yanks home the lump. Chisholm and Kelley/Nature

Chisholm, of Centre Scientifique de Monaco in Monaco, can now rest assured of his sanity. After a series of frustations, Chisholm and filmmaker Russell Kelley have finally captured infrared images of a bootlace-size eunicid worm poking out of a rock, yanking lumps of coral back to its fortress, and gluing them in place.

“This is the first time to our knowledge anybody’s seen this,” Chisholm says. He and Kelley, from Watermark Films in Townsville, Australia, describe the worm’s feat in the Jan. 11 Nature.

Worm work could play a major role in building coral reefs, particularly the puzzling ones on soft sediment, Chisholm suggests. Scientists haven’t known how creatures starting such structures avoid being buried.

Now, Chisholm speculates that worms assemble coral bits and whatever hard materials they can find and thus provide more stability in an unsteady world. That relative solidity increases the odds that fish larvae seek shelter there, and their wastes fuel coral growth. With the structure’s increasing stability, chances rise that waterborne juvenile corals will join the heap, further stabilizing it. “You get a chain reaction,” Chisholm sums up.

Reef specialist John Pandolfi at the Smithsonian Institution in Washington, D.C., welcomes the new study: “I think it’s great.” He says, “Nobody knows very much about how a coral reef gets off the ground.”

The researchers’ worm started with a rock, Pandolfi notes. So, he continues to wonder about reef start-up on shifting sediments free of even small rocks. Regardless of whether the worm scenario explains such an extreme case, Pandolfi says, “it provides a nice mechanism.”

At first, Chisholm says, he had no idea a worm was moving the three lumps of coral, which weigh 5 to 20 grams. In the morning, lumps would appear right-side-up, 6 to 16 centimeters from where he’d left them at night. He returned the corals to their original spots to have them moved again, 21 times in a month.

When he and Kelley tried to film the process with dim white or red light, nothing happened. When 1-minute light pulses alternated with 15 minutes of darkness, coral moved only during the blackouts.

An infrared setup finally revealed the worm at work. “It all happens so fast,” Chisholm says. “It’s like a rubber band.”

Eunicid worm specialist Kristian Fauchald of the Smithsonian comments, “This is a bit of a surprise.” He adds, “Clearly, the consequences of a dense population of worms will be substantial in changing the outlines of the reef.”

The carnivorous worms, which live in tropical oceans and grow several meters long, foil predators by avoiding light, Chisholm speculates. He adds, “We think there’s a whole world of biology out there happening in complete darkness.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.