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Noisy Eaters Are Cause of Mysterious Ocean Sounds

Dave Hansford in Wellington, New Zealand
for National Geographic News
August 18, 2008
 
If noisy chewing bothers you, never date a sea urchin.

New Zealand scientists say they have confirmed that the spiny sea creatures are responsible for a mysterious, twice-daily uproar heard underwater.

The 20- to 30-decibel noise is caused by the spiny sea creatures' teeth scraping on reefs as the hungry starfish relatives feed on algae and invertebrates.

Auckland University marine biologists Craig Radford and Andrew Jeffs solved the mystery during a study of ambient noise around northern New Zealand reefs.

The pair recorded two massive spikes in sound intensity each day: The first occurs just before dusk, the other before sunrise.

Radford said urchins had long been suspected of creating the din, but it took a series of experiments to confirm it.

"We put some urchins in a tank and got them feeding on algae, then we recorded them. The noise they were producing caused spikes at certain frequencies."

Those frequencies matched the sonic peaks the team had recorded at sea. The recorded frequencies also confirmed a series of earlier Australian experiments using Helmholtz resonance, the phenomenon of air resonating as it moves in and out of a cavity—as when you blow across a bottle opening to create a tone.

Radford said the nocturnal animals' hard shells produced just such a resonance as they fed with their five-toothed, calcium-carbonate mouths. An urchin's chewing apparatus is called an Aristotle's lantern, due to the ancient Greek philosopher's reference to the mouth's resemblance to a type of five-sided lamp.

Noisy Eaters

"The noise they make is the sound of those teeth scraping on the rocks," Radford said. "A large urchin will have a low resonance frequency, while a small urchin will have a higher frequency.

"When they emerge from their crevices at dusk, they're probably really hungry, munching away quite rapidly," he said, adding that the din drops off as the night progresses.

"They have another big feed before they go to sleep" at dawn, the biologist said.

On heavily fished reefs—where depleted fish stocks have led to an increase in urchin numbers—the noise was much greater than in reserves where fishing was banned, the researchers found.

Coastal noise of similar frequency and bandwidth has been recorded near the Bahamas; San Diego, California; and Australia.

Chris Tindle, a physicist at the University of Auckland, said the urchins made more noise on dark nights around the new moon.

"It's a huge increase—20 to 30 decibels—which is an increase of a hundred to a thousand times the background level."

Sonic Signposts?

Biologists believe the noise of reefs—not just the munching of urchins, but also the pops of snapping shrimps and the grunts of fish—acts as a beacon. The sound may guide larval fish and crustaceans, which hatch in plankton swarms many miles out at sea, to suitable habitats.

"They have to find their way back somehow," study co-leader Radford said. "They've got really impressive swimming abilities—7.8 inches [20 centimeters] a second for a 0.2-inch-long [5-millimeter-long] animal. But they need some kind of cue to swim towards."

In a featureless blue world, he said, "there are no visual cues. So we reckon it's sound."

Tindle, the physicist and marine sound expert, noted that fish larvae have been attracted toward sound in laboratory experiments.

"Whether they are attracted by sound from way out at sea, we still don't know. But we think sea creatures use the sounds coming from different directions to find their way around, to navigate."
 

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