Why Amazing Corals Spawn Simultaneously

When corals spawn, they cast millions of sperm and eggs into the sea, where they drift up to the ocean surface, collide, form larvae and float away to form new coral reefs. The coral polyps will "blow" their eggs and sperm simultaneously in quick frenzies for just one, or maybe a few, consecutive nights a year -- usually shortly after sunset on evenings closely following a full moon. A reef generally picks one day during a full moon in summer to blow, for 20 minutes or so, during the twilight hours. Although scientists have yet to agree on how corals know which month to spawn, Alison Sweeney, an evolutionary biologist at the University of California at Santa Barbara, chose a narrower question: How do corals select the precise moment to blow?

Sweeney suspected that a hue shift in the twilight sky away from red, toward blue, was the polyps' cue. Prior to a full moon, the moon reaches the sky before sunset and, reflecting the ruddy light of the setting sun, makes the whole sky slightly redder. Just after a full moon, when sunset precedes moonrise, the moon is no longer there to reflect the pinkish tint, so twilight turns bluer.

Sweeney led a research team to the Virgin Islands in August 2009. They observed elkhorn coral for six evenings. Nearby they suspended an optical cable to reef depth, about eight feet below the surface, from a floating spectrometer. They noted shifts in the ocean's color each twilight. Consistently, it reflected the sky's color. The coral spawned during twilights of radiant blue: the third and fourth nights after a full moon, between 9:20 p.m. and about 9:50 p.m.

Sweeney, whose team reported its results in the February Journal of Experimental Biology, believes that like sea urchins, which link reproduction to lunar cycles, elkhorn "see" color shifts through their skin, which contains photo receptors of the kind found in human retinas. She is unsure why they prefer blue hues to red. However, when the receptors recognize the right color, a biochemical reaction probably ripples through the entire reef

- Rebecca Coffey, Scientific American, May 2011