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
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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 ().
The urchins hide in crevices during the day, out of sight from predators, and emerge to feed at dusk. 鈥淲hen they first come out I guess they鈥檙e hungry, so they鈥檙e eating with lots of gusto and making lots of munching noises,鈥� says co-author Andrew Jeffs. He suggests that the peak in urchin feeding just before dawn may be their 鈥渟upper鈥�, before they tuck themselves back into crevices for the day.
Listen to a recording of the chorus on the reef
This regular noise could even help the larvae of fish and crabs find their way to reefs, says Jeffs, as previous studies have found that some larvae can orient towards sound.
