A Staff Report from the Straight Dope Science Advisory Board

Why do coral only have sex on one specific night a year?

September 25, 2000

Dear Straight Dope:

I have been a diver for a long time now and I keep wondering why coral reproduce only once a year on the first full moon in August (or something like that). I think the bright light from a full moon interrupts their sleep, but some are nocturnal, right? What gives?

SDStaff Doug replies:

The sex life of corals and other marine organisms tends to be pretty intermittent for a number of reasons, most of which relate to their habit of simply releasing sperm and eggs into the water instead of having physical contact with a partner and delivering gametes in an efficient manner. Sexual synchrony is the only way to ensure a density of sperm and eggs sufficient to permit a decent number to find one another in that vast volume of seawater. As for why a particular night should be better than any other, it certainly will relate to things like tides and the life cycle of the species involved — after all, what good is fertilizing 30 billion eggs if they all get washed up on a beach a few hours later?

Some critters actually mate (that is, have physical contact), but do so only in certain moon phases. This has to do with predator saturation and vulnerability. Palolo worms, for instance, come out of the coral crevices they live in just one night a year for an orgiastic frenzy. There's no way that predators can eat all the palolo worms in a single night. If the worms mated more often, a greater proportion of them would fall victim. Same thing happens with 17-year cicadas. No predator can eat an infinite amount of food in a finite amount of time. Timing is everything.

SDStaff Jill comments:

"most of which relate to their habit of simply releasing sperm and eggs into the water instead of having physical contact with a partner and delivering gametes in an efficient manner … . Some critters actually mate (that is, have physical contact), but do so only in certain moon phases. This has to do with predator saturation and vulnerability."

Oooh, Doug. I love it when you talk like this.

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