Reproduction can be a haphazard event, especially when you simply cast your gametes into the sea and hope for the best. However, some organisms in coral reefs have improved their odds by synchronising when they spawn. These mass spawning events on reefs can last for as little as 20 min and only occur during twilight for a few nights each year. So, how do animals that lack even the simplest of nervous systems coordinate such sophisticated behaviour? Alison Sweeney and her colleagues from the University of California, Santa Barbara, and Duke University wondered whether mass spawning events might be synchronised by fluctuations in the twilight spectrum. They explain that the spectrum of twilight is deep blue before the moon rises, but after moonrise the spectrum becomes redder. As the moon is already in the sky at sunset during the first half of a lunar month, the twilight spectrum is always red shifted, but at full moon (when the moon is just below the horizon at sunset) there is a brief period when the spectrum of skylight is deep twilight blue before the moon rises. Sweeney and her colleagues realised that corals and other reef residents could use this brief period of pure twilight to synchronise spawning, but only if the spectrum of light in the ocean followed the same pattern as skylight (p. 770).

Measuring the spectrum of light in the ocean above a coral reef in the US Virgin Islands over a 6 day period around full moon, the team found that the twilight spectrum shifted significantly depending on whether or not the moon had risen. At full moon, the twilight spectrum was deep blue just after sunset but gained red wavelengths as soon as the moon rose. Also, the length of the blue twilight period increased on subsequent evenings as the moon rose later each day. While recording the light spectrum, the team also monitored elkhorn coral colonies for spawning events and found that the corals spawned simultaneously between 21:30 h and 21:50 h on the third and fourth nights after the full moon.

Of course, this does not confirm that corals use shifts in the twilight spectrum to synchronise spawning – at present there is only a correlation. However, Sweeney and her colleagues point out that corals are capable of detecting changes in light quality and synchronise successfully even on overcast days, so they are keen to test the effects of skylight spectra on spawning events to find out whether twilight synchronises mass spawning.

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Twilight spectral dynamics and the coral reef invertebrate spawning response
J. Exp. Biol.