AMPLIFIED chewing sounds from ravenous sea urchins create a dawn and dusk chorus beneath the waves.

The ambient underwater noise on rocky reefs becomes a hundred times louder just before dawn and just after dusk. To find out why, Craig Radford and his colleagues at the University of Auckland, New Zealand, recorded the sounds made by individual reef animals in the lab, and then compared them with the dominant sound in the natural reef din.

Listen to a recording of the sea urchins in the lab

They found that grazing sea urchins produced the noise as they scraped algae off rocks. The hard, dome-shaped bodies of the animals act like resonance chambers, amplifying the sound of their chewing (Marine Ecology Progress Series, vol 362, p 37).

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