Washington, August 19 : A team of scientists has confirmed that urchins - spiny sea creatures, are responsible for a mysterious, twice-daily uproar heard underwater around northern New Zealand reefs.
According to a report in National Geographic News, the mystery was solved by Auckland University marine biologists Craig Radford and Andrew Jeffs, who conducted a study of ambient noise around the reefs.
The pair recorded two massive spikes in sound intensity each day: The first occurs just before dusk, the other before sunrise.
They found that 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.
Radford said that 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," he said.
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 that the nocturnal animals' hard shells produced just such a resonance as they fed with their five-toothed, calcium-carbonate mouths.
"The noise they make is the sound of those teeth scraping on the rocks. A large urchin will have a low resonance frequency, while a small urchin will have a higher frequency," he said.
"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.
The researchers found that 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. Coastal noise of similar frequency and bandwidth has been recorded near the Bahamas; San Diego, California; and Australia.
According to Chris Tindle, a physicist at the University of Auckland, 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," he said.
Biologists believe that 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.