Craig Radford of the University of Auckland, New Zealand, curious about why ambient underground water noises became louder twice a day, recorded sounds made by individual reef animals then compared them with the sound in the natural reef, New Scientist reported Tuesday.

Radford and his colleagues found grazing sea urchins produced the noise as they scraped algae off rocks. Their bodies act like resonance chambers, amplifying the sound of their chewing, he said.

"When they first come out I guess they're hungry, so they're eating with lots of gusto and making lots of munching noises," says co-author Andrew Jeffs.

He suggests that the peak in urchin feeding just before dawn may be their "supper," before they retreat into crevices for the day.