"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 increase20 to 30 decibelswhich is an increase of a hundred to a thousand times the background level."
Biologists believe the noise of reefsnot just the munching of urchins, but also the pops of snapping shrimps and the grunts of fishacts 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 abilities7.8 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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