Eventually these rear segments become little more than sacks engorged with either sperm or eggs. At exactly the right moment, Fauchald said, "the rear end starts some very heavy muscle contractions and eventually breaks off."
The liberated segments then start spiraling toward the surface. They float for up to an hour until the outer casings split open, spilling out their contents. Sperm fertilizes the floating eggs in a vast reproductive frenzy that happens just once a year and lasts only for a few hours.
But successful fertilization is not guaranteed. "There are several complicating factors," Fauchald said. "You must have an adequate sperm concentration. There must be enough mucous present to keep everything together, so that the spawning mass is not fragmented or washed apart. A storm would be a big problem." So would large quantities of predatory fish.
A few hungry islanders, by comparison, are the least of the worm's problems.
Once a successful swarming is over, the zygotesfertilized eggslive in the open water for only a few days before sinking to the bottom. There they burrow into the coral and grow into the next generation.
But what happens to the parent worms who so recently lost three quarters of their back end? "They don't necessarily die," Fauchald said. "Once the posterior has broken off, the anterior end promptly starts with wound-healing. They have to get their digestive tracts working fairly quickly, otherwise they won't be able to swallow." It takes about a week until they're fully healed, and then they start producing new segments to make up for the ones that were lost.
Timing Is Everything
Successful reproduction depends on getting all those packets to the surface at exactly the same time. But how does a worm that spends its life in a darkened burrow know when to release its sperm and eggs?
It's a question that interests the Samoans as well, and everyone has a favorite date.
Some say it happens three days after the new moon in October or November. Or a little after the last quarter of the first full moon in October.
Everyone agrees that the spawning follows the lunar cycle, and that it usually happens somewhere around the seventh night after the full moon that follows the autumnal equinox. If it's a weak showing, then a second rising can be expected in November.
To help find their way to the surface, the worms have a row of light-sensitive eyes along one side of their bodies. "Even on a cloudy night the surface will be lighter than the ground behind them," Fauchald said, "and that's enough to get them to the surface.
"Once the first worm goes, the presence of spawn in the water sets off all the others."
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