for National Geographic News
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.
"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.
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