Coral inhabitants “sing” to the rhythm of the moon

It is far from calm at sea. Healthy coral reefs – which unfortunately are becoming increasingly rare due to warming and acidifying ocean waters – are teeming with life. And that coral life loves to make its voice heard. It creates a cacophony of sounds, from fish but also from other coral inhabitants such as molluscs. By studying these sounds, researchers can measure the health of coral reefs. Creating ‘soundscapes’ using subsea recording equipment also has the advantage that coral reefs can be studied much more continuously than is the case with inspection diving for example. This non-invasive auditory monitoring can also be done at night, when coral life is usually most active.

Thanks to audio clips recorded at three coral reefs off the coast of Hawaii, biologists have discovered that coral inhabitants have different night shifts. The division of metamorphoses appears to depend on moonlight penetrating the seawater to the coral. When moonlight fell on the reef, the researchers noticed the fish’s high-frequency sounds (high tones) being stronger than during the night hours without moonlight. Low-frequency sounds made by fish (low tones) were much weaker when the moon was shining, and the same was true for sounds made by molluscs. Fish in particular seem to be stimulated by moonlight to make their call more powerfully heard with higher pitches. What exactly does this mean – for example, are they the same fish that produce high and low tones or are they different? – Needs further investigation.

The research shows that coral noise varies not only with the monthly lunar cycle, but also with daily changes in moonlight falling on the reef.

Source: Naval Warfare Center, Rhode Island, US PLOS ONE