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Infrared Camera Exposes Reef Building

Photonics Spectra
Apr 2001
Daniel S. Burgess

Marine scientist John R.M. Chisholm was scratching his head over this one. The coral in his aquarium, it seemed, suffered from somnambulism. On 21 nights over the course of a month, the colonies moved across the sandy bottom to a rock as far as 6 inches away. He decided to record this activity with an infrared camera, and so solved not only the case of the wandering coral, but also the mystery of how coral reefs form on the shifting seabed.

"Reef corals are not thought to be capable of self-propulsion; thus, we were very surprised when we found that a number of corals had moved location during the night," said Chisholm, the director of research at the Centre Scientifique de Monaco. He enlisted the help of his longtime colleague and friend, Russell Kelley of Watermark Films Pty. Ltd. in Townsville, Australia, to catch the coral in the act.

Infrared video has exposed the nocturnal domestic habits of eunicid worms. A sequence of images illustrates how the marine organism (seen in false color) buttresses its home with pieces of coral; this suggests a mechanism for reefs to form on shifting seabeds. Courtesy of John R.M. Chisholm.

At first, they were stumped. Under weak red or white light, the coral refused to move. When the researchers cycled the illumination, one minute on to 15 minutes off, the coral sometimes would make the trip, but only in the dark when the camera could not see. Chisholm and Kelley turned to the infrared. Using a Hi-8 video camera from Sony Corp. of Tokyo, equipped with a standard near-IR illuminator, they waited.

"The major problem is that near-IR is very rapidly absorbed by seawater [and] thus is only useful for close-up work," Chisholm said. He noted that pure water, with optical characteristics similar to those of seawater, has a transmittance of more than 60 percent per meter at 700 nm, but falls to 9 percent at 775 nm. Nevertheless, Chisholm and Kelley found that, after the aquarium went dark, a eunicid worm as thick as a shoelace emerged from the rock, grabbed the bits of coral, pulled them back home in its jaws and glued them to the surface with a proteinaceous secretion.

Mystery under the sea

The worm's behavior may explain how coral reefs form on largely sandy patches of the seafloor. Reporting their observations in the Jan. 11 issue of Nature, Chisholm and Kelley posit that, by cementing bits of coral to the available hard substrata -- whether rocks or shell and rock fragments -- the eunicids and related marine worms create stable conditions for reefs to take hold. The hypothesis also explains why corals grow in patches, because their establishment would be dependent on the foraging radius of the worms.

"If worms have been responsible for seeding reef growth on continental shelf margins and patchiness in benthic communities in general, as our observations suggest," Chisholm said, "this will be another example of how living organisms cooperate, by accident or design, to create conditions which favor the continuance and evolution of life.

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