Resonating sea urchin skeletons create coastal choruses

Abstract

Ambient sound intensity in coastal waters typically increases by as much as 2 to 3 orders of magnitude (20 to 30 dB) immediately after sunset and before sunrise in what is known as a dawn and evening chorus. The dominant feature of the chorus is most often a dramatic increase in spectrum level usually in a narrow frequency range of around 400 to 4000 Hz. While the sources of some choruses have been identified, the sources of many choruses remain unidentified. Here we confirm that in New Zealand, the sound is the feeding noises of sea urchins for which frequencies in the range of 800 to 2800 Hz are amplified by the ovoid calcareous skeleton, or 'test', of urchins acting as a Helmholtz resonator. Furthermore, the timing of the dawn and dusk choruses is related to the crepuscular feeding activity of sea urchins. Underwater sound recordings from individual sea urchins of a range of sizes confirm earlier speculation that the urchin test acts as a Helmholtz resonance chamber capable of generating sufficient acoustic power to create these choruses. These results indicate the potential importance of coastal urchin populations as a major contributor to the underwater choruses, which appear to be important in assisting the larvae of key reef species, such as fishes, crabs, and lobsters, to locate suitable settlement sites.